Patent Publication Number: US-7917702-B2

Title: Data prefetch throttle

Description:
BACKGROUND 
     The present invention relates generally to the field of processors and in particular to a system and method for controlling data prefetching in processors. 
     Processors use caching to relieve memory-associated processing bottlenecks. Instruction caching works, for example, by using faster-access memory to hold selected portions of a larger set of program instructions stored in slower memory, such as main memory or a higher-level of cache memory. 
     Instructions present in the cache are thus accessed with lower delays than would be required for access to the slower memory, and processors commonly employ some form of hardware-based instruction prefetching to keep the instruction cache filled with needed lines of instructions from the slower memory. Prefetching places lines of instructions from slower memory into the instruction cache before instructions within those lines are needed. 
     Hardware-based prefetching also may be applied to data. However, successfully prefetching data can be more difficult than successfully prefetching instructions. For example, data values may be more scattered or spread out in memory than program instructions, making predictive-based prefetching more challenging. As such, data prefetching may or may not improve performance, and the performance of data prefetching may change dramatically during processor operation. 
     Thus, it is known for example to “filter” prefetch operations. Prefetch filtering represents a “pollution” avoidance mechanism, where the data cache is considered polluted when it contains prefetched data lines that are never used, i.e., data lines that are prefetched but ultimately replaced before ever being accessed (hit). As such, prefetch filtering implies carrying out data prefetching on an ongoing basis but selectively skipping certain data prefetches that otherwise would be carried out absent such filtering. 
     In more detail, individual data prefetches may or may not be performed in dependence on the applied filtering criteria. The filtering criteria may reflect a prefetching performance history developed, for example, over some range of program execution. However, the determination of appropriate filtering may require undesirable hardware complexity or resource consumption, particularly to yield meaningful performance improvements over data prefetching without filtering. 
     SUMMARY 
     According to one or more embodiments, a method of controlling data prefetching for a data cache comprises tracking prefetch hits for the data cache and disabling data prefetching for the data cache based on the tracking of prefetch hits. The method further includes tracking overall hits for data cache and enabling data prefetching for the data cache based on the tracking of overall hits. In this context, disabling data prefetching comprises disabling all data prefetching for the data cache, although data lines are still fetched into the data cache as needed, e.g., on data cache misses, irrespective of whether data prefetching is enabled. 
     In at least one embodiment taught herein, a processor includes a data cache comprising cache memory and a cache controller. The cache controller disables data prefetching for the data cache based on tracking prefetch hits for the data cache and enables data prefetching for the data cache based on tracking overall hits for the data cache. In at least one such embodiment, the cache controller tracks the prefetch hits by tracking a prefetch hit rate and tracks the overall hits by tracking an overall hit rate (or, equivalently, an overall miss rate). 
     With the above examples in mind, data prefetching control as taught herein offers, among other things, the performance and power advantages of data prefetching on a conditional basis, while simultaneously offering simple and efficient hardware implementations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of one embodiment of a processor. 
         FIG. 2  is a state diagram for an embodiment of data prefetch control. 
         FIG. 3  is a functional block diagram of one embodiment of counting and control circuits useful in controlling data prefetch. 
         FIG. 4  is a functional block diagram of one embodiment of data cache memory, including indicators to denote prefetched data lines. 
         FIG. 5  is a functional block diagram of another embodiment of indicators to denote prefetched data lines in a data cache. 
         FIGS. 6 and 7  are logic flow diagrams of one embodiment of processing logic for controlling data prefetch. 
     
    
    
     DETAILED DESCRIPTION 
     As a non-limiting example,  FIG. 1  illustrates one embodiment of a processor  100  that includes an instruction execution pipeline  102 , status/control registers  104 , and a data cache  106 , which includes a cache controller  108  and associated cache memory  110 . In operation, the data cache  106  caches lines of data from one or more higher levels of memory  112 , which may include higher-level caches and/or main (system) memory. In at least one embodiment, the data cache  106  comprises a Level  1  (“L 1 ”) data cache. 
     Advantageously, the (data) cache controller  108  is configured to enable and disable data cache prefetching dynamically according to logical control mechanisms that are implemented in the data cache  106  with low hardware complexity.  FIG. 2  is a state diagram illustrating one embodiment of this advantageous prefetching control. 
