Abstract:
A method and system for implementing a generalized system stutter are disclosed. Specifically, one embodiment of the present invention sets forth a method, which includes the steps of blocking a first request received from a first of a plurality of bus masters during a low power state of a computing device for as long as permissible by the timing requirements of the computing device, wherein the first request is capable of triggering the computing device to transition out of the low power state, and during an active state of the computing device, servicing the first request along with other pending requests from the rest of the plurality of bus masters before the computing device transitions back to the low power state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to power management and more specifically to a method and system for implementing generalized system stutter. 
     2. Description of the Related Art 
     Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Energy efficiency is becoming an increasingly important consideration in many system designs. Memory manufacturers have developed memory systems with multiple power states, such as active, active idle, power-down, and self-refresh. A memory system typically needs to be in the active state to service a request, and the remaining power states are in order of decreasing power consumption but increasing time to transition back to active. In other words, the active state consumes the most power, and the self-refresh state incurs the most delay for clock resynchronization. Similarly, system interconnect links are also associated with multiple power states, with the lowest power state again corresponding to the highest latency. Thus, one approach of achieving energy efficiency is to maintain a memory system, a system interconnect link, or both in the lowest power state for as long as possible, while effectively managing the high latencies associated with entering and exiting such a state. 
     To illustrate, suppose a display system  110  is the only active agent in a computing device  100  during a low power state, in which system memory  106  is in the self-refresh state, and a system link  108  is in the power-down state.  FIG. 1A  is a simplified block diagram of a computing device  100  capable of displaying data in this low power state. The display system  110  of the computing device  100  includes a display engine  112 , a display device  114 , and a display first-in-first-out (“FIFO”) buffer  116 . The display engine  112  utilizes the display FIFO buffer  116  to decouple the stringent timing requirements of the display device  114  from the memory system  106 . So, to be able to survive potentially significant latency associated with “waking up” the system memory  106  from the low power state just to retrieve data, the display engine  112  ensures that the display FIFO buffer  116  stores sufficient pixel data to satisfy the timing requirements of the display device  114  during the low power state. Specifically, while the computing device  100  resides in the low power state, the display engine  110  processes and drains the data in the display FIFO buffer  116  in a direction  118 . When the display engine  110  hits a pre-determined critical watermark in the display FIFO buffer  116 , the display engine  110  initiates the process of exiting the low power state and fetching data from the system memory  106  to fill up the display FIFO buffer  116  in a direction  120 . This filling up process is also referred to as “topping off” the display FIFO buffer  116 . 
       FIG. 1B  is a timing diagram illustrating one pattern of system memory accesses by a display system without a display FIFO buffer to optimize power efficiency and a display engine, whereas  FIG. 1C  is a timing diagram illustrating a different pattern of system memory accesses by the display system  110  with the display FIFO buffer  116  to optimize power efficiency and the display engine  112 . Without the power efficiency optimization, the gap between any two memory accesses, denoted as access gap  150 , is typically less than the latency associated with entering or exiting a low power state, such as the self-refresh state. On the other hand, with an appropriated sized display FIFO buffer  116 , the memory accesses can be clustered, and an access gap  160  can be lengthened to be at least equal to the latency associated with entering or exiting the self-refresh state. This clustering of memory access requests and lengthening of access gaps are collectively referred to as “display stutter.” With the pattern shown in  FIG. 1C , the computing device  100  is able to achieve the desired energy efficiency. 
     However, in addition to the display system  110 , the computing device  100  has various input/output (“I/O”) agents that request to access the system memory  106  via the system link  108  and a chipset  104 . Some examples of these I/O agents include, without limitation, an Integrated Driver Electronics (“IDE”) device, a Universal Serial Bus (“USB”) device, a network controller, a Peripheral Component Interconnect Express (“PCI Express”) controller, a PCI bridge, and a PCI-X controller. Each of the N I/O agents has its own distinct timing requirements, and many of the I/O agents do not support stutter requirements. Although redesigning each of the I/O agents to issue memory access requests leading to a similar memory access pattern as the one shown in  FIG. 1C  may improve the energy efficiency of the computing device  100 , the risks and the costs of tinkering with multiple working devices, especially the legacy I/O agents that have already been widely adopted, are likely to far outweigh any such improvement. 
