# Patent

## Title
Method and apparatus for scheduling packets

## Abstract

A method and apparatus for scheduling packets using a pre-sort scheduling array having one or more smoothing registers. The scheduling array includes a number of round buffers, each round buffer having an associated smoothing register. To schedule a packet for transmission, the packet's transmission round and relative position within that round are determined, and an identifier for the packet is placed at the appropriate position within the scheduling array. A bit of the associated smoothing register is set, the set bit corresponding to the entry receiving the packet identifier. During transmission, the set bits of the smoothing register associated with a current round buffer are read to identify packets that are to be dequeued.

## Background
RELATED APPLICATIONS
This application is related to U.S. patent application Ser. No. 10/640,206, entitled “METHOD AND APPARATUS FOR SCHEDULING PACKETS”, filed on Aug. 12, 2003, and to U.S. patent application Ser. No. 10/819,818, entitled “METHOD AND APPARATUS FOR SCHEDULING PACKETS”, filed on even date herewith.
FIELD OF THE INVENTION
The invention relates generally to computer networking and, more particularly, to a method and apparatus for scheduling packets.
BACKGROUND OF THE INVENTION
A network switch (or router or other packet forwarding or data generating device) may receive packets or other communications at rates exceeding hundreds, if not thousands, of packets per second. To insure a fair allocation of network resources (e.g., bandwidth), or to insure that resources are allocated in accordance with a desired policy, a network switch typically implements some type of packet transmission mechanism that determines when packets are selected for transmission. A conventional packet transmission mechanism will generally attempt to allocate bandwidth amongst all packet flows in a consistent manner, while preventing any one source from usurping too large a share—or an unauthorized share—of the network resources (e.g., by transmitting at a high data rate and/or by transmitting packets of relatively large size).
A typical network switch includes a number of packet queues, wherein each queue is associated with a specific flow or class of packet flows. As used herein, a “flow” is a series of packets that share at least some common header characteristics (e.g., packets flowing between two specific addresses). When packets arrive at the switch, the flow to which the packet belongs is identified (e.g., by accessing the packet's header data), and the packet (or a pointer to a location of the packet in a memory buffer) is stored in the corresponding queue. Enqueued packets are then selected for transmission according to a desired policy. For example, a number of queues may be serviced one-by-one in a round-robin manner; however, such a simple round-robin transmission scheme does not take into account packet size, and a source of packets could acquire bandwidth by simply transmitting packets of a large size.
To overcome the deficiencies of the simple round-robin method described above, a method known as “deficit round-robin” has been proposed. In a deficit round-robin packet transmission scheme, a number of packet queues are serviced one by one in a round-robin manner, wherein one pass through each active queue (i.e., a non-empty queue) is referred to as a “round.” For each round, a queue is allocated a quantum of bandwidth, which may be expressed as a specified quantity of data (e.g., 256 bytes). The quantums for a number of queues (or packet flows) may be the same or, alternatively, a set of queues may have different quantums (e.g., to implement quality of service or other resource reservation schemes). Also, a “deficit count” is maintained for each queue, and the deficit count for a queue during any given round is equal to the sum of any remaining deficit count from the preceding round plus the quantum for that queue. Accordingly, the usable bandwidth by a queue during a round is the current value of the deficit count (i.e., the prior deficit count plus the quantum).
When a queue is serviced during a round, the packet at the head of the queue is accessed to determine the packet's size, and if the packet's size is less than or equal to the current deficit count, the packet is dequeued for transmission. The size of the dequeued packet is subtracted from the deficit count, and if the updated value of the deficit count is greater than or equal to the next packet in that queue (if any), the next packet is also dequeued for transmission and the deficit count again updated. So long as a queue has remaining bandwidth in the current round—as determined by the queue's deficit count—the queue may continue to dequeue packets for transmission. Conversely, if the packet at the head of the queue (or any subsequent packet, if the lead packet has already been dequeued) has a size exceeding the current deficit count of the queue—i.e., the queue has utilized its allocated bandwidth in the current round—a packet (or an additional packet) is not dequeued from this queue during the current round. However, the queue's unutilized bandwidth, as expressed by the queue's deficit count, is carried over to the next round. An example of the above-described deficit round-robin method is presented in M. Shreedhar et al., Efficient Fair Queuing Using Deficit Round Robin, Proceedings ACM SIGCOMM, pp. 231-242, 1995.
The deficit round-robin scheme described above provides an approximation of another technique for scheduling packets known as Weighted Fair Queuing (WFQ). The WFQ scheduling technique may provide guaranteed services for a given number of sessions sharing the same link. WFQ is itself an approximation of Generalized Processor Sharing (GPS). GPS is defined using a fluid model in which packets are considered to be infinitely divisible. According to the GPS scheme, at any instant, each active session is served with a minimum rate equal to that session's reserved rate. Excess bandwidth available from inactive sessions (e.g., those with empty queues) is distributed among all active sessions at each instant in proportion to their individual reservations. For example, where each session is apportioned an equal share of the available bandwidth on a link and there are N active queues, the active sessions are served simultaneously, each at 1/N the link speed. Because GPS serves a small portion of a packet at a time, this scheme can, in practice, be difficult to implement.
WFQ is a packet-by-packet version of the GPS algorithm. A virtual finish time is calculated for each packet, and packets are sorted in a priority order according to their finish times. Packets are then transmitted according to this ordering. However, the sorting of transmission candidates (e.g., packets) according to their finish times and the WFQ finish time calculations are complex and require a large computational overhead, making this scheme costly to implement.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating an embodiment of a network having a switch including a packet scheduler.
FIG. 2A is a schematic diagram illustrating an embodiment of the switch shown in FIG. 1.
FIG. 2B is a schematic diagram illustrating an embodiment of per-port data maintained by the switch of FIG. 2A.
FIG. 3 is a schematic diagram illustrating an embodiment of a processing device shown in the switch of FIG. 2A.
FIG. 4 is a schematic diagram illustrating one embodiment of the packet scheduler shown in FIGS. 1-3.
FIG. 5 is a schematic diagram further illustrating the packet scheduler shown in FIG. 4.
FIG. 6 is a schematic diagram further illustrating the packet scheduler shown in FIG. 4.
FIG. 7 is a block diagram illustrating an embodiment of a method of scheduling enqueued packets using a pre-sorted scheduling array, as may be performed by the packet scheduler of FIGS. 1-6.
FIG. 8 is a block diagram illustrating an embodiment of a method of calculating a transmission round for a packet, as may be performed by the packet scheduler of FIGS. 1-6.
FIG. 9 is a block diagram illustrating an embodiment of a method of dequeuing packets for transmission using a pre-sorted scheduling array, as may be performed by the packet scheduler of FIGS. 1-6.
FIGS. 10A-10H are schematic diagrams illustrating an example of the methods of FIGS. 7 through 9.
FIG. 11 is a schematic diagram illustrating an embodiment of a pre-sorted scheduling array including smoothing registers.
FIG. 12 is a block diagram illustrating an embodiment of a method of scheduling packets using a pre-sorted scheduling array having smoothing registers.
FIG. 13 is a block diagram illustrating an embodiment of a method of dequeuing packets for transmission using a pre-sorted scheduling array having smoothing registers.
FIG. 14 is a schematic diagram illustrating an embodiment of a round buffer and its associated smoothing register.



DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a packet scheduler are disclosed herein. The disclosed embodiments of the packet scheduler are described below in the context of a network switch. However, it should be understood that the disclosed embodiments are not so limited in application and, further, that the embodiments of a packet scheduler described in the following text and figures are generally applicable to any device, system, and/or circumstance where scheduling of packets or other communications is needed. For example, the disclosed embodiments may find application in a switch on a high speed backplane fabric.
Illustrated in FIG. 1 is an embodiment of a network 100. The network 100 includes a switch 200 having a number of ports, including ports 280a, 280b, . . . , 280n. Associated with each of the ports 280a-n is a set of queues (i.e., port 280a is associated with queues 290a, port 280b is associated with queues 290b, and so on). Each of the switch ports 280a-n may be coupled with a node (or nodes) 110 via a corresponding link 120 (i.e., port 280a is coupled with node 110a via link 120a, and so on). A node 110 comprises any addressable device. For example, a node 110 may comprise a computer system or other computing device—such as a server, a desktop computer, a laptop computer, or a hand-held computing device (e.g., a personal digital assistant or PDA)—or a node 110 may comprise another switch coupled with other nodes (e.g., a collection of other nodes comprising a subnet). Each of the links 120a-n may be established over any suitable medium—e.g., wireless, copper wire, fiber optic, or a combination thereof—using any suitable protocol—e.g., TCP/IP (Transmission Control Protocol/Internet Protocol), HTTP (Hyper-Text Transmission Protocol), as well as others.
The network 100 may comprise any type of network, such as a Local Area Network (LAN), a Metropolitan Area Network (MAN), a Wide Area Network (WAN), a Wireless LAN (WLAN), or other network. The switch 200 also couples the network 100 with another network (or networks) 5, such as, by way of example, the Internet and/or another LAN, MAN, LAN, or WLAN. Switch 200 may be coupled with the other network 5 via any suitable medium, including a wireless, copper wire, and/or fiber optic connection using any suitable protocol (e.g., TCP/IP, HTTP, etc.).
The switch 200 receives communications (e.g., packets, frames, cells, etc.) from other network(s) 5 and routes those communications to the appropriate node 110, and the switch 200 also receives communications from the nodes 110a-n and transmits these communications out to the other network(s) 5. Generally, a communication will be referred to herein as a “packet”; however, it should be understood that the disclosed embodiments are applicable to any type of communication, irrespective of format or content. To schedule a packet for transmission, whether the packet is addressed to a node in another network 5 or is destined for one of the nodes 110a-n in network 100, the switch 200 includes a packet scheduler 400. The packet scheduler 400 implements what is referred to herein as a “pre-sort deficit round-robin” scheduling method, and various embodiments of this packet scheduler are described below in greater detail.
The switch 200 may be implemented on any suitable computing system or device (or combination of devices), and one embodiment of the switch 200 is described below with respect to FIG. 2A and the accompanying text. However, although the disclosed embodiments are explained below in the context of a switching device, it should be understood that the disclosed embodiments of a packet scheduler may be implemented on any device that routes, generates, forwards, or otherwise manipulates communications between two or more devices (e.g., communications between two nodes interconnected by a computer network, or communications between two devices interconnected by a high speed backplane fabric, etc.).
It should be understood that the network 100 shown in FIG. 1 is intended to represent an exemplary embodiment of such a system and, further, that the network 100 may have any suitable configuration. For example, the network 100 may include additional nodes 110, as well as other networking devices (e.g., routers, hubs, gateways, etc.); which have been omitted from FIG. 1 for ease of understanding. Further, it should be understood that the network 100 may not include all of the components illustrated in FIG. 1.
In one embodiment, the switch 200 comprises any suitable computing device, and the packet scheduler 400 comprises a software application that may be implemented or executed on this computing device. An embodiment of such a switch is illustrated in FIG. 2A.
Referring to FIG. 2A, the switch 200 includes a bus 205 to which various components are coupled. Bus 205 is intended to represent a collection of one or more buses—e.g., a system bus, a Peripheral Component Interface (PCI) bus, a Small Computer System Interface (SCSI) bus, etc.—that interconnect the components of switch 200. Representation of these buses as a single bus 205 is provided for ease of understanding, and it should be understood that the switch 200 is not so limited. Those of ordinary skill in the art will appreciate that the switch 200 may have any suitable bus architecture and may include any number and combination of buses.
Coupled with bus 205 is a processing device (or devices) 300. The processing device 300 may comprise any suitable processing device or system, including a microprocessor, a network processor, an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA), or similar device. An embodiment of the processing device 300 is illustrated below in FIG. 3 and the accompanying text.
Also coupled with the bus 205 is program memory 210. Where the packet scheduler 400 is implemented as a software routine comprising a set of instructions, these instructions may be stored in the program memory 210. Upon system initialization and/or power up, the instructions may be transferred to on-chip memory of the processing device 300, where they are stored for execution on the processing device. The program memory may comprise any suitable non-volatile memory. In one embodiment, the program memory 210 comprises a read-only memory (ROM) device or a flash memory device. In another embodiment, the switch 200 further includes a hard-disk drive (not shown in figures) upon which the packet scheduler software may be stored. In yet another embodiment, the switch 200 also includes a device (not shown in figures) for accessing removable storage media—e.g., a floppy-disk drive, a CD-ROM drive, and the like—and the packet scheduler software is downloaded from a removable storage media into memory of the processing device 300 (or downloaded into the program memory 210). In yet a further embodiment, upon power up or initialization of the switch 200, the packet scheduler software is downloaded from one of the nodes 110a-n or from another network 5 and stored in memory of the processing device 300 (in which case, program memory 210 may not be needed).
Switch 200 also includes system memory 220, which is coupled with bus 205. The system memory 210 may comprise any suitable type and/or number of memory devices. For example, the system memory 210 may comprise a DRAM (dynamic random access memory), a SDRAM (synchronous DRAM), a DDRDRAM (double data rate DRAM), and/or a SRAM (static random access memory), as well as any other suitable type of memory. During operation of switch 200, the system memory 220 provides one or more packet buffers 280 to store packets received from another network 5 and/or that have been received from the nodes 110a-n. 
Also, in one embodiment, the system memory 220 stores per-port data 270, which is described below in more detail with respect to FIG. 2B and the accompanying text. In one embodiment, the packet buffers 260 are stored in a DRAM device (or SDRAM or DDRDRAM), and the per-port data 270 is stored in a SRAM device. In another embodiment, the per-port data 270 is stored in a DRAM device (or SDRAM or DDRDRAM), and in a further embodiment, the per-port data 270 is stored in a combination of SRAM and DRAM (or SDRAM or DDRDRAM). For example, as will be explained below (see FIG. 2B), a pre-sorted scheduling array 420 (or 1120) associated with a per-port data 270 may be stored in any type of DRAM, whereas other portions of this per-port data 270 (e.g., per-queue data 410) may be stored in SRAM. In yet another embodiment, the per-port data 270 (or a portion thereof) is stored in an on-chip memory subsystem 330 of processing device 300, as described below. For example, the pre-sorted scheduling array and queues for each port may be stored in an on-chip DRAM, and the per-queue data may be stored in an on-chip SRAM.
The switch 200 further comprises a network/link interface 230 coupled with bus 205. The network/link interface 230 comprises any suitable hardware, software, or combination of hardware and software that is capable of coupling the switch 200 with the other network (or networks) 5 and, further, that is capable of coupling the switch 200 with each of the links 120a-n. 
It should be understood that the switch 200 illustrated in FIG. 2A is intended to represent an exemplary embodiment of such a device and, further, that this switch may include many additional components, which have been omitted for clarity and ease of understanding. By way of example, the switch 200 may include a chip set associated with the processing device 300, additional memory (e.g., a cache memory), one or more input devices (e.g., a keyboard, a pointing device such as a mouse, and a scanner or other data entry device), one or more output devices (e.g., a video monitor or an audio output device), as well as additional signal lines and buses. The switch 200 may also include a hard-disk drive and/or a device for accessing removable storage media, both as noted above. Also, it should be understood that the switch 200 may not include all of the components shown in FIG. 2A.
Turning now to FIG. 2B, the per-port data 270 is illustrated in greater detail. Per-port data 270 includes data associated with each of the ports 280a-n (i.e., per-port data 270a is maintained for port 280a, per-port data 270b is maintained for port 280b, and so on). For each port 280a-n, a number of queues are provided, as previously described (e.g., for port 280a, per-port data 270a includes a set of queues 290a comprising queues 291a, 291b, . . . , 291j). Each of the packet queues 291a-j is associated with a specified flow of packets or a group of packet flows (e.g., those flows having the same class of service, those flows that map to the same queue based upon a hashing scheme, etc.). Any suitable number of queues 291a-j may be associated with each of the ports 280a-n and, although illustrated as having the same number of queues 291a-j, it should be understood that the ports 280a-n may not have equal numbers of associated queues 291a-j. A queue may store pointers to packets stored in the packet buffers 280 (i.e., packets that correspond to the specified packet flow or flows).
The per-port data for any given port also includes two data structures maintained by the packet scheduler 400: per queue data and a pre-sorted scheduling array (e.g., per-port data 270a for port 280a includes per-queue data 410a and pre-sorted scheduling array 420a, and so on). The per queue data 410a-n and pre-sorted scheduling arrays 420a-n will be described in greater detail below.
As previously noted, an embodiment of processing device 300 is illustrated in FIG. 3 and the accompanying text. It should be understood, however, that the processing device 300 shown in FIG. 3 is but one embodiment of a processing device upon which the disclosed embodiments of a packet scheduler 400 may be implemented. Those of ordinary skill in the art will appreciate that the disclosed embodiments of packet scheduler 400 may be implemented on many other types of processing systems and/or processor architectures.
