Abstract:
Methods and apparatuses for reducing power consumption of a system cache within a memory controller. The system cache includes multiple ways, and each way is powered independently of the other ways. A target active way count is maintained and the system cache attempts to keep the number of currently active ways equal to the target active way count. The bandwidth and allocation intention of the system cache is monitored. Based on these characteristics, the system cache adjusts the target active way count up or down, which then causes the number of currently active ways to rise or fall in response to the adjustment to the target active way count.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to data caches, and in particular to methods and mechanisms for dynamically sizing a system cache located in a memory controller. 
     2. Description of the Related Art 
     Modern day mobile electronic devices often include multiple components or agents sharing access to one or more memory devices. These multiple agents may make large numbers of requests to memory, and as the number of these requests increases, the power consumption of the device increases, which limits the battery life of the device. One approach for reducing power consumption is to try to reduce the number of times that off-chip memory is accessed by caching data in or near the processor. 
     Conventional caches are typically coupled to or nearby a processor and store data that is frequently accessed by the processor to reduce latency. Caches tend to consume large amounts of power, which is a valuable commodity in mobile electronic devices. Therefore, techniques to decrease the power consumption of caches are desired for reducing the overall power consumption of ICs and other electronic devices. 
     SUMMARY 
     Systems, memory controllers, caches, and methods for reducing the power consumption of a system cache are disclosed. 
     In one embodiment, the system cache may have a multi-way set associative configuration. Each way of the multi-way system cache may be powered separately from the other ways, allowing individual ways to be powered up or powered down during the operation of the system cache. The system cache may include a cache control unit, and the cache control unit may include logic to track various metrics related to the performance of the system cache. 
     In one embodiment, the cache control unit may maintain a target active way count, which specifies the desired number of active ways in the system cache. The cache control unit may also track the replacement and allocation failure count of requests and the hit count of requests that are received by the system cache. In addition, multiple programmable threshold values may be compared to these metrics. Based on the relationship between these metrics and the various threshold values, the target active way count may be adjusted. The cache control unit may detect a change to the target active way count, and then the cache control unit may increase or decrease the number of currently active ways in the system cache to match the change to the target active way count. 
     In one embodiment, the cache control unit may utilize a low-pass filter to avoid oscillation of ways powering up and down. To avoid oscillation, when a given way is powered down, a timer may be started. The cache control unit may prevent any of the ways from being powered up until the timer has expired. Similarly, when a given way is powered up, the timer may be started, and the cache control unit may prevent any of the ways from being powered down until the timer has expired. 
     These and other features and advantages will become apparent to those of ordinary skill in the art in view of the following detailed descriptions of the approaches presented herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the methods and mechanisms may be better understood by referring to the following description in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates one embodiment of a portion of an electronic device. 
         FIG. 2  illustrates one embodiment of a portion of an integrated circuit. 
         FIG. 3  is a block diagram illustrating one embodiment of a system cache. 
         FIG. 4  is a block diagram illustrating one embodiment of a pair of tag memory ways. 
         FIG. 5  illustrates one embodiment of a requesting agent conveying a request to a system cache. 
         FIG. 6  illustrates one embodiment of a set of configuration registers. 
         FIG. 7  is a block diagram illustrating one embodiment of a cache control unit controlling power supplies for ways of a system cache. 
         FIG. 8  is a generalized flow diagram illustrating one embodiment of a method for using coarse-grained power management techniques in a system cache. 
         FIG. 9  is a generalized flow diagram illustrating one embodiment of a method for dynamically sizing a multi-way set associative system cache. 
         FIG. 10  is a block diagram of one embodiment of a system. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the methods and mechanisms presented herein. However, one having ordinary skill in the art should recognize that the various embodiments may be practiced without these specific details. In some instances, well-known structures, components, signals, computer program instructions, and techniques have not been shown in detail to avoid obscuring the approaches described herein. It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements. 
     This specification includes references to “one embodiment”. The appearance of the phrase “in one embodiment” in different contexts does not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. Furthermore, as used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     Terminology. The following paragraphs provide definitions and/or context for terms found in this disclosure (including the appended claims): 
     “Comprising.” This term is open-ended. As used in the appended claims, this term does not foreclose additional structure or steps. Consider a claim that recites: “A memory controller comprising a system cache . . . .” Such a claim does not foreclose the memory controller from including additional components (e.g., a memory channel unit, a switch). 