     As shown in  FIG. 2 , State  200  represents an operational state of the data cache  106  where data prefetching is enabled, while State  202  represents an operational state of the data cache  106  where prefetching is disabled. Rather than screening or otherwise filtering individual prefetches, the cache controller  108  advantageously ceases all prefetching when operating in State  202 . Thus, the prefetching control embodied in  FIG. 2  operates like an on/off switch for data prefetching. 
     In one or more embodiments, the cache controller  108  transitions from State  200  (prefetching enabled) to State  202  (prefetching disabled) as a function of tracking “prefetch hits”. Further, the cache controller  108  transitions from State  202  back to State  200  as a function of tracking “overall hits”. In this context, “prefetch hits” are hits on prefetched data lines held in the cache memory  110  of the data cache  106 , while “overall hits” are hits on any data line (prefetched or not) held in the cache memory  110  of the data cache  106 . In this sense, the prefetch hits reflect the percentage of data cache hits that involve prefetched data lines and the overall hits reflect the overall percentage of cache hits. Equivalently, the cache controller  108  tracks cache misses. For example, if an overall hit rate for the data cache  106  is ninety percent, then the overall miss rate is ten percent. 
     In more detail, during program execution the processor  100  first looks for needed data in the data cache  106 . A data cache hit represents the case where the needed data resides in the data cache  106 . Conversely, a data cache miss represents the case where the needed data does not reside in the data cache  106 . The cache controller  108  performs data fetches in response to data cache misses, which are often referred to as “compulsory fetches”. On the other hand, assuming that prefetching is enabled, the cache controller  108  prefetches lines of data from the higher-level memory  112  into the cache memory  110  of the data cache  106  according to one or more prefetching strategies (“policies”). As a non-limiting example, the cache controller  108  may use sequence-based and/or pointer-based prefetching policies. 
     In any case, those skilled in the art will appreciate that the cache memory  110  contains a mix of prefetched and fetched (non-prefetched) data lines, assuming that the data cache  106  is operating with prefetching enabled (State  200 ). Thus, individual hits on the data cache  106  involve either a prefetched data line or a non-prefetched data line, and tracking the prefetch hits provides the cache controller  108  with insight regarding prefetching performance. Put simply, a low number of data cache hits involving prefetched data lines suggests that data prefetching is not helpful for current program execution conditions. 
     Disabling prefetching under these conditions is advantageous because it eliminates prefetching overhead (memory bus accesses and control). Shutting down prefetching—transitioning from State  200  to State  202 —thus reduces processor operating power and reduces resource loading. Turning prefetching off for such conditions provides the further advantage of preventing the pollution of the data cache  106  with data lines that probably will not be used. 
     On the other hand, program execution conditions are subject to change such that prefetching again becomes desirable. To that end, the cache controller  108  tracks the overall hits for the data cache  106  while operating in State  202 , and enables prefetching if the overall hits for the data cache  106  become too low, for example. (Equivalently, the overall misses become too high.) In other words, if the overall hit rate for the data cache  106  begins to suffer with data prefetching turned off, the cache controller  108  turns prefetching back on by transitioning back to State  200 . 
     For example, the cache controller  108  tracks the prefetch hits as a prefetch hit rate and tracks the overall hits as an overall hit rate. In this manner, a defined disable threshold may be established for the prefetch disable decision at a default or dynamically calculated value. Likewise, a defined enable threshold may be established for the prefetch enable decision at a default or dynamically calculated value. As a non-limiting example, the cache controller  108  may be configured to turn off prefetching if the prefetch hit rate falls below two percent, and may be configured to turn on prefetching if the overall hit rate falls below ninety-nine percent. Of course, these are just example values, and the thresholds can be adjusted or otherwise tuned according to the particular processor characteristics and data cache size, and according to other considerations such as prefetching overhead, miss penalties, etc. 
     Regardless of the particular decision thresholds used,  FIG. 3  illustrates one embodiment of tracking mechanisms that can be used by the cache controller  108  to track the prefetch hits and overall hits. More particularly,  FIG. 3  illustrates a counter control circuit  300 , a first counter  302 , and a second counter  304 . These circuits may be included in or associated with the cache controller  108 . 