     As the foregoing illustrates, what is needed in the art is a generalized system stutter that can be easily deployed and addresses at least the shortcomings of the prior art approaches set forth above. 
     SUMMARY OF THE INVENTION 
     A method and system for implementing a generalized system stutter are disclosed. Specifically, one embodiment of the present invention sets forth a method, which includes the steps of blocking a first request received from a first of a plurality of bus masters during a low power state of a computing device for as long as permissible by the timing requirements of the computing device, wherein the first request reaching a limit of the timing requirements is capable of triggering the computing device to transition out of the low power state, and during an active state of the computing device, servicing the first request along with other pending requests from the rest of the plurality of bus masters before the computing device transitions back to the low power state. 
     One advantage of the disclosed method and system is that without any redesign of various I/O agents in a computing device, memory accesses for this computing device can be managed to enhance the energy efficiency of the computing device 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1A  is a simplified block diagram of a computing device capable of displaying data in a low power state; 
         FIG. 1B  is a timing diagram illustrating one pattern of system memory accesses by a display system without a display FIFO buffer; 
         FIG. 1C  is a timing diagram illustrating a different pattern of system memory accesses by another display system, which includes a display FIFO buffer; 
         FIG. 2  is a simplified block diagram of some components in a computing device configured to implement generalized system stutter, according to one embodiment of the present invention; 
         FIG. 3A  is an exploded view of a centralized stutter unit, according to one embodiment of the present invention; 
         FIG. 3B  is a state transition diagram of a blocker in the centralized stutter unit, according to one embodiment of the present invention; 
         FIG. 4A  is a timing diagram of handling a memory access request from a bus master having a high latency tolerance, according to one embodiment of the present invention; 
         FIG. 4B  is a timing diagram of handling a memory access request from a bus master having a significantly lower latency tolerance than an access gap, according to one embodiment of the present invention; 
         FIG. 4C  is another timing diagram of handling a memory access request from a bus master having a significantly lower latency tolerance than an access gap, according to one embodiment of the present invention; 
         FIG. 4D  is a timing diagram of handling multiple memory access requests from different bus masters, according to one embodiment of the present invention; and 
         FIG. 4E  is a timing diagram of handling a memory access request from a bus master during the processing of a cluster of memory accesses for a display system, according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this disclosure, one embodiment of the present invention is implemented as a software component for use with a computing device. The software component defines functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computing device) on which information is permanently stored; (ii) writable storage media (e.g., writeable memory devices such as flash memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the present invention, are embodiments of the present invention. It should however be apparent to a person with ordinary skills in the art to implement other embodiments of the present invention using hardware components or a combination of hardware components and software components. 
       FIG. 2  is a simplified block diagram of some components in a computing device  200  configured to implement generalized system stutter, according to one embodiment of the present invention. The computing device  200  includes a processing unit  202 , a chipset  204  with an arbiter  205 , system memory  206 , a display system  210 , and a centralized stutter unit  221  coupled to a system link  208  and a number of I/O agents. The display system  210  typically includes a display engine  212 , local video memory (not shown in  FIG. 2 ), and a display FIFO buffer  216  to process video data and to drive a display device  214 . The display device  214  is an output device capable of emitting a visual image corresponding to a data signal generated by the display engine  212 . Some examples of the display device  214  include, without limitation, a cathode ray tube (CRT) monitor, a liquid crystal display, a plasma display, a projector, or any other suitable display system. 
     The system memory  206  stores programming instructions and data, including screen data, for the processing unit  202  and even the display engine  212  to execute and operate on. As shown, the processing unit  202  communicates with the system memory  206  and the display system  210  via the chipset  204 . Alternatively, the processing unit  202  includes a dedicated memory port to connect to the system memory  206 . In other implementations, the processing unit  202 , the display engine  212  in the display system  210 , the chipset  204 , or any combination thereof, may be integrated into a single processing unit. Further, the functionality of the display engine  212  may be included in the chipset  204  or in some other type of special purpose processing unit or co-processor. In such embodiments, software instructions may reside in other memory systems than the system memory  206  and may be executed by processing units other than the processing unit  202 . It should also be apparent to a person with ordinary skills in the art to recognize that the chipset  204  may include multiple discrete integrated circuits that work together to serve different types of I/O agents, such as a northbridge and a southbridge. 