Turning now to FIG. 3, the processing device 300 includes a local bus 305 to which various functional units are coupled. Bus 305 is intended to represent a collection of one or more on-chip buses that interconnect the various functional units of processing device 300. Representation of these local buses as a single bus 305 is provided for ease of understanding, and it should be understood that the processing device 300 is not so limited. Those of ordinary skill in the art will appreciate that the processing device 300 may have any suitable bus architecture and may include any number and combination of buses.
A core 310 and a number of processing engines 320 (e.g., processing engines 320a, 320b, . . . , 320k) are coupled with the local bus 305. In one embodiment, the core 310 comprises a general purpose processing system. Core 310 may execute an operating system and control operation of processing device 300, and the core 310 may also perform a variety of management functions, such as dispensing instructions to the processing engines 320 for execution.
Each of the processing engines 320a-k comprises any suitable processing system, and each may include an arithmetic and logic unit (ALU), a controller, and a number of registers (for storing data during read/write operations). Each processing engine 320a-k may, in one embodiment, provide for multiple threads of execution (e.g., four). Also, each of the processing engines 320a-k may include a memory (i.e., processing engine 320a includes memory 322a, processing engine 320b includes memory 322b, and so on). The memory 322a-k of each processing engine 320a-k can be used to store instructions for execution on that processing engine. In one embodiment, one or more of the processing engines (e.g., processing engines 320b, 320c) stores instructions associated with the packet scheduler 400 (or instructions associated with certain components of the packet scheduler 400). The memory 322a-k of each processing engine 320a-k may comprise SRAM, ROM, EPROM (Erasable Programmable Read-Only Memory), or some type of flash memory (e.g., flash ROM). Further, although illustrated as discrete memories associated with a specific processing engine, it should be understood that, in an alternative embodiment, a single memory (or group of memories) may be shared by two or more of the processing engines 320a-k (e.g., by a time-division multiplexing scheme, etc.).
Also coupled with the local bus 305 is an on-chip memory subsystem 330. Although depicted as a single unit, it should be understood that the on-chip memory subsystem 330 may—and, in practice, likely does—comprise a number of distinct memory units and/or memory types. For example, such on-chip memory may include SRAM, SDRAM, DDRDRAM, and/or flash memory (e.g., flash ROM). It should be understood that, in addition to on-chip memory, the processing device 300 may be coupled with off-chip memory (e.g., system memory 220, off-chip cache memory, etc.). As noted above, in one embodiment, the packet scheduler 400 is stored in the memory of one or more of the processing engines 320a-k. However, in another embodiment, a set of instructions associated with the packet scheduler 400 may be stored in the on-chip memory subsystem 330 (shown in dashed line in FIG. 3).
Processing device 300 further includes a bus interface 340 coupled with local bus 305. Bus interface 340 provides an interface with other components of switch 200, including bus 205. For simplicity, bus interface 340 is depicted as a single functional unit; however, it should be understood that, in practice, the processing device 300 may include multiple bus interfaces. For example, the processing device 300 may include a PCI bus interface, an IX (Internet Exchange) bus interface, as well as others, and the bus interface 340 is intended to represent a collection of one or more such interfaces.
It should be understood that the embodiment of processing device 300 illustrated and described with respect to FIG. 3 is but one example of a processing device that may find use with the disclosed embodiments of a packet scheduler and, further, that the processing device 300 may have other components in addition to those shown in FIG. 3, which components have been omitted for clarity and ease of understanding. For example, the processing device 300 may include other functional units (e.g., an instruction decoder unit, an address translation unit, etc.), a thermal management system, clock circuitry, additional memory, and registers. Also, it should be understood that a processing device may not include all of the elements shown in FIG. 3.
An embodiment of the packet scheduler 400 is illustrated in FIG. 4. The packet scheduler 400 can schedule packets for transmission as the packets are enqueued, and the packet scheduler 400 can also dequeue packets for transmission. An embodiment of a method 700 for scheduling packets is illustrated and described with respect to FIG. 7, and an embodiment of a method 900 of dequeuing packets is illustrated and described with respect to FIG. 9. Also, an embodiment of a method 800 of determining a transmission round, as may be performed by the packet scheduler 400, is illustrated in FIG. 8 and the accompanying text.
Turning now to FIG. 4, the packet scheduler 400 includes the per-queue data 410 and the pre-sorted scheduling arrays 420 associated with each of the ports 280, as set forth above. Again, the per-queue data 410 for each port is associated with a set of queues 290 for that port, as illustrated in FIG. 4. Per-queue data 410 includes data 500 for each of the individual queues 291a-j of the set of queues 290 (i.e., per-queue data 500a is associated with queue 291a, per-queue data 500b is associated with queue 291b, and so on). Packet scheduler 400 also includes a scheduling agent 405 to schedule received packets for transmission, as will be described in greater detail below with respect to FIGS. 7 through 9. The scheduling agent 405 schedules packets for transmission based on the notion of “rounds”, as with conventional deficit round-robin (DRR) schemes. However, in contrast to conventional DRR methods, the scheduling agent 405 may make scheduling decisions when packets are enqueued, and entries for scheduled packets are placed into the pre-sorted scheduling arrays 420. By forecasting scheduling decisions into the future, transmit scheduling is simply a matter of dequeuing previously scheduled packets from the pre-sorted scheduling arrays 420, and the computational waste associated with analyzing a queue that does not possess the necessary bandwidth allocation during any given round is eliminated. When a packet is dequeued from the pre-sorted scheduling array 420, the packet is passed to a transmit process 490 for transmission.
Referring to FIG. 5, the per-queue data 500 for a single queue is illustrated. The per-queue data 500 includes a quantum 510, a packet count 520, a deficit count 530, and a last scheduled round 540. The quantum 510 is a per-queue constant that represents the bandwidth (expressed, for example, as a number of bytes) allocated to a queue per scheduling round. The quantum 510 for all queues may be the same or, as part of a quality of service or resource reservation scheme, the quantum 510 may be different for the various queues. Because the quantum may be set to a value that is larger than the minimum packet size, multiple packets associated with the same flow may be scheduled for transmission in the same scheduling round.
The packet count 520 represents the number of packets in that queue. The packet count 520 for a given queue is incremented when a packet is enqueued in the queue, and the packet count is decremented when a packet is dequeued therefrom.
The deficit count 530 represents the unused “transmission credit” that a populated queue possesses. During any round, the deficit count equals the remaining deficit count from the prior round plus the quantum for that queue. The number of packets scheduled for a queue during any given round cannot exceed the current deficit count for that queue (in other words, the number of packets cannot exceed the queue's allocated bandwidth). When a queue becomes empty (i.e., its packet count is zero), the deficit count 530 may be set to zero. However, in another embodiment, the deficit count is not reset to zero when a queue is emptied, and that queue is allowed to hold onto its unused transmission credit for a future packet.
The last scheduled round 540 for a queue is a number representing the round for which the packet at the end of that queue has been scheduled for transmission. As will be explained in more detail below, the scheduling round for a packet being enqueued to a non-empty queue is based on the last scheduled round 540 for that queue. If a packet is being enqueued to an empty queue, the scheduling round for the packet is based on a “current round” maintained by the scheduling agent 405.
An embodiment of a pre-sorted scheduling array 420 is shown in FIG. 6. The pre-sorted scheduling array 420 includes a number of memory buffers 422, including buffers 422a, 422b, . . . , 422m. Each of the buffer 422a-m corresponds to a scheduling round (i.e., rounds 1, 2, . . . , M). The number of rounds M—and, hence, the number of buffers 422a-m—is generally a function of the throughput of the ports 280a-n. Where the ingress rate and egress rate at a port are approximately the same, the number of rounds may be low; however, as the expected backlog in the queues of a port increases, the number of scheduling rounds also increases. In one embodiment, each of the memory buffers 422a-m comprises a FIFO (first-in-first-out) buffer capable of storing a number of entries 424a, 424b, . . . , 424s. Each of the entries 424a-s may store a pointer or other identifier to a packet that has been scheduled for transmission. Thus, during operation of the switch 200 and packet scheduler 400, each buffer 422a-m of the pre-sorted scheduling array 420 associated with a port will include a list of one or more (or none) packets that have been scheduled for transmission in the round corresponding to that buffer.
In one embodiment, the packet scheduler 400 of FIGS. 4 through 6 comprises a set of instructions (i.e., a software application) run on a computing device—e.g., the switch architecture illustrated in FIG. 2A or other suitable computing-device—as noted above. The set of instructions may be stored locally in program memory 210 or, in another embodiment, the instructions may be stored in a remote storage device (e.g., one of the nodes 110a-n) and accessed via network 100 (or from another network 5). The set of instructions is downloaded from the program memory, or the remote storage media, and stored on the processing device 300 for execution. In a further embodiment, the packet scheduler 400 comprises a set of instructions stored on a machine accessible medium, such as, for example, a magnetic media (e.g., a floppy disk or magnetic tape), an optically accessible disk (e.g., a CD-ROM disk), a flash memory device, etc. To run packet scheduler 400 on switch 200, a device for accessing removable storage media may access the instructions on the machine accessible medium, and the instructions may then be downloaded to processing device 300 and executed.