     In one or more embodiments, the counter control circuit  300  increments the first counter  302  in response to the cache controller  108  detecting hits on prefetched data lines in the cache memory  110 , and decrements the first counter  302  in response to the cache controller  108  detecting hits that are not on prefetched data lines in the cache memory  110 . In this manner, the value of the first counter  302  reflects the percentage of hits on the data cache  106  that are on prefetched data lines. As such, the counter control circuit  300  or another circuit element within the cache controller  108  can compare the value of the first counter  302  to the defined disable threshold as the basis for determining whether to transition to State  202 . 
     Further, the counter control circuit  300  increments the second counter  304  in response to hits on the data cache  106  (any hits), and decrements the second counter  304  in response to data cache misses. In this manner, the value of the second counter  304  reflects the overall percentage of hits on the data cache. More particularly, by counting up on data cache hits and counting down on data cache misses, the value of the second counter  304  reflects a hit/miss percentage for the data cache  106 . As such, the counter control circuit  300  or another circuit element within the cache controller  108  can compare the value of the second counter  304  to the defined enable threshold as the basis for determining whether to transition to State  200 . 
     The above processing involves detecting whether individual data cache hits are on prefetched data lines in the cache memory  110 .  FIGS. 4 and 5  illustrate different embodiments of providing for that detection. In both figures, one sees that the cache controller  108  stores or otherwise maintains indicators that denote which data lines in the cache memory  110  were prefetched. 
     In particular,  FIG. 4  illustrates an embodiment where the cache memory  110  comprises, for each data line held in the cache memory  110 , tag memory  400  for holding memory address information, data memory  402  for holding the line of cached data, and a prefetch flag  404  to indicate the status of the data line as prefetched (e.g., “1”) or not prefetched (e.g., “0”). 
     Conversely,  FIG. 5  illustrates an alternative embodiment, wherein the stored (prefetch) indicators are implemented as a register set  500 , including a line identifier (ID) entry  502  for each prefetched data line in the cache memory  110 . For example, entries may be added to the register set  500  for each data line prefetched into the cache memory  110 , such that only prefetched data lines are represented in the register set  500 . Alternatively, the register set  500  may include entries for all data lines in the cache memory  110 , each entry indicating whether or not the corresponding data line in the cache memory  110  is prefetched. 
       FIGS. 6 and 7  together illustrate one embodiment of processing logic that exploits use of the stored indicators ( 404  or  502 ) to detect prefetch hits. As a non-limiting example, the illustrated processing may be implemented by the cache controller  108  via digital processing logic, e.g., in a state machine. Further, it should be noted that one or more of the illustrated processing steps may be performed in other than the illustrated sequence, or may be performed concurrently with other steps, and/or may be performed as part of other processing tasks. 
     In broad accordance with the illustrated processing, the cache controller  108  uses a first count (e.g., the value of the first counter  302 ) to track prefetch hits on the data cache  106  and uses a second count (e.g., the value of the second counter  304 ) to track overall hits on the data cache  106 . The first and second counters  302  and  304  may comprise saturating counters, such that the corresponding first and second count values saturate at respective maximums. Regardless of that detail, one or more embodiments of the cache controller  108  transition the data cache  106  between the prefetched enabled condition and the prefetch disabled condition as a function of the first and second count values. These counts may be initialized as part of beginning operations. 
     In more detail, the illustrated processing begins with enabling data prefetching for the data cache  106  (Block  600 ). In at least one embodiment, the cache controller  108  is configured to start operations with data prefetching enabled by default, such that starts or restarts of the processor  100  turn on data prefetching. 
     With prefetching enabled, the data cache controller  108  fetches data lines as needed into the data cache  106 , and prefetches data lines into the cache according to the active prefetching policy (Block  602 ). Processing continues on a looping or otherwise ongoing basis with the cache controller  108  determining whether a data cache hit occurs (Block  604 ). If a data cache hit occurs (yes from Block  604 ), the cache controller  108  detects whether the hit was a prefetch hit (Block  606 ), e.g., it uses the stored (prefetch) indicators ( 404  or  502 ) to determine whether the particular data line involved in the cache hit was or was not a prefetched data line. 
     If the hit was a prefetch hit (yes from Block  606 ), the data cache controller  108  increments the first count (Block  608 ). If the hit was not a prefetch hit (no from Block  606 ), the data cache controller  108  decrements the first count (Block  610 ). The first count may be maintained in this manner by operating on the first counter  302  via the counter control circuit  300 . 
     Operations continue with evaluating the first count (Block  612 ) to determine whether the value of the first count is above a defined disable threshold for prefetching. With that arrangement, the disable threshold may be set at a percentage value corresponding to the point at which prefetching is deemed undesirable. In any case, for a binary count value, that determination may be made by comparing the count value to a binary pattern corresponding to the desired threshold value. In at least one embodiment, the first counter  302  is sized according to the desired count resolution for tracking prefetch hits. Note, too, that the evaluation of the first count may be performed on each cache hit, or may be performed according to another schedule or triggering condition. 
     In any case, if the value of the first count indicates that the prefetch hit rate is too low (yes from Block  614 ), the cache controller  108  disables prefetching (Block  616 ). From there, processing optionally continues with resetting the first count and/or resetting the second count (Block  618 ). That is, one or both counts may be set in conjunction with making the transition from prefetched enabled to prefetched disabled in a manner that reinforces that state change. 
     In at least one such embodiment, the second count is reset to a maximum value as part of changing to the prefetch disabled state and the first count is reset to a maximum value as part of changing to the prefetch enabled state. Doing so prevents rapid state reversals (sometimes referred to as “ping-ponging”). More particularly, the example counter resetting represents one form of control hysteresis contemplated herein. It is broadly contemplated in one or more embodiments herein to implement enable/disable control hysteresis, such as by resetting the tracking mechanisms (counters or otherwise) used for tracking prefetch hits and overall hits, adjusting control thresholds, suspending state change processing temporarily after making a state change, etc. 
     Returning to the illustrated processing by following connector “B” to  FIG. 7 , one sees that the processing continues with prefetching turned off. While prefetching is disabled, the cache controller  108  continues monitoring for data cache accesses (Block  700 ). If there is a data cache access (yes from Block  700 ), the cache controller  108  detects whether the access resulted in a cache hit (Block  702 ). If the access resulted in a hit (yes from Block  702 ), processing continues with the cache controller  108  incrementing the second count (Block  704 ). Conversely, if the cache access resulted in a cache miss (no from Block  702 ), processing continues with the cache controller  108  decrementing the second count (Block  706 ) and fetching data lines as needed into the cache memory  110  (Block  708 ). 
     Processing then continues with evaluating the second count (Block  710 ). Cache accesses and/or counter updates may be used as the trigger for count evaluation, or another schedule or trigger may be used. In any case, the evaluation may comprise comparing the value of the second count to a defined enable threshold. In at least one such embodiment, the defined enable threshold represents a lower percentage value for data cache hits. With that arrangement, the overall hit rate is deemed low if the percentage of cache hits as tracked by the second count is at or below the lower percentage. 
     If the overall hit rate is not low (no from Block  712 ), processing loops back to Block  700 . On the other hand, if the overall hit rate is low (yes from Block  712 ), processing continues through connector “A” back to Block  600  of  FIG. 6  for prefetch enabling. (Note that the first and/or second counts may be reset as part of transitioning back to the prefetch enabled condition (Block  714 )). 
     In an alternative embodiment, the cache controller  108  is configured to track prefetch hits based on counting or otherwise determining the number of prefetched data lines in the cache memory  110  (as compared to the overall number of data lines in the cache memory  110 ). The cache controller  108  may use the first counter  302  for counting prefetched data lines, or it may be configured with other counters and/or registers for tracking that information. In any case, the count of prefetched data lines in comparison to the overall count of data lines still reflects a prefetch hit rate in the sense that the number of prefetched data lines in the cache memory  110  will, because of the data cache replacement policy, decrease over time if prefetch hits are relatively infrequent. 
     With the above embodiments and other variations in mind, data cache prefetching control as taught herein broadly comprises tracking prefetch hits and tracking overall hits, such that transitions from the prefetch enabled condition are based on the prefetch hits and transitions from the prefetch disabled condition are based on the overall hits. In at least one embodiment, prefetching is disabled if the prefetch hit rate falls below a defined disable threshold, and prefetching is enabled if the overall hit rate falls below a defined enable threshold. Stored indicators may be used to denote which data lines are prefetched, and various counters or other registers may be used for the prefetch hit and overall hit tracking. 
     Therefore, although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.