     “Configured To.” Various units, circuits, or other components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs the task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the “configured to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, sixth paragraph, for that unit/circuit/component. Additionally, “configured to” can include generic structure (e.g., generic circuitry) that is manipulated by software and/or firmware (e.g., an FPGA or a general-purpose processor executing software) to operate in a manner that is capable of performing the task(s) at issue. “Configured to” may also include adapting a manufacturing process (e.g., a semiconductor fabrication facility) to fabricate devices (e.g., integrated circuits) that are adapted to implement or perform one or more tasks. 
     “First,” “Second,” etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, in a cache with a plurality of cache lines, the terms “first” and “second” cache lines can be used to refer to any two of the plurality of cache lines. 
     “Based On.” As used herein, this term is used to describe one or more factors that affect a determination. This term does not foreclose additional factors that may affect a determination. That is, a determination may be solely based on those factors or based, at least in part, on those factors. Consider the phrase “determine A based on B.” While B may be a factor that affects the determination of A, such a phrase does not foreclose the determination of A from also being based on C. In other instances, A may be determined based solely on B. 
     Referring now to  FIG. 1 , a block diagram illustrating one embodiment of a portion of an electronic device is shown. In the illustrated embodiment, electronic device  10  includes a memory  12 , memory controller  14 , coherence point  18 , processor complex  20 , graphics engine  22 , non real-time (NRT) peripherals  24 , and real-time (RT) peripherals  26 . It is noted that electronic device  10  may also include other components not shown in  FIG. 1 . Furthermore, in another embodiment, one or more of the components shown in  FIG. 1  may be omitted from electronic device  10 . In various embodiments, electronic device  10  may also be referred to as an apparatus, mobile device, or computing device. 
     Memory  12  is representative of any number and type of memory devices, such as dynamic random access memory (DRAM), synchronous DRAM (SDRAM), double data rate (DDR, DDR2, DDR3, etc.) SDRAM (including mobile versions of the SDRAMs such as mDDR3, etc., and/or low power versions of the SDRAMs such as LPDDR2, etc.), RAMBUS DRAM (RDRAM), static RAM (SRAM), etc. One or more memory devices may be coupled onto a circuit board to form memory modules such as single inline memory modules (SIMMs), dual inline memory modules (DIMMs), etc. 
     Memory controller  14  may include circuitry configured to interface to memory  12 , and various components may be coupled to memory controller  14  via coherence point  18 . In other embodiments, one or more of the other devices shown in  FIG. 1  may be coupled directly to memory controller  14  rather than coupled through coherence point  18 . In various embodiments, memory controller  14  may include any number of ports for coupling to various peripherals, components, and/or requesting agents. 
     Memory controller  14  may include system cache  16  for storing data retrieved from or intended for memory  12 . System cache  16  may be configured to process memory requests from multiple requesting agents. One or more requesting agents may be included within any of the devices shown connected to coherence point  18 . In one embodiment, cache lines may be allocated in system cache  16  with either a sticky state or a non-sticky state. When deciding which data to retain in system cache  16 , system cache  16  may base the decisions on the sticky status of the cache lines. As a result of using the sticky allocation for data that is going to be reused, the number of accesses that are made to memory  12  may be reduced, which reduces latency of memory requests and power consumption of electronic device  10 . 
     Coherence point  18  may be configured to route coherent and non-coherent traffic to and from memory controller  14 . Coherence point  18  may also be referred to as a coherence switch. Although not shown in  FIG. 1 , coherence point  18  may be coupled to other devices, such as a flash controller, camera, display, and other devices. 
     Processor complex  20  may include any number of central processing units (CPUs) (not shown) and various other components (e.g., caches, bus interface unit). The CPU(s) of processor complex  20  may include circuitry to run an operating system (OS). In various embodiments, the OS may be any type of OS (e.g., iOS). Each of the CPUs may include a level one (L1) cache (not shown), and each L1 cache may be coupled to a level two (L2) cache. Other embodiments may include additional levels of cache (e.g., level three (L3) cache). 