     Although the arbiter  205  and the centralized stutter unit  221  are shown to be two discrete components in  FIG. 2 , it should be apparent to a person with ordinary skills in the art to implement generalized system stutter using other configurations while remaining within the scope of the present invention. For example, in one implementation, the functionality of the centralized stutter unit  221  is included in the arbiter  205 . 
       FIG. 3A  is an exploded view of the centralized stutter unit  221 , according to one embodiment of the present invention. For each I/O agent with a bus master, the centralized stutter unit  221  includes a corresponding blocker to potentially block the requests of the bus master from propagating to the arbiter  205 . For instance, a blocker 1    302  corresponds to an I/O agent 1    222 , and a blocker N    304  corresponds to an I/O agent N    224 . In one implementation, each blocker has a programmable field, MAX_BLOCK_TIME, which can be expressed mathematically as follows:
 
MAX_BLOCK_TIME=latency tolerance associated with I/O agent−(latency associated with exiting a low power state+amount of time arbitrating among I/O agents)
 
     To illustrate, in conjunction with  FIG. 2 , suppose the bus master of the I/O agent 1    222  has a built-in  100  usec of latency tolerance, and the bus master requests data from the system memory  206  while in a low power state. Suppose further that it takes 20 usec for both the system memory  206  to transition from the low power state to an active state and the arbiter  205  to select a request to service. So, for the bus master of the I/O agent 1    222  to meet its timing constraints, it needs to receive the requested data within 100 usec. However, since it takes 20 usec for the arbiter  205  and the system memory  206  to service the request, the blocker 1    302  can at most block the request for (100-20) or 80 usec. In other words, on the 80 th  usec, the blocker 1    302  needs to start propagating the request to arbiter  205  and initiate the process of transitioning the system memory  206  to the active state. Alternatively, the latency tolerance used in the above equation further depends on the type of software safety nets, if any, configured to operate on the computing device  200 . For example, one software safety net configures the computing device  200  to resend packets if packet losses are detected during a transmission. With such a software safety net, the latency tolerance may be lengthened to exceed the built-in latency tolerance of the bus master. 
     Moreover, each blocker is connected to one another. So, the propagation of one bus master request from a single I/O agent to the arbiter  205  triggers the “unblocking” of all the other blockers in the centralized stutter unit  221  and releases all the pending bus master requests to the arbiter  205 . The arbiter  205  is configured with policies to select among the requests from various bus masters to service. It should be apparent to a person with ordinary skills in the art to recognize that the arbiter  205  can adopt any of the known arbitration schemes without exceeding the scope of the present invention. 
       FIG. 3B  is a state transition diagram  350  of a blocker in the centralized stutter unit  221 , according to one embodiment of the present invention. Using the blocker 1    302  shown in  FIG. 3A  as an illustration, the blocker 1    302  typically stays in an idle state  352 , especially during a low power state of the computing device  200  of  FIG. 2 . This state indicates that the blocker 1    302  does not have any pending bus master request. Suppose during the low power state, the blocker 1    302  receives a bus master request from the I/O agent 1    222 . If the aforementioned MAX_BLOCK_TIME for the blocker 1    302  contains a non-zero value, then the blocker 1    302  transitions to a block state  354  and starts blocking the request. The block remains in effective until either the I/O agent 1    222  deasserts the request or the blocker 1    302  transitions to a request pending state  356 . To transition to the request pending state  356 , one triggering condition is when the bus master request has already been blocked for MAX_BLOCK_TIME, and another triggering condition is when the computing device  200  exits the low power state. This transitioning out of the low power state can occur prior to the expiration of MAX_BLOCK_TIME in a number of scenarios. For instance, another blocker in the centralized stutter unit  221  unblocks its pending request before the blocker 1    302  reaches its MAX_BLOCK_TIME and triggers the computing device  200  to enter an active state. In another scenario, the display system  210  begins requesting for data via the system link  208  and triggers the computing device  200  to transition out of the low power state before the expiration of MAX_BLOCK_TIME of the blocker 1    302 . 