Upon system initialization and/or power up, the set of instructions of packet scheduler 400 may be downloaded to and stored in the memory 322a-k of one of the processing engines 320a-k for execution in that processing engine. Alternatively, the set of instructions may be downloaded to the memories of two or more of the processing engines 320a-k. Where multiple processing engines 320 are utilized to run packet scheduler 400, each of the multiple processing engines 320 may independently perform packet scheduling or, alternatively, the components of packet scheduler 400 may be spread across the multiple processing engines 320, which function together to perform packet scheduling. As noted above, the packet scheduler 400 may also be downloaded to and stored in an on-chip memory subsystem 330 of processing device 300. For a processing device having multiple processing engines, as shown in FIG. 3, packet scheduling operations may be performed in parallel with other functions (e.g., routing, security, error detection, etc.). Where multiple processing engines execute packet scheduler 400, packet scheduling for multiple ports may also be performed in parallel. In addition, for a processing device having one or more multi-threaded processing engines, this multi-threaded functionality may allow packet scheduling activities for multiple ports to be conducted in parallel. Although many of the above-described embodiments make use of the processing device 300 of FIG. 3, which includes a number of processing engines 320a-k, each of which may be capable of multi-threaded operation, it should be understood that the disclosed embodiments of packet scheduler 400 are not limited to execution on such a processing device and, further, that the packet scheduler software 400 may be executed on any suitable processing device.
In yet a further embodiment, the packet scheduler 400 is implemented in hardware or a combination of hardware and software (e.g., firmware). For example, the packet scheduler 400 may be implemented in an ASIC, an FPGA, or other similar device that has been programmed in accordance with the disclosed embodiments.
Illustrated in FIG. 7 is an embodiment of a method 700 for scheduling packets, as may be performed by packet scheduler 400. It should be understood, as suggested above, that the packet scheduling method set forth in FIG. 7 (and FIG. 8) is performed on a per-port basis. Referring now to block 710 in FIG. 7, a packet is received, and the received packet is accessed to identify the queue (and flow) with which the packet is associated and to determine the size of the packet, which is shown at block 720. As set forth at block 730, a transmission round for the packet is determined. An embodiment of a method 800 for determining the transmission round is illustrated and described below with respect to FIG. 8.
Once the transmission round for the received packet has been determined, an entry for the packet is created in one of the memory buffers of the pre-sorted scheduling array, as set forth at block 740. The entry is created in the memory buffer associated with the transmission round of the packet, and the entry will include an identifier for the packet (e.g., a pointer to a memory location of the packet). Referring to block 750, the per-queue data for the identified queue (i.e., the queue associated with the flow to which the packet belongs) is updated. As will be described below, the deficit count, packet count, and last scheduled round of the queue may be updated. As set forth at block 760, the received packet (or a pointer to the packet) is then stored in the identified queue. Thus, at the time the received packet has been enqueued and is awaiting transmission, that packet's scheduling time in some future transmission round has already been determined and, at the time of transmission, the packet will not need to be accessed.
As noted above, at block 730, a transmission round is determined for the received packet. Turning now to FIG. 8, illustrated is an embodiment of a method 800 for determining the transmission round of a packet. Referring to block 810 in FIG. 8, after the received packet has been accessed to identify the associated queue and packet size—see block 720—the number of rounds needed to schedule the packet is determined. In one embodiment, as set forth at blocks 812 through 816, if the deficit count is less than the packet size—see block 812—the number of rounds (# RNDS) is determined by subtracting the deficit count (DFCT CNT) from the packet size (PCKT SZ) and dividing that result by the quantum (QNTM) for the identified queue, wherein the final result is rounded up to the nearest whole number, which is set forth in block 814. However, as shown at block 816, where the deficit count exceeds the packet size, the number rounds needed to schedule the packet is zero (in other words, the queue still has sufficient allocated bandwidth remaining in the last scheduled round to schedule the current received packet).
Referring to block 820 in FIG. 8, if the identified queue contains one or more enqueued packets—i.e., the queue is not empty, as indicated by the packet count (PCKT CNT) for the identified queue—the scheduled transmission round (SCLD RND) for the packet equals the last scheduled round (LST SCLD RND) plus the previously calculated number of rounds (# RNDS). On the other hand, if the identified queue is empty, as indicated by a packet count of zero (see block 820), the scheduled transmission round equals the current round (CRNT RND) maintained by the scheduling agent 405 plus the calculated number of rounds. The current round corresponds to the round number of the memory buffer from which the scheduling agent is dequeuing packets for transmission. In other words, packets cannot be entered into the pre-sorted scheduling array at a round behind that which is presently being dequeued for the transmit process. Stated another way, packets should be scheduled ahead of the transmit process. Prior to commencement of transmit activities, the current round is initially set to zero.
The per-queue data is also updated after the scheduling of a packet, as noted above with respect to block 750. This is further illustrated in FIG. 8 in blocks 752 through 756. Referring to block 752, the last scheduled round (LST SCLD RND) is set equal to the scheduled transmission round (SCLD RND). As illustrated by block 754, the packet count (PCKT CNT) for the identified queue is updated by incrementing the packet count by one. Further, as set forth in block 756, an updated value for the deficit count (DFCT CNTNEW) is determined by subtracting the packet size from the product of the quantum (QNTM) and the number of rounds (# RNDS), which result is then added to the previous deficit count (DFCT CNTOLD).
Illustrated in FIG. 9 is a method 900 of dequeuing packets from the pre-sorted scheduling array associated with any given port. Referring to block 910 in FIG. 9, the buffer of the pre-sorted scheduling array corresponding to the current round is accessed. As set forth at block 920, an entry of the current round buffer is accessed. Typically, for a FIFO type memory buffer, the “oldest” entry in the buffer will be accessed. The packet identified by the accessed entry of the current round buffer is then dequeued for transmission, as set forth at block 930. Referring to block 940, the per-queue data for the queue from which the packet has been dequeued is updated. Specifically, as shown in block 940, the packet count (PCKT CNT) of this queue is decremented by one. Also, if the queue is empty after dequeuing of the packet (i.e., the updated packet count equals zero), the deficit count (DFCT CNTNEW) is, in one embodiment, set equal to zero. However, it is not required by the disclosed method to force the deficit count to zero when a queue becomes empty, and in another embodiment, a queue is allowed to “hold” on to its remaining deficit count when its packet count reaches zero.
Moving to block 950 in FIG. 9, if there are other entries for scheduled packets in the current round buffer, another entry is accessed and the above-described process (i.e., blocks 920, 930, and 940) is repeated. The packets scheduled for transmission in the current round buffer are dequeued for transmission, and when these scheduled packets have been dequeued—see block 960—the method advances to the next buffer. Generally, the current round will be incremented by one, and the next buffer will correspond to the buffer having a round number one higher than the previously accessed buffer. However, in another embodiment, empty buffers are skipped, and the method will advance to the next populated buffer, irrespective of its round number. After advancing to the next buffer, the above-described process (i.e., blocks 910, 920, 930, 940, and 950) is repeated to dequeue all packets from this buffer.
The packet scheduler 400 set forth with respect to FIGS. 1 through 6, as well as the methods 700, 800, 900 illustrated and described in FIGS. 7 through 9, respectively, may be better understood by the example set forth in FIGS. 10A-10H, which is now described. The example illustrated in FIGS. 10A-10H is based on a single port having three queues. It should, however, be understood that FIGS. 10A-10H present a simplified example including a small number of queues for ease of explanation and illustration. As will be appreciated by those of ordinary skill in the art, a network switch may, in practice, include hundreds, or even thousands, of queues per port.
Referring now to FIG. 10A, a port has three associated queues 291a, 291b, 291c (QUEUE 1, QUEUE 2, QUEUE 3), each queue having per-queue data 410a-c, respectively. The quantum for queues 291a, 291b is 256 bytes, whereas the quantum for queue 291c is 512 bytes. Initially, all other per-queue values (i.e., packet count, deficit count, and last scheduled round) are set equal to zero. The current round is also initially set equal to zero (i.e., transmission has not yet started). A pre-sorted scheduling array 420 includes a number of buffers 422, wherein buffers 422a-d corresponding to rounds one through four (RND 1-RND 4) are initially shown. All buffers 422a-d of the pre-sorted scheduling array are initially empty.
Turning to FIG. 10B, a first packet has been received. This packet is accessed (see FIG. 7, block 710), and it is found that the packet is associated with the first queue 291a and has a size of 350 bytes. To determine the scheduled transmission round for the first packet (see block 730), a number of rounds needed to schedule the packet is calculated (see blocks 810, 812, 814, and 816 in FIG. 8). Subtracting the deficit count (initially zero) from the packet size (350) and dividing by the quantum (256), and then rounding up to the nearest whole number, yields a value of two. Because the first queue is initially empty (see block 820), the scheduled transmission round equals the current round (initially zero) plus the calculated number of rounds (see block 840), which is two. An entry (“Q1-PCKT 1”) for the first packet is then created in the second buffer 422b (RND 2) of the presorted scheduling array for the first packet (see block 740 in FIG. 7), which is shown in FIG. 10B. The per-queue data 410a for the first queue 291a is then updated (see blocks 750, 752, 754, and 756 in FIG. 8), such that the packet count is now one, the deficit count is now equal to 162, and the last scheduled round is equal to two, all as illustrated in FIG. 10B. The first packet is then enqueued to the first queue 291a, which is also shown in FIG. 10B.
Referring next to FIG. 10C, another packet has been received, and this packet is found to belong to a flow associated with the second queue 291b and to have a size of 256 bytes (see block 720). Subtracting the deficit count for queue 291b (initially zero) from the packet size (256) and dividing by the quantum (256) yields a number of rounds equal to one (see blocks 810, 812, 814, 816). The second queue is initially empty, and the scheduled transmission round equals the current round (still zero) plus the calculated number of rounds, which was one (see block 820, 840). An entry (“Q2-PCKT 1”) of this packet is then placed in the first buffer 422a of the presorted scheduling array 420 (see block 740), and the packet has been stored in the second queue 291b (see block 760), both as illustrated in FIG. 10B. The per-queue data 410b for second queue 291b is also updated (see blocks 750, 752, 754, 756), as shown in FIG. 10B (i.e., packet count equal to one, deficit count equal to zero, last scheduled round equal to one).
With continued reference to FIG. 10C, a third packet has been received, and this packet has a size of 150 bytes and is associated with the third queue 291c. The number of rounds necessary to schedule this packet is one, and an entry (“Q3-PCKT 1”) is created in the fist buffer 422a of the pre-sort scheduling array 420, as shown in FIG. 10C. The per-queue data 410c for the third queue 291c is updated (i.e., packet count equal to one, deficit count equal to 362, and last scheduled round equal to one), and the packet is stored in the third queue 291c, all as illustrated in FIG. 10C.
Turning now to FIG. 10D, another packet having a size of 256 bytes that is associated with the second queue 291b has been received. Performing the above-described process for determining the scheduled transmission round will yield a scheduled transmission round of two, and an entry (“Q2-PCKT 2”) for this packet is placed in the second buffer 422b of the pre-sorted scheduling array 420. As further illustrated in FIG. 10D, an additional packet has been received that is associated with the third queue 291c. This packet has a size of 150 bytes. Note that the size of this packet (150) is less than the current deficit count (362) of the third queue (see FIG. 10C). Thus, the number of rounds needed to schedule this packet is zero (see blocks 812 and 816 in FIG. 8), and the scheduled transmission round is equal to the last scheduled round (see blocks 820 and 830). Accordingly, an entry (“Q3-PCKT 2”) for this packet is placed in the first buffer 422a of the pre-sorted scheduling array 420, and multiple packets associated with the third queue 291c will be transmitted in the first scheduling round (RND 1). Also, the per-queue data 410c for the third queue 291c is again updated, as shown in FIG. 10D (packet count equal to two, deficit count now equal to 212, last scheduled round remaining at one).
In FIG. 10E, two additional packets have been received, including a packet associated with the third queue 291c and having a size of 300 bytes, as well as a packet associate with the first queue and also having a size of 300 bytes. Note that the size (300) of the packet associated with the third queue 291c is greater than the remaining deficit count (212), and the scheduled transmission round is determined to be round two. Therefore, an entry (“Q3-PCKT 3”) for this packet is created in the second buffer 422b of the presorted scheduling array, and the per-queued data 410c for the third queue 291c is updated, all as shown in FIG. 10E. The received packet associated with the first queue 291a can be scheduled for transmission in the third round—i.e., subtracting the deficit count (162) from the packet size (300) and dividing the result by the quantum (256) and rounding up to the nearest whole number yields the quantity one, which when added to the last scheduled round (2) for this queue yields a scheduled transmission round of three—and an entry (“Q1-PCKT 2”) for this packet is placed in the third buffer 422c of pre-sorted scheduling array 420, as shown in FIG. 10E.
Referring now to FIG. 10F, another packet associated with the first queue 291a has been received. This packet has a size of 300 bytes. Applying the above-described procedure to determine a scheduled transmission round yields a transmission round of four, and an entry (“Q1-PCKT 3”) for this packet is created in the fourth buffer 422d of the pre-sorted scheduling array 420. The per-queue data 410a for first queue 291a is also updated, as illustrated in FIG. 10F. Still referring to FIG. 10F, an additional packet associated with the second queue 291b has been received, this packet having a size of 768 bytes. Once again, applying the above-described procedure, it is found that the number of rounds needed to schedule this packet is two, which is added to the last scheduled round (2) for the second queue, thereby resulting in a scheduled transmission round of four. Accordingly, an entry (“Q2-PCKT 3”) is created in the fourth buffer 422d of pre-sorted scheduling array 420, and the per-queue data 420b for second queue 291b is updated, as shown in FIG. 10F.
With reference to both FIGS. 10F and 10G, the scheduling agent 405 has commenced dequeuing of packets. The packets in the first buffer 422a (i.e., the RND 1 buffer) of the pre-sorted scheduling array 420 have been dequeued (see FIG. 9, blocks 910 through 950). The dequeue process has advanced to the next buffer (see block 960), which is the RND 2 buffer 422b, and the packet stored in this buffer have also been dequeued. Thus, in FIG. 10G, packets remain in only the third and fourth buffers 422c, 422d of the pre-sorted scheduling array 420, and the current round now equals two (although all packets have been dequeued from the RND 2 buffer, dequeuing from the third buffer has not yet started). In FIG. 10G (and FIG. 10H), the first and second buffers 422a, 422b are no longer shown, but a fifth buffer 422e (RND 5) and a sixth buffer 422f (RND 6) are now shown. Also, the per-queue data 410a-c for the queues 291a-c has been updated (see FIG. 9, block 940).
Turning now to FIG. 10H, another packet associated with the third queue has been received. This packet has a size of 1664 bytes. Because the third queue 291c became empty after the first and second rounds of dequeuing, the deficit count of the third queue was reset to zero (see FIG. 10G). Dividing the packet size by the quantum yields a number of rounds required for scheduling of four. Because the third queue 291c is currently empty (see blocks 820 and 840 in FIG. 8), this number is added to the current round (2) to determine the scheduled transmission round for this packet, which yields a scheduled transmission round of six. Accordingly, an entry (“Q3-PCKT 4”) is entered into the sixth buffer 422f of the pre-sorted scheduling array, as shown in FIG. 10H. The packet is stored in the third queue, and the per-queue data 410c for this queue is updated, as also shown in FIG. 10H. Still referring to FIG. 10H, an additional packet associated with the second queue 291b has been received, this packet having a size of 500 bytes. Based on the current deficit count, the last scheduled round, and the quantum for this queue, the scheduled transmission round is determined to be round five, and an entry (“Q2-PCKT 4”) for this packet is created in the fifth buffer 422e of pre-sorted scheduling array 420, as shown in FIG. 10H. The packet is stored in the second queue 291b, and the per-queue data 410b of this queue updated, which is also shown in this figure. Additional enqueues with scheduling entries made in the pre-sorted scheduling array, as well as additional dequeues, would be processed in a similar manner.
In the embodiments described above in FIGS. 4 through 10H, packets are placed in the round buffers in the order in which they are received (e.g., in a FIFO-like manner). This FIFO-like entry of packets into the pre-sort array may introduce some unfairness into the scheduling of packets. In a further embodiment, however, one or more smoothing registers are included in the pre-sort array to provide fine grain sorting of packets within a scheduling round. Such fine grain sorting of packets using a smoothing register can reduce the unfairness that may result from the FIFO-like ordering of packets in a scheduling round and, further, this additional sorting may provide a relatively closer approximation of that result which could theoretically be achieved by the WFQ scheduling technique.
Turning now to FIG. 11, illustrated is an embodiment of a pre-sorted scheduling array 1120 that includes smoothing registers. In one embodiment, the pre-sorted scheduling array 420 of FIG. 4 comprises the pre-sorted array 1120 of FIG. 11, which will now be described in greater detail.
Referring to FIG. 11, the pre-sorted scheduling array 1120 includes a number of round buffers 1122, including round buffers 1122a, 1122b, . . . , 1122m. During operation of the switch 200 and packet scheduler 400, each round buffer 1122a-m of the pre-sorted scheduling array 1120 associated with a port will include a list of one or more (or none) packets that have been scheduled for transmission in the round corresponding to that buffer. Each of the round buffers 1122a-m corresponds to a scheduling round (i.e., rounds 1, 2, 3, . . . , M). Again, the number of rounds M—and, hence, the number of buffers 1122a-m—is generally a function of the throughput of the ports 280a-n. Where the ingress rate and egress rate at a port are approximately the same, the number of rounds may be low; however, as the expected backlog in the queues of a port increases, the number of scheduling rounds also increases. Each of the round buffers 1122a-m includes a number of entries 1123a, 1123b, . . . , 1123x, and each of the entries 1123a-x of a round buffer 1122 can each store a packet (or a pointer to a packet or other identifier).
Associated with each of the round buffers 1122a-m is a smoothing register 1124 (e.g., associated with round buffer 1122a is smoothing register 1124a, associated with round buffer 1122b is smoothing register 1124b, and so on). When a packet is placed in a round buffer, the smoothing register bit corresponding to that entry is set (e.g., switched from a first state such as a “0” to a second state such as a “1”). Thus, each of the smoothing registers 1124a-m contains a one-to-one mapping of the entries 1123a-x of that smoothing register's corresponding round buffer 1122. In other words, each entry 1123a-x of a round buffer 1122 that contains a packet (or packet identifier) will have its corresponding bit in the associated smoothing register set. Thus, during packet scheduling, the smoothing registers enable the entry of packets into a round buffer at selected positions (as opposed to a FIFO ordering), which facilitates the sorting of packets within any scheduling round.
During the scheduling of packets, packets will be placed into the round buffers 1122 at selected positions, as noted above. Conversely, when packets (or pointers to packets) are read out of the entries 1123a-x of a round buffer 1122 during the dequeue process, the packets may be read out in order (e.g., in a manner similar to a FIFO). However, because the round buffers 1122 need to be amenable to the random placement of packets in the entries 1123a-x (e.g., in a manner similar to a DRAM), the round buffers 1122 will not generally be implemented as a FIFO (first-in first-out) data structure (although, in other embodiments, the round buffers 1122 may be implemented as FIFO buffers).
To read packets out of a round buffer 1122, the bits 1125a-x of the associated smoothing register are read, and where a bit of the smoothing register has been set (e.g., it state changed), the corresponding entry of the round buffer is read. In one embodiment, the processing device 300 includes an embedded instruction set having a “find first bit set” instruction, which will find the first (or next) bit in a smoothing register that is set. Thus, in one embodiment, the “find first bit set” instruction is utilized, and the bits of a smoothing register are not examined one-by-one in order. Rather, the “find first bit set” instruction is used to directly find the first (or next) non-zero bit. However, it is within the scope of the disclosed embodiments to examine the bits of a smoothing register one-by-one in order without the use of the “find first bit set” instruction. In summary, through the use of the smoothing registers, FIFO-like behavior is approximated at the round buffers 1122 during the dequeue process, although packets may be placed within a round buffer at any selected location during enqueuing.
A round buffer may have any suitable number of entries (and its smoothing register a corresponding number of bits), and a pre-sorted scheduling array 1120 may include any suitable number of round buffers-1122. Also, as noted above, the sizes of the smoothing registers 1124a-m corresponds to the respective sizes of the round buffers 1122a-m. For example, if a round buffer were to include 64 entries, that round buffer's corresponding smoothing register would have a length of 64 bits.
Illustrated in FIG. 12 is an embodiment of a method 1200 for scheduling packets using a pre-sort scheduling array including smoothing registers to provide for the fine grain ordering of packets within any given scheduling round. It should be understood, as previously suggested, that the packet scheduling method set forth in FIG. 12 is performed on a per-port basis. Referring now to block 1210 in FIG. 12, a packet is received, and the received packet is accessed to identify the queue (and flow) with which the packet is associated and to determine the size of the packet, which is shown at block 1220. As set forth in block 1230, the scheduled transmission round for the packet is determined. In one embodiment, the transmission round is determined in a manner as described in FIG. 8 and the accompanying text above.
Referring to block 1240, the per-queue data for the identified queue (i.e., the queue associated with the flow to which the packet belongs) is updated. As previously described, the deficit count, packet count, and last scheduled round of the queue may be updated (see FIG. 8, blocks 750-756).
Referring to block 1250, the relative position of the packet within the packet's scheduled transmission round is determined. In one embodiment, the scheduling array 1120 as a whole could be viewed as presenting a time window that is divided into a number of time slots. For example, if a scheduling array includes four round buffers, each including 64 entries, the scheduling array provides an aggregate time window of 256 time slots (the actual length of time represented by each time slot would be a function of the particular application). This time window would be a sliding window, as a new round buffer is added when dequeue of the current round is complete. Packets associated with a given queue are then spaced apart within this time window based, at least in part, on the queue's quantum and the sizes of the packets. Referring to block 1255, according to one embodiment, the relative position of a packet within its transmission round is calculated by subtracting the new value of the deficit count (e.g., the updated deficit count for the received packet's associated queue that was calculated after determining the transmission round for the packet) from the quantum to determine a numerator. This numerator is then divided by the quantum, and this result is multiplied by the number of entries in the transmission round (rounded to the nearest whole number). The above-described embodiment for determining the relative position of a packet will be illustrated in more detail using the example presented in FIG. 14 and the accompanying text below.
The technique for placing packets within the scheduling array that was described in the preceding paragraph provides a result that may approximate that result that could be achieved using WFQ. However it should be understood that this is but one method for placing packet within a scheduling array that may find application to the disclosed embodiments. Thus, the reader will appreciate that, in other embodiments, alternative techniques and/or parameters may be used to determine the relative position of a packet within that packet's round buffer.
Once the transmission round and relative position for the received packet has been determined, an entry for the packet is created in one of the round buffers of the pre-sorted scheduling array, as set forth at block 1260. The entry is created in the round buffer associated with the transmission round of the packet. Further, the position of the entry within the transmission round will be the previously determined relative position o that packet. To create the entry for the packet, an identifier for the packet (e.g., a pointer to a memory location of the packet) is inserted in that entry of the round buffer corresponding to the desired relative position of the packet (as determined in block 1250). Note that, should the desired position of a packet already be occupied by a previously received packet, the packet can be placed in the round buffer at the next-closest available entry.
The round buffer in which the received packet is placed will have an associated smoothing register. The bit of this smoothing register that corresponds to the round buffer entry in which the packet was placed is then set, as set forth in block 1270. Again, the round buffers will generally receive packets at any selected entry. Those bits of a smoothing register that have been set (e.g., their state changed) will indicate which entries of a round buffer will need to be read during dequeue, and the smoothing register will also dictate the order in which packets are read out of the round buffer.
As set forth at block 1280, the received packet (or a pointer to the packet) is then stored in the identified queue. Thus, at the time the received packet has been enqueued and is awaiting transmission, that packet's scheduling time in some future transmission round has already been determined and, at the time of transmission, the packet will not need to be accessed.
Illustrated in FIG. 13 is an embodiment of a method 1300 of dequeuing packets from a pre-sorted scheduling array associated with any given port, wherein the pre-sorted array includes smoothing registers. Referring to bock 1310 in FIG. 13, the smoothing register associated with the round buffer in the scheduling array that corresponds to the current round is accessed. In one embodiment, the associated smoothing register is accessed using the “find first bit set” instruction, as described above. The “find first bit set” instruction will find the first (or next) bit in a smoothing register that is set. However, in other embodiments, the bits of a smoothing register may be examined one-by-one in order.
If a set bit is found in the smoothing register—see block 1320—the entry of the current round buffer that corresponds to the set bit is read, as set forth in block 1330. This entry of the round buffer will identify a packet, and this packet may then be dequeued, which is set forth in block 1340. Referring to block 1350, the per-queue data for the queue from which the packet has been dequeued is updated. As previously described, the packet count (PCKT CNT) of this queue is decremented by one, and if the queue is empty after dequeuing of the packet (i.e., the updated packet count equals zero), the deficit count (DFCT CNTNEW) may be set equal to zero (again, it is not required by the disclosed method to force the deficit count to zero when a queue becomes empty, and in another embodiment, a queue is allowed to “hold” on to its remaining deficit count when its packet count reaches zero).
The above-described procedure is then repeated for each set bit found in the smoothing register associated with the current round buffer. However, should an additional set bit not be found (or should no set bits have been present in the associated smoothing register), the method advances to the next round, as set forth in block 1360. The process of accessing the set bits of the smoothing register to identify those entries of the current round that have stored packets is then repeated for subsequent rounds.
The methods 1200, 1300 of FIGS. 12 and 13, respectively, may be better understood by reference to the example set forth in FIG. 14. The example illustrated in FIG. 14 is based on a single port having four queues and a scheduling array including three round buffers, each comprising 32 entries, and their respective smoothing registers (each also 32 bits). However, it should be understood that FIG. 14 presents a simplified example including a small number of queues and relatively small round buffers for ease of explanation and illustration. Again, as will be appreciated by those of ordinary skill in the art, a network switch may, in practice, include hundreds or even thousands of queues per port and, further, that such a switch may include round buffers having 64 entries or more.
Referring to FIG. 14, a port has four queues 1410a (Q1), 1410b (Q2), 1410c (Q3), and 1410d (Q4). These queues have quantums of 512 bytes, 256 bytes, 256 bytes, and 128 bytes, respectively. A pre-sort scheduling array 1420 including three round buffers 1422a, 1422b, 1422c (initially corresponding to rounds 1 through 3) and three associated smoothing registers 1424a, 1424b, 1424c (e.g., smoothing register 1424a is associated with round buffer 1422a, and so on). A number of packets have been received in the queues 1410a-d and scheduled for transmission. In the following text, transmission rounds are determined as set forth in FIG. 8, but details of these calculations are not presented here.
Turning first to Q1, a first packet (P1) having a size of 384 bytes has been received. This packet will be scheduled in the first round (buffer 1410a), and the updated deficit count is 128. Determining the position of the packet within the first round according to the embodiment of block 1255 indicates that the packet should be places in the 24th entry of the first round (e.g., [(512−128)/512][32]=24). Thus, an identifier for P1 is placed in the 24th entry of the first round buffer 1410a. Note that, for the first packet received in Q1 and scheduled into the first round, the position of P1 is also related to the ratio of the packet size to the quantum (e.g., 384/512). A second packet (P2) is then received in Q1, the packet also having a size of 384 bytes. The packet will be scheduled in the second transmission round, and the new deficit count is 256, and this packet will be placed in the 16th entry of the second round buffer 1410b (e.g., [(512−256)/512][32]=16). A third packet (P3) of 384 bytes is also received in Q1. Packet P3 will be entered into the third transmission round, and the new deficit count for Q1 is 384. The third packet will, therefore, be placed in the 8th entry of the third round, as shown in FIG. 14 (e.g., [(512−384)/512][32]).
Thus, at this juncture, of the three packets received in Q1, P1 has been placed in the 24th entry of the first round, P2 has been placed in the 16th entry of the second round, and P3 has been placed in the 8th entry of the third round (the packets of Q1 are also identified by arrows in FIG. 14). However, although placed in different entries of their respective rounds, when the scheduling array is viewed as a whole, the packets P1, P2, and P3 are equally spaced apart in time, a result that is expected because these packets are of equal size. In other words, if the scheduling array is viewed in the aggregate as providing 96 time slots (3 rounds, each providing 32 time slots), the first packet is in the 24th time slot, the second packet is 24 time slots behind the first, and the third packet is 24 time slots behind the second.
With reference still to FIG. 14, additional packets have been received in the queues and placed in the scheduling array 1420 in a manner similar to that described above. Consider, for example, the first packet (P1) received in Q2. This packet has a size of 240 bytes, and it is positioned in the 30th entry of the first round. Note that although P1 of Q2 is smaller in size that P1 of Q1, this result is expected because Q1 has a greater quantum (e.g., 512 bytes) than that of Q2 (e.g., 256 bytes). As can also be seen in FIG. 14, the corresponding bit of the smoothing registers has been set (e.g., switched from a “0” to a “1”) for each entry of a round buffer that has received a packet. During dequeue, packets are read from the round buffer in the order provided by the set bits in the smoothing register, as described above (see FIG. 13 and the accompanying text above).
The foregoing detailed description and accompanying drawings are only illustrative and not restrictive. They have been provided primarily for a clear and comprehensive understanding of the disclosed embodiments and no unnecessary limitations are to be understood therefrom. Numerous additions, deletions, and modifications to the embodiments described herein, as well as alternative arrangements, may be devised by those skilled in the art without departing from the spirit of the disclosed embodiments and the scope of the appended claims.

## Claims

1. A method comprising:
accessing a received packet to determine a size of the packet and a queue associated with the packet;
determining a transmission round for the packet, wherein determining a transmission round comprises determining a number of rounds needed to schedule the packet;
determining a position within the transmission round based, at least in part, on a quantum of the associated queue;
storing an identifier for the packet in a scheduling array, the scheduling array including a number of round buffers, each round buffer including a number of entries, the identifier stored in one of the round buffers corresponding to the transmission round at one of the entries corresponding to the position;
setting a bit of a smoothing register associated with the one round buffer, the set bit corresponding to the one entry;
accessing the set bit in the associated smoothing register;
accessing the one entry of the one round buffer corresponding to the set bit;
reading the packet identifier from the one entry of the one round buffer; and
dequeuing the packet from the associated queue.

2. A method comprising:
accessing a received packet to determine a size of the packet and a queue associated with the packet;
determining a transmission round for the packet, wherein determining a transmission round comprises determining a number of rounds needed to schedule the packet;
determining a position within the transmission round based, at least in part, on a quantum of the associated queue;
storing an identifier for the packet in a scheduling array, the scheduling array including a number of round buffers, each round buffer including a number of entries, the identifier stored in one of the round buffers corresponding to the transmission round at one of the entries corresponding to the position, wherein determining a transmission round comprises:
if the associated queue has a previously enqueued packet, setting the transmission round equal to a sum of a last scheduled round and the number of needed rounds; and
if the associated queue is empty, setting the transmission round equal to a sum of a current round and the number of needed rounds.

3. The method of claim 2, further comprising updating per-queue data of the associated queue, the per-queue data including a packet count, a deficient count, and the last scheduled round.

4. The method of claim 3, wherein updating the data comprises:
setting the last scheduled round equal to the transmission round;
incrementing the packet count; and
determining a new value of the deficit count based, at least in part, on an old value of the deficit count.

5. The method of claim 4, wherein determining the position comprises:
subtracting the new value of the deficit count from the quantum of the associated queue to determine a numerator;
dividing the numerator by the quantum of the associated queue to determine a result; and
multiplying the result by the number of entries in the one round buffer.

6. The method of claim 4, wherein determining the new value for the deficient count comprises:
determining a product of the quantum and the number of needed rounds;
determining a sum of the product and the old value of the deficit count; and
subtracting the packet size from the sum.

7. The method of claim 4, wherein determining a number of rounds needed to schedule the packet comprises:
if the old value of the deficit count exceeds the packet size, setting the number of needed rounds equal to zero; and
if the old value of the deficit count is less than the packet size, subtracting the old value of the deficit count from the packet size to determine a result, and dividing the result by the quantum to determine the number of needed rounds.

8. The method of claim 1, wherein the associated queue comprises one of a number of queues and the number of queues is associated with one of a number of ports of a switching device.

9. A method comprising:
providing a number of queues, each of the queues associated with a port;
providing per-queue data for each of the queues, the per-queue data for each queue including a quantum;
providing a number of round buffers, each round buffer including a number of entries;
providing a number of registers, each of the registers associated with one of the round buffers, each register having a number of bits equal to the number of entries in the associated round buffer;
accessing a received packet to determine a size of the packet and which of the number of queues the packet is associated with;
determining a transmission round for the packet, the transmission round corresponding to one of the round buffers;
setting a bit of the register associated with the corresponding round buffer, the set bit corresponding to the one entry; and
dequeuing a packet identified by a packet identifier from the associated queue.

10. The method of claim 9, further comprising:
determining a position within the corresponding round buffer based, at least in part, on the quantum of the associated queue.

11. The method of claim 10, further comprising:
reading the set bit in the associated register;
accessing the one entry of the corresponding round buffer that corresponds to the set bit; and
reading the packet identifier from the one entry of the corresponding round buffer.

12. The method of claim 10, wherein the transmission round is determined based, at least in part, on the size of the packet, the quantum of the associated queue, a packet count of the associated queue, and a deficit count of the associated queue.

13. The method of claim 10, further comprising updating the packet count and the deficit count of the associated queue.

14. The method of claim 13, wherein determining the position comprises:
subtracting the updated value of the deficit count from the quantum of the associated queue to determine a numerator;
dividing the numerator by the quantum of the associated queue to determine a result; and
multiplying the result by the number of entries in the one round buffer.

15. An apparatus comprising:
a processing device;
a memory system coupled with the processing device, the memory system having stored therein
a number of queues, each of the queues associated with a port, per-queue data for each of the queues, the per-queue data for each queue including a quantum,
a number of round buffers, each round buffer including a number of entries, and
a number of registers, each of the registers associated with one of the round buffers, each register having a number of bits equal to the number of entries in the associated round buffer;

accessing a received packet to determine a size of the packet and which of the number of queues the packet is associated with;
determining a transmission round for the packet, the transmission round corresponding to one of the round buffers;
storing an identifier for the packet in the corresponding round buffer at one of the entries corresponding to the position;
setting a bit of the register associated with the corresponding round buffer, the set bit corresponding to the one entry; and
dequeuing a packet identified by a packet identifier from the associated queue.

16. The apparatus of claim 15, wherein the processing device is programmed to perform operations including:
determining a position within the corresponding round buffer based, at least in part, on the quantum of the associated queue.

17. The apparatus of claim 16, wherein the processing device is programmed to perform operations further including:
reading the set bit in the associated register;
accessing the one, entry of the corresponding round buffer that corresponds to the set bit; and
reading the packet identifier from the one entry of the corresponding round buffer.

18. The apparatus of claim 16, wherein the per-queue data for each queue includes a packet count and a deficit count, and wherein the transmission round is determined based, at least in part, on the size of the packet, the quantum of the associated queue, the packet count of the associated queue, and the deficit count of the associated queue.

19. The apparatus of claim 18, wherein the processing device is programmed to perform operations further including updating the packet count and the deficit count of the associated queue.

20. The apparatus of claim 19, wherein to determine the position within the corresponding round buffer, the processing device is programmed to perform operations including:
subtracting the updated value of the deficit count from the quantum of the associated queue to determine a numerator;
dividing the numerator by the quantum of the associated queue to determine a result; and
multiplying the result by the number of entries in the one round buffer.

21. A system comprising: a bus;
a processing device coupled with the bus; and
a system memory coupled with the bus, the system memory including a dynamic random access memory (DRAM);
wherein the processing device is programmed to perform operations including providing
a number of queues stored in the system memory, each of the queues associated with a port,
providing per-queue data for each of the queues, the per-queue data for each queue including a quantum,
providing a number of round buffers stored in the system memory, each round buffer including a number of entries, and
providing a number of registers stored in the system memory, each of the registers associated with one of the round buffers, each register having a number of bits equal to the number of entries in the associated round buffer.

22. The system of claim 21, wherein the per-queue data is stored in a memory of the processing device.

23. The system of claim 21, wherein the DRAM comprises one of a double data rate DRAM (DDRDRAM) and a synchronous DRAM (SDRAM).

24. The system of claim 21, wherein the processing device is programmed to perform operations including:
accessing a received packet to determine a size of the packet and which of the number of queues the packet is associated with;
determining a transmission round for the packet, the transmission round corresponding to one of the round buffers;
determining a position within the corresponding round buffer based, at least in part, on the size of the packet and the quantum of the associated queue;
storing an identifier for the packet in the corresponding round buffer at one of the entries corresponding to the position; and
setting a bit of the register associated with the corresponding round buffer, the set bit corresponding to the one entry.

25. The system of claim 24, wherein the processing device is programmed to perform operations further including:
reading the set bit in the associated register;
accessing the one entry of the corresponding round buffer that corresponds to the set bit; reading the packet identifier from the one entry of the corresponding round buffer; and dequeuing the packet identified by the packet identifier from the associated queue.

26. The system of claim 24, wherein the per-queue data for each queue including a packet count and a deficit count, and wherein the transmission round is determined based, at least in part, on the size of the packet, the quantum of the associated queue, the packet count of the associated queue, and the deficit count of the associated queue.

27. The system of claim 26, wherein the processing device is programmed to perform operations further including updating the packet count and the deficit count of the associated queue.

28. The system of claim 27, wherein to determine the position within the corresponding round buffer, the processing device is programmed to perform operations including:
subtracting the updated value of the deficit count from the quantum of the associated queue to determine a numerator;
dividing the numerator by the quantum of the associated queue to determine a result; and
multiplying the result by the number of entries in the one round buffer.

29. An article of manufacture comprising:
a computer readable medium having embodied heron a computer program product, when accessed by a computer, causes the computer to:
access a received packet to determine a size of the packet and a queue associated with the packet;
determine a transmission round for the packet based on a determination of a number of rounds needed to schedule the packet;
determine a position within the transmission round based, at least in part, on a quantum of the associated queue; and
store an identifier for the packet in a scheduling array, the scheduling array including a number of round buffers, each round buffer including a number of entries, the identifier stored in one of the round buffers corresponding to the transmission round at one of the entries corresponding to the position, wherein the computer program product, when accessed by a computer, causes the computer to set a bit of a smoothing register associated with the one round buffer, the set bit corresponding to the one entry,

wherein the computer program product, when accessed by a computer, causes the computer to:
access the set bit in the associated smoothing register;
access the one entry of the one round buffer corresponding to the set bit;
read the packet identifier from the one entry of the one round buffer; and
dequeue the packet from the associated queue.

30. An article of manufacture comprising:
a computer readable medium having embodied heron a computer program product, when accessed by a computer, causes the computer to:
access a received packet to determine a size of the packet and a queue associated with the packet;
determine a transmission round for the packet based on a determination of a number of rounds needed to schedule the packet;
determine a position within the transmission round based, at least in part, on a quantum of the associated queue; and
store an identifier for the packet in a scheduling array, the scheduling array including a number of round buffers, each round buffer including a number of entries, the identifier stored in one of the round buffers corresponding to the transmission round at one of the entries corresponding to the position, wherein the

computer program product, when accessed by a computer, causes the computer, when determining a transmission round, to:
if the associated queue has a previously enqueued packet, set the transmission round equal to a sum of a last scheduled round and the number of needed rounds; and if the
associated queue is empty, set the transmission round equal to a sum of a current round and the number of needed rounds.

31. The article of manufacture of claim 30, wherein the computer program product, when accessed by a computer, causes the computer to update per-queue data of the associated queue, the per-queue data including a packet count, a deficient count, and the last scheduled round.

32. The article of manufacture of claim 31, wherein the computer program product, when accessed by a computer, causes the computer, when updating the data, to:
set the last scheduled round equal to the transmission round;
increment the packet count; and
determine a new value of the deficit count based, at least in part, on an old value of the deficit count.

33. The article of manufacture of claim 32, wherein the computer program product, when accessed by a computer, causes the computer, when determining the position, to:
subtract the new value of the deficit count from the quantum of the associated queue to 3 determine a numerator;
divide the numerator by the quantum of the associated queue to determine a result; and multiply the result by the number of entries in the one round buffer.

34. The article of manufacture of claim 32, wherein the computer program product, when accessed by a computer, causes the computer, when determining the new value for the deficient count, to:
determine a product of the quantum and the number of needed rounds;
determine a sum of the product and the old value of the deficit count; and
subtract the packet size from the sum.

35. The article of manufacture of claim 32, wherein the computer program product, when accessed by a computer, causes the computer, when determining a number of rounds needed to schedule the packet, to:
if the old value of the deficit count exceeds the packet size, set the number of needed rounds equal to zero; and
if the old value of the deficit count is less than the packet size, subtract the old value of the deficit count from the packet size to determine a result, and divide the result by the quantum to determine the number of needed rounds.

36. The method of claim 2, wherein the associated queue comprises one of a number of queues and the number of queues is associated with one of a number of ports of a switching device.