     Graphics engine  22  may include any type of graphics processing circuitry. Generally, the graphics engine  22  may be configured to render objects to be displayed into a frame buffer (not shown). Graphics engine  22  may include graphics processors that execute graphics software to perform a part or all of the graphics operation, and/or hardware acceleration of certain graphics operations. The amount of hardware acceleration and software implementation may vary from embodiment to embodiment. NRT peripherals  24  may include any non-real time peripherals. Various embodiments of the NRT peripherals  24  may include video encoders and decoders, scaler/rotator circuitry, image compression/decompression circuitry, etc. RT peripherals  26  may include any number and type of real-time peripherals. 
     It is noted that other embodiments may include other combinations of components, including subsets or supersets of the components shown in  FIG. 1  and/or other components. While one instance of a given component may be shown in  FIG. 1 , other embodiments may include two or more instances of the given component. Similarly, throughout this detailed description, two or more instances of a given component may be included even if only one is shown, and/or embodiments that include only one instance may be used even if multiple instances are shown. 
     Turning now to  FIG. 2 , one embodiment of a portion of an integrated circuit is shown. Integrated circuit (IC)  30  may include requesting agents  32 A-C, switch interface  34 , coherence points  36  and  38 , and memory controller  40 . Memory controller  40  may include memory controller caches  42  and  44 , memory channel switch  46 , and memory channel units  48  and  50 . Memory controller  40  may be coupled to one or more memory devices (not shown). In various embodiments, IC  30  may be included within any of various types of electronic devices, including mobile, battery-powered devices. IC  30  may also be referred to as a system on chip (SoC) or an apparatus. It is noted that IC  30  may include other components and interfaces not shown in  FIG. 2 . 
     The requesting agents  32 A-C may be configured to perform various operations in the system, and may access memory as part of performing these operations. For example, requesting agents  32  may be processors (either general purpose processors, or special purpose processors such as graphics processors). The processors may be configured to access memory to fetch instructions for execution, and may also be configured to access various data operands of the instructions in memory in response to executing the instructions. Other requesting agents may include fixed function circuitry (e.g., DMA controllers, peripheral interface controllers). The requesting agents  32  may be physically separate circuitry, such as a separate instance of a processor. Alternatively, a requesting agent may be a logical entity such as a process or thread executing on a processor, such that a single physical processor may include multiple logical requestors. The number of requesting agents  32 A-C included in a given embodiment may vary, from one to any number of requesting agents. 
     A given requesting agent (physical or logical) may be identified by a requesting agent identifier (ID). In various embodiments, the requesting agent may add a transaction identifier (TID) to track each individual request separately. Each request generated by a requesting agent  32 A-C may be accompanied by a group ID. The group ID may also be referred to as dataset ID. The group ID may be a separate identifier from the requesting agent ID and the TID, and the number of bits used to represent the group ID value may vary depending on the embodiment. For example, in one embodiment, four bits may be used to represent the group ID value, and there may be 16 separate group IDs. The group ID may be assigned to a request based on the dataflow to which the request belongs. The OS or device driver, depending on the embodiment, may assign the group ID. For some types of dataflows, the same group ID may be shared by multiple requesting agent IDs. In one embodiment, requests to page translation tables may be considered part of the same dataflow, and any of these requests, regardless of the requesting agent ID, may be assigned to a common group ID. For other types of dataflows, a group ID may be utilized by only a single requesting agent. 
     Coherence points  36  and  38  may be configured to manage the coherency of requests that are conveyed to the memory controller  40  from the requesting agents  32 A-C. In one embodiment, traffic from requesting agents  32 A-C may be split up in switch interface  34  and traverse a specific coherence point depending on the address that is being targeted by the specific memory request. Other embodiments may include other numbers of coherence points. 
     Memory controller caches  42  and  44  may be separate physical caches but may be considered a single logical memory controller cache. More specifically, memory controller caches  42  and  44  may share a single address space, and memory requests that reference the address space of cache  42  may be routed by switch interface  34  to cache  42  via coherence point  36  and memory requests that reference the address space of cache  44  may be routed by switch interface  34  to cache  44  via coherence point  38 . Switch interface  34  may be any type of communication medium (e.g. a bus, a point-to-point interconnect, etc.) and may implement any protocol. An interface may refer to the signal definitions and electrical properties of the interface, and the protocol may be the logical definition of communications on the interface (e.g., including commands, ordering rules, coherence support). It is noted that memory controller caches  42  and  44  may also be referred to as system caches. In other embodiments, memory controller  40  may include other numbers of memory controller caches. For example, in another embodiment, memory controller  40  may include four separate memory controller caches. 
     Memory controller caches  42  and  44  may be configured to maintain a sticky status for each cache line stored in the caches. The sticky status may be implemented via a sticky state, sticky flag, sticky bit, sticky tag, or other similar field. In one embodiment, a tag memory may be utilized to store tag entries that correspond to cache lines stored in a data memory. The tag entries may include multiple fields including a sticky status field and a group ID field. The group ID field may be used to identify the dataflow source of the request which caused the cache line to be allocated in the cache. 
     Memory controller switch  46  may route traffic between memory controller caches  42  and  44  and memory channel units  48  and  50 . There may be one memory channel unit  48  and  50  for each memory channel included in a given embodiment, and other embodiments may include one channel or more than two channels. The memory channel units  48  and  50  may be configured to schedule memory operations to be transmitted on the memory channel. The memory channel units  48  and  50  may be configured to queue read memory operations (or reads) and write memory operations (or writes) separately, and may be configured to arbitrate between reads and writes using a credit based system, for example. In the credit-based system, reads and writes may be allocated a certain number of credits. 
     In an embodiment, the memory channel units  48  and  50  may schedule memory operations in bursts of operations. To create bursts of memory operations for scheduling, the memory channel units  48  and  50  may group memory operations into affinity groups. A memory operation may be said to exhibit affinity with another memory operation if the operations may be performed efficiently on the memory interface when performed in close proximity in time. 
     It should be understood that the distribution of functionality illustrated in  FIG. 2  is not the only possible architecture which may be utilized for an integrated circuit. Other integrated circuits may include other components, omit one or more of the components shown, and/or include a different arrangement of functionality among the components. 
     Referring now to  FIG. 3 , a block diagram of one embodiment of a system cache is shown. In one embodiment, system cache  60  may include tag memory  62 , data memory  64 , cache control unit  66 , and configuration register  68 . It is noted that system cache  60  may also include other components and logic not shown in  FIG. 3 . For example, in other embodiments, system cache  60  may include arbitration circuitry to arbitrate among requests. It is to be understood that the system cache architecture shown in  FIG. 3  is merely one possible architecture that may be implemented. In other embodiments, other system cache architectures may be utilized with the methods and mechanisms disclosed herein. 
     In one embodiment, tag memory  62  may be coupled to receive addresses for memory requests from requesting agents. It is noted that the terms “memory request” and “transaction” may be used interchangeably throughout this disclosure. Data memory  64  may be coupled to receive data or provide data for transactions. In various embodiments, tag memory  62  and data memory  64  may include multiple ways, and each way may be addressable by index. For example, in one embodiment, tag memory  62  and data memory  64  may each include 16 ways. In other embodiments, tag memory  62  and data memory  64  may include other numbers of ways. Cache control unit  66  is coupled to tag memory  62  and data memory  64 , and cache control unit  66  may be configured to receive various control data related to the received transactions and to respond to the received control data. It is noted that although cache control unit  66  is shown in  FIG. 3  as a single unit, in other embodiments, cache control unit  66  may be split up into multiple units within system cache  60 . Configuration register  68  may include configuration information for the various group IDs associated with the data stored in system cache  60 . Configuration register  68  may be programmed by software commands sent to cache control unit  66  from the OS and/or various requesting agents. 
     Configuration register  68  is representative of any number of configuration registers which may be utilized as part of system cache  60 . For example, in one embodiment, there may be a separate configuration register  68  for each group identifier (ID) assigned by the OS to use system cache  60 . In this embodiment, each configuration register may define a status, quota, and replacement policy for a respective group ID. The status may be set to either active or inactive by a software command sent to system cache  60 . When the status is set to inactive, this may trigger the cache control unit  66  to invalidate all of the lines that are allocated for this particular group ID. The quota may be set to limit the amount of lines that may be allocated for the respective group ID in system cache  60 . 
     Data memory  64  may comprise a set of data entries, each having capacity to store a cache line of data. The cache line may be the unit of allocation and deallocation in data memory  64 . The cache line may be any desirable size, such as 32 bytes or 64 bytes, although larger and smaller cache line sizes may be supported in other embodiments. In another embodiment, the cache lines of data memory  64  may be referred to as “cache blocks”. 
     In various embodiments, data memory  64  may utilize any type of memory device. In one embodiment, data memory  64  may comprise a RAM, for example, indexed by entry number. Data memory  64  may be arranged so that a set of cache line storage locations may be selected for read/write operation responsive to an index portion of the input address (e.g., a number of bits of the address that may be decoded to uniquely select a set among the number of implemented sets). The cache line storage location that is to be accessed may be identified by the cache control unit  66  (e.g., responsive to detecting a cache hit for a request, responsive to allocating the cache line storage location to store a missing cache line). Data may be read from the accessed cache line storage location to return to the requestor for a read cache hit, or to transmit to the memory for a cache line evicted from system cache  60 . Data may be written to the accessed cache line storage location for a write cache hit from a requestor or to complete a cache fill of a missing cache line into an allocated cache line storage location. In some embodiments, data memory  64  may be a banked implementation and bank selection control may be provided from the cache control unit  66  as well. 
     Tag memory  62  may utilize any type of memory device, such as for instance, a RAM. Alternatively, tag memory  62  may comprise a content addressable memory (CAM) for snooping purposes, or a RAM/CAM combination. The tag memory  62  may comprise a plurality of tag entries, each entry selected by a different value of the index mentioned above. The selected tag entry may store the tags that correspond to the set of cache line storage locations in system cache  60  that are selected by the index. Each tag corresponds to a cache line in the respective cache line storage location, and may include the tag portion of the address of the corresponding cache line (i.e., the address, less the least significant bits that define an offset within the cache line and the bits that are used for the index), and various other state information. In response to a request, the tag memory  62  may be configured to decode the index and output the tags to the cache control unit  66  for processing. In an embodiment, the tag memory  62  may also include tag comparison circuitry configured to compare the tags to the tag portion of the request address, and may provide the comparison results to the cache control unit  66 . In another embodiment, the cache control unit  66  may compare the tags. The cache control unit  66  may also be configured to perform various tag updates by writing the tag entry. 
     System cache  60  may have any configuration. In some embodiments, a direct mapped or set associative configuration may be implemented. In typical direct mapped and set associative caches, there is a preconfigured, one-to-one correspondence between tag entries and data entries. In a direct mapped configuration, each address maps to one possible entry (tag memory  62  and data memory  64 ) in system cache  60 , at which the corresponding cache line would be stored. In one embodiment, system cache  60  may be associative, in which a given address maps to two or more cache line storage locations in the data memory  64  that may be eligible to store the cache line. System cache  60  may be set associative, in which each address maps to two or more possible entries (dependent on the associativity of the cache). In one embodiment, N cache line storage locations are mapped to addresses having the same value in a subset of the address bits referred to as an index, where N is an integer greater than one and less than the total number of cache line storage locations in data memory  64 . The N cache line storage locations forming a set corresponding to a given index are often referred to as “ways”. Other embodiments may be fully associative, in which any cache line storage location may be mapped to any address. 
     Cache control unit  66  may dynamically allocate a data entry in data memory  64  to store data for a transaction received by system cache  60 . The transaction may be a write to memory, for example. The transaction may also be a read completion (with data) provided from the memory (not shown) in response to a read previously received from a requesting agent and targeting the memory. 
     In one embodiment, each transaction received by system cache  60  from a requesting agent may include a group ID number, a cache allocation hint, and one or more other attributes. The cache allocation hint may be utilized by system cache  60  and cache control unit  66  to determine how to allocate a cache line for the transaction if the transaction misses in the system cache  60 . If a new cache line is allocated for the transaction, the group ID number may be stored in a corresponding entry in tag memory  62 . 