     Instead of reaching the request pending state  356  via the block state  354  as described above, the blocker 1    302  may reach the request pending state  356  directly from the idle state  352 . To illustrate, suppose the blocker 1    302  again receives a bus master request from the I/O agent 1    222 . If the computing device  200  is not in a low power state or MAX_BLOCK_TIME of the blocker 1    302  is configured to be zero, then the blocker 1    302  directly transitions to the request pending state  356 . After propagating the pending request to the arbiter  205  for further processing, the blocker 1    302  transitions back to the idle state  352 . 
     Furthermore, because the display system  210  is typically the main consumer of data during a low power state of the computing device  200  of  FIG. 2 , one implementation of the generalized system stutter is to manipulate the aforementioned blockers to cluster as many memory access requests from various I/O agents with the memory access requests from the display system  210  as possible.  FIG. 4A  to  FIG. 4E  and the following discussions describe the handling of various bus masters of I/O agents with different latency tolerance limits. 
       FIG. 4A  is a timing diagram of handling a memory access request  400  from a bus master having a high latency tolerance, according to one embodiment of the present invention. Here, the blocker responsible for this bus master causes the memory access request  400  to be serviced after a cluster  402  of memory accesses for the display system  210  is performed. In one implementation, the cluster  402  of memory accesses is for the display engine  212  to fill up the display FIFO  216 . By grouping a memory access  404  with the cluster  402 , the system memory  206  does not need to separately transition out of the low power state to just satisfy the memory access request  400 . It is worth noting that an access gap  406  is limited by the minimum latency tolerance among all the I/O agents in the computing device with pending requests to access the system memory  206 . In the example shown in  FIG. 4A , however, the minimum latency tolerance equals to the latency tolerance of the display system  210 , which in one implementation, is dictated by the size of the display FIFO  216 . 
       FIG. 4B  is a timing diagram of handling a memory access request  410  from a bus master having a significantly lower latency tolerance than an access gap  416 , according to one embodiment of the present invention. The blocker responsible for this bus master causes the unblocking of the arbiter  205  and the transitioning of the system memory  206  out of the low power state. In this specific example, a memory access  414  corresponding to the memory access request  410  is injected in a cluster  412  of memory accesses for the display system  210 . Similar to the process illustrated in  FIG. 4A  and detailed above, processing the memory access  414  along with the cluster  412  prevents the system memory  206  from separately transitioning out of the low power state just to service the memory access request  410 . 
       FIG. 4C  is another timing diagram of handling a memory access request  420  from a bus master having a significantly lower latency tolerance than an access gap  428  Here, because of the low latency tolerance, the system memory  206  transitions out of the lower power state to service a memory access  424  corresponding to the memory access request  420 . To take full advantage of the system memory  206  being in an active state during a period  430 , one implementation of the display engine  212  causes the display FIFO  216  to top off in a direction  434 . More precisely, as the display engine  212  drains and processes pixel data in the display FIFO  216  in a direction  432  during the low power state, the occurrence of the memory access request  420  triggers the waking up of the system memory  206  and the servicing of a cluster  426  of memory accesses to top off the display FIFO  216  in the direction  434 . 
       FIG. 4D  is a timing diagram of handling multiple memory access requests from different bus masters, according to one embodiment of the present invention. Suppose the latency tolerance of either of the two bus masters is significantly longer than an access gap  450 . The two blockers responsible for a memory access request  440  and a memory access request  442  cause the corresponding memory accesses  446  and  448 , respectively, to be grouped with a cluster  444  of memory accesses for the display system  210 . It should be apparent to a person with ordinary skills in the art to recognize that the clustering of the memory accesses  446  and  448  shown in  FIG. 4D  is for illustrative purposes only and can be modified according to the arbitration policies adopted by the arbiter  205  without exceeding the scope of the present invention. 
     Lastly,  FIG. 4E  is a timing diagram of handling a memory access request  460  from a bus master during the processing of a cluster  462  of memory accesses for the display system  210 , according to one embodiment of the present invention. In one implementation, the blocker responsible for this bus master propagates the request to the arbiter  205  without further blocking and causes a memory access  464  to be injected in a cluster  462  of memory accesses for the display system  210 . 
     The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples, embodiments, and drawings should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims.