Computer system with multiple classes of transaction IDs

A computer system may include a sending device, a receiving device, and a network coupling the devices. The sending device may be configured to send a packet on the network in order to initiate a transaction. The sending device is configured to only encode a portion of a transaction ID identifying the transaction in the packet. The receiving device is configured to receive the packet from the network and to send a responsive packet to the sending device as part of the transaction. The receiving device is configured to encode all of the transaction ID in the responsive packet. The receiving device may generate the portion of the transaction ID not encoded in the packet by the sending device in response to the packet having a particular characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of multiprocessor computer systems and, more particularly, to communication within multiprocessor computer systems.

2. Description of the Related Art

Multiprocessing computer systems include two or more processors that may be employed to perform computing tasks. A particular computing task may be performed upon one processor while other processors perform unrelated computing tasks. Alternatively, components of a particular computing task may be distributed among multiple processors to decrease the time required to perform the computing task as a whole.

Various components within a multiprocessing computer system may communicate with each other during operation. For example, various components may participate in a coherency protocol that involves sending and receiving communications. A popular architecture in commercial multiprocessing computer systems is a shared memory architecture in which multiple processors share a common memory. In shared memory multiprocessing systems, a cache hierarchy is typically implemented between the processors and the shared memory. In order to maintain the shared memory model, in which a particular address stores exactly one data value at any given time, shared memory multiprocessing systems employ cache coherency. Generally speaking, an operation is coherent if the effects of the operation upon data stored at a particular memory address are reflected in each copy of the data within the cache hierarchy. For example, when data stored at a particular memory address is updated, the update may be supplied to the caches that are storing copies of the previous data. Alternatively, the copies of the previous data may be invalidated in the caches such that a subsequent access to the particular memory address causes the updated copy to be transferred from main memory or from a cache.

Various communications may be sent between components of a multiprocessing computer system in order to, for example, implement a coherency protocol. As the size of each communication increases, the amount of network bandwidth necessary to send communications also increases, which may in turn increase the cost of the multiprocessing system. Accordingly, it is desirable to be able to reduce the amount of information included in communications sent between components.

SUMMARY

Various embodiments of systems and methods that implement multiple classes of transaction IDs are disclosed. In one embodiment, a computer system may include a sending device, a receiving device, and a network coupling the devices. The sending device may be configured to send a packet on the network in order to initiate a transaction. The sending device is configured to only encode a portion of a transaction ID identifying the transaction in the packet. The receiving device is configured to receive the packet from the network and to send a responsive packet to the sending device as part of the transaction. The receiving device is configured to encode all of the transaction ID in the responsive packet. The receiving device may generate the portion of the transaction ID not encoded in the packet by the sending device in response to the packet having a particular characteristic. In some embodiments, the sending device may be configured to assign a transaction ID, which includes a subset ID and a packet characteristic ID, to each transaction initiated by the sending device. A subset ID portion of a transaction ID assigned to one transaction may have the same value as a subset ID portion of another transaction's transaction ID.

DETAILED DESCRIPTION OF EMBODIMENTS

Computer System

FIG. 1shows a block diagram of one embodiment of a computer system140that may implement multiple classes of transaction IDs. Computer system140includes processing subsystems142A and142B, memory subsystems144A and144B, and an I/O subsystem146interconnected through an address network150and a data network152. Each of processing subsystems142, memory subsystems144, and I/O subsystem146may be referred to as a client device or subsystem. It is noted that although five client devices are shown inFIG. 1, embodiments of computer system140employing any number of client devices are contemplated. Elements referred to herein with a particular reference number followed by a letter may be collectively referred to by the reference number alone. For example, processing subsystems142A-142B may be collectively referred to as processing subsystems142.

Each of processing subsystems142and I/O subsystem146may access memory subsystems144. Devices configured to perform accesses to memory subsystems144are referred to herein as “active” devices. Because each active device within computer system140may access data in memory subsystems144, potentially caching the data, memory subsystems144and active devices such as processing systems142may implement a coherency protocol in order to maintain data coherency. Each client inFIG. 1may be configured to participate in the coherency protocol by sending address messages on address network150and data messages on data network152using split-transaction packets. Similar address and/or data packets may be used to participate in other protocols.

Memory subsystems144are configured to store data and instruction code for use by processing subsystems142and I/O subsystem146. Memory subsystems144may include dynamic random access memory (DRAM), although other types of memory may be used in some embodiments.

I/O subsystem146is illustrative of a peripheral device such as an input-output bridge, a graphics device, a networking device, etc. In some embodiments, I/O subsystem146may include a cache memory subsystem similar to those of processing subsystems142for caching data associated with addresses mapped within one of memory subsystems144.

In one embodiment, data network152may be a logical point-to-point network. Data network152may be implemented as an electrical bus, a circuit-switched network, or a packet-switched network. In embodiments where data network152is a packet-switched network, packets may be sent through the data network using techniques such as wormhole, store and forward, or virtual cut-through. In a circuit-switched network, a particular client device may communicate directly with a second client device via a dedicated point-to-point link that may be established through a switched interconnect mechanism. To communicate with a third client device, the particular client device utilizes a different link as established by the switched interconnect than the one used to communicate with the second client device. Messages upon data network152are referred to herein as data packets.

Address network150accommodates communication between processing subsystems142, memory subsystems144, and I/O subsystem146. Messages upon address network150are generally referred to as address packets. When an address packet references a storage location within a memory subsystem144, the referenced location may be specified via an address conveyed within the address packet upon address network150. Subsequently, data corresponding to the address packet on the address network150may be conveyed upon data network152. In one embodiment, address packets may correspond to requests for an access right (e.g., a readable or writable copy of a cacheable coherency unit) or requests to perform a read or write to a non-cacheable memory location. Thus, address packets may be sent by an active device in order to initiate a coherency transaction. Subsequent address packets may be sent by other devices in order to implement the access right and/or ownership changes needed to satisfy the coherence request. In the computer system140shown inFIG. 1, a coherency transaction may include one or more packets upon address network150and data network152. Typical coherency transactions involve one or more address and/or data packets that implement data transfers, ownership transfers, and/or changes in access privileges. Communications upon address network150may be point-to-point or broadcast, depending on the embodiment. Note that in some embodiments, address network and data network152may be implemented using the same physical interconnect.

Classes of Transaction IDs

As noted above, transactions may involve several address and/or data packets. When communicating with other client devices, client devices may encode transaction IDs into packets to identify the transactions of which those packets are a part. The client device that initiates a transaction may initially assign a unique transaction ID to that transaction. The initiating client device may encode at least a portion of the assigned transaction ID into the packet sent to initiate the transaction. Other client devices may encode the transaction ID in other packets that are sent as part of that transaction. If the initiating device has several outstanding transactions, the initiating device may use the transaction IDs included in subsequently received packets to match those packets to outstanding transactions. As a transaction completes, the initiating client device may free the transaction ID associated with that transaction. Free transaction IDs may then be assigned to new transactions initiated by that device. Since each outstanding transaction has a unique transaction ID with respect to other outstanding transactions initiated by the same client device, the number of bits included in each transaction ID may control how many outstanding transactions a client device can have.

The same transaction IDs may be used by different client devices in some embodiments. For example, processing subsystem142A may have outstanding transactions identified by transaction IDs1,5,8,9, and12at the same time that processing subsystem142B has outstanding transactions identified by transaction IDs2,5,6,7,8, and9.

FIG. 2illustrates an exemplary transaction ID200that may be used in one embodiment. The transaction ID200ofFIG. 2includes a subset ID202and a packet characteristic ID204. The packet characteristic ID204identifies a particular characteristic of a packet in which all or part of the transaction ID200is encoded. Each transaction initiated by a packet having the same characteristic is identified by a transaction ID having the same packet characteristic ID.

Transactions may be grouped into classes based on whether each transaction is initiated by sending a packet having a particular characteristic. For example, if a transaction is initiated by sending a packet having that characteristic, the transaction belongs to one class. If the transaction cannot be initiated by sending a packet having that characteristic, the transaction belongs to another class. More than one packet characteristic may be used to classify transactions in some embodiments. Packet characteristic ID204may identify features such as a type of command encoded in the packet and/or an address range encoded in the packet. For example, packet characteristic ID204may identify whether a packet includes a read command or a write command. Packet characteristic ID204may instead (or additionally) indicate whether the packet includes an address within a particular address range.

Each transaction initiated by sending a packet having a particular characteristic may have a unique subset ID202with respect to other outstanding (i.e., uncompleted) transactions in the same transaction class that are initiated by the same initiating device. Transactions in different classes, which have different packet characteristic IDs204, may have the same subset ID202. For example, if there are seven bits of subset ID information in each transaction ID, there may be up to 128 uniquely identifiable outstanding transactions in each transaction class at each client device. An outstanding transaction within each transaction class may include a transaction ID having a particular subset ID202, such as 0001101. Accordingly, the same subset ID202may be used to identify transactions in different classes.

The amount of packet characteristic ID204and subset ID202information included in a transaction ID200may also vary among embodiments. The subset ID202may be sized to be able to uniquely identify a desired number of outstanding transactions at a particular client device. For example, if it is desirable to be able to uniquely identify up to 256 outstanding transactions per transaction class, there may be at least eight bits of subset ID202information. Note that some client devices may tend to have substantially fewer outstanding transactions than other client devices. However, subset IDs202may be uniformly sized for all of the client devices in many embodiments. The amount of packet characteristic ID204information may depend, at least in part, on the number of transaction classes included in a particular embodiment. For example, a single bit of packet characteristic204information may be used to differentiate between classes if only two transaction classes are implemented.

Use of transaction IDs200that include packet characteristic IDs204and subset IDs202may allow more efficient communications in some embodiments. For example, in many embodiments, only packets having a particular characteristic may be used to initiate a particular class of transaction. Accordingly, whenever a device sends a packet to initiate that particular class of transaction, that device may be configured to only encode the subset ID202of that transaction ID in the packet. Recipient devices may regenerate the excluded packet characteristic ID204based on the class of transaction initiated by that packet. When responding to the initiating device (e.g., by sending other address and/or data packets), these devices may encode the full transaction ID in the responsive packets. For example, in one embodiment, two transaction classes, read and write transactions, may be defined. The type of command (read or write) encoded in a packet is the packet characteristic that indicates the transaction class. Since only two transaction classes are implemented, packet characteristic IDs204may be a single bit in size. A packet characteristic ID204value of 1 may indicate a read transaction and a value of 0 may indicate a write transaction. If a client device sends a read-to-own address packet initiating a read transaction, the client device may only encode the subset ID202in the packet. Recipient devices may identify that the packet initiates a read transaction (e.g., by examining the type of command included in the packet) and responsively identify the packet characteristic ID204of that packet as having a value of 1. When encoding the packet's transaction ID in responsive packets that are part of the read transaction, these devices may encode both the packet characteristic ID204(having a value of 1) and the subset ID202received in the initiating packet.

As another example of how a recipient device may regenerate the packet characteristic ID202portion of a transaction ID200, consider another embodiment in which the most significant bit of all read commands has a value of 1 and the most significant bit of all write commands has a value of 0. Two transaction classes, read transactions (packet characteristic204=1) and write transactions (packet characteristic204=0), may be implemented. In such an embodiment, the most significant bit of the command encoding may always have the same value as the packet characteristic204bit. Based on whether the command encoding in a received packet identifies a read or write transaction (as indicated by the most significant bit of the command encoding), a recipient device may regenerate the packet characteristic204bit of the transaction ID as having the same value as the most significant bit of the command encoding. If the recipient device sends any responsive packets as part of the transaction, the recipient device may thus encode the full transaction ID in any responsive packets.

Since the packet characteristic ID204may not be included in at least some packets, the size of those packets may be reduced. Alternatively, at least some of the room freed by not including the packet characteristic ID204in such a packet may be used to include other information, such as additional bit(s) used in an error detection and/or correcting code.

FIGS. 3-5illustrate how devices may use transaction IDs200when communicating exemplary address and data packets in one embodiment.FIG. 3illustrates how a device142B may send a data packet to device142A in response to receiving an address packet from device142A. The initiating device142A may initiate a coherency transaction by requesting access to a particular cache line by sending an address packet specifying that cache line and the requested access right. If the receiving device142B is responsible for providing the initiating device with a copy of the specified cache line in response to such a request, the receiving device142B may send a responsive data packet containing a copy of the specified cache line to the initiating device. Note that the address packet sent by the initiating device142A may be handled by one or more intervening devices (e.g., a memory subsystem144) before being provided to receiving device142B in some embodiments.

In the embodiment ofFIG. 3, characteristics of the address packet sent by device142A depend on the class of coherency transaction being initiated. Accordingly, any address packet having that characteristic belongs to a particular transaction class. Since the transaction class may be readily determined by receiving devices based on characteristics of the packet, the sending device142A may only encode the subset ID202portion of the packet's transaction ID200in the address packet300, as shown inFIG. 4.

Since the initiating device may have sent more than one packet with the same subset ID202, the receiving device144B may be configured to encode the entire transaction ID200in the data packet400returned to the initiating device, as shown inFIG. 5. The transaction ID200included in the data packet400is used by device142A in order to identify the transaction of which the data packet400is a part.

FIG. 6is a flowchart of one embodiment of a method of communicating between devices in a computer system. A sending device may identify each transaction initiated by that device by a transaction ID. At601, the sending device sends a packet to initiate a transaction. The packet only includes a portion of that packet's transaction ID. In particular, the packet does not include a packet characteristic ID portion of the transaction ID. The packet may have a particular characteristic, and all transactions in a particular class may be initiated by sending packets having that particular characteristic. In contrast, no transaction in any other class may be initiated by sending packets having that particular characteristic. Accordingly, the transaction class of the packet may be determined based on the presence or lack of the packet characteristic in that packet.

At603, a receiving device receives the packet sent by the sending device. The receiving device may generate a responsive packet that includes the full transaction ID of the sending device. The receiving device may generate the full transaction ID of the sending device from the subset ID included in the packet received at603and transaction class implied by the packet having the particular characteristic. The receiving device may send the responsive packet to the sending device, as indicated at605.

In some embodiments, the sending device may selectively encode either a full transaction ID or only a subset ID in a packet dependent on whether the packet has that characteristic. For example, when the sending device is sending a packet to initiate a transaction and the packet has a characteristic identifiable by the packet characteristic ID, the sending device may encode only a portion of the transaction ID. In contrast, when the sending device is sending a packet as part of transaction initiated by another device, the sending device may encode the entire transaction ID.