     Tag memory  62  may be configured to store various tags for the cache lines cached in the system cache  60 . For example, in one embodiment, the tags may include the coherence state, the sticky state, a dirty indicator, least recently used (LRU) data, a group identification (ID), and other data. Depending on the embodiment, some or all of these tags may be included in each entry of tag memory  62 . 
     Turning now to  FIG. 4 , a block diagram of one embodiment of a pair of tag memory ways is shown. Tag memory ways  70 A-B are representative of any number of ways that may be included within a tag memory, such as tag memory  62  (of  FIG. 3 ). In one embodiment, each tag memory way  70 A-B may include any number of entries for data corresponding to cache lines stored in a corresponding data memory way. A sample entry is shown in each of tag memory ways  70 A-B. 
     Each tag entry may include the tag portion of the address (tag address  72 A-B), to be compared against input request addresses. Tag address  72 A-B may include the most significant bits of the physical address field for a received transaction. The number of bits used for the tag address  72  field may vary depending on the embodiment. State  74 A-B may represent the state of the corresponding cache line stored in the data memory. There may be multiple different values which the state  74 A-B may take, depending on the embodiment. For example, in one embodiment, the different possible states may include the following: invalid, clean, dirty, data pending, sticky clean, sticky dirty, and LRU dirty. The requesting agent may also provide a hint as to the sticky status of the transaction. The data pending state may indicate that data for the cache line is currently being fetched from memory. Any entries with an invalid state may be chosen as the best candidates for replacement when a new line is allocated in the system cache. The next best candidates for replacement may be any entries with the LRU dirty state. It is noted that in another embodiment, each entry in tag memory ways  70 A-B may include a sticky flag or sticky bit, and this may indicate if the entry is sticky, rather than the state field. 
     The requesting agent responsible for generating the transaction may convey a hint with the transaction that determines the state that will be assigned to the corresponding tag entry. This hint may determine if the data associated with the transaction is stored in the system cache. For example, in one scenario, for a specific transaction, the hint accompanying the transaction may indicate that the transaction is sticky. If the transaction is accompanied by a sticky hint, and the transaction misses in the system cache, then the data may be retrieved from memory and allocated in the system cache with a tag state  74  set to sticky. Setting the state to sticky indicates that this data will “stick” in the cache and will not be removed by the system cache. If data for another sticky transaction from a different group ID were attempting to allocate space in the system cache, this data would be prevented from replacing sticky lines from other group IDs. 
     The LRU  76 A-B field may store a value indicating a usage status associated with the corresponding line. This LRU  76 A-B field may indicate how recently and/or how often the corresponding line has been accessed, and the number of bits in this field may vary depending on the embodiment. The group ID  78 A-B field may store a group ID identifying the group that owns the corresponding line in the data memory of the system cache. The group may refer to a specific dataflow that is being used by one or more requesting agents. It is noted that a “group ID” may also be referred to as a “dataset ID” in some embodiments. Depending on the embodiment, various numbers of bits may be utilized to represent the group ID. 
     In some cases, a single group ID may be shared by two or more requesting agents. For example, page translation tables may be utilized by multiple requesting agents, and any transactions referencing the page translation tables may be assigned a common group ID. This common group ID may span multiple requesting agents. Also, each requesting agent may use multiple separate group IDs for the different dataflows being utilized by the requesting agent. A group ID may be assigned to a dataflow for one or more requesting agents by the OS of the host electronic device. In one embodiment, a device driver may request a group ID from the OS. As part of the request, the device driver may identify which type of data the request corresponds to. Then, in response to receiving the request from the device driver, the OS may specify the group ID to be used for this request based on the type of data being accessed. 
     Each group represented by a group ID may be assigned a specific quota of cache lines in the system cache. When a group reaches the total amount of its quota, the group may not be able to allocate any more lines in the system cache. Instead, the specific group may replace its existing lines in the cache with the newly allocated lines. In one embodiment, the first lines that are replaced for a given group ID may be the lines which have an invalid state followed by the lines which have a LRU dirty state. 
     The parity  80 A-B field may include any number of parity bits to provide an indication of the accuracy of the data in the entire entry across all of the fields. It is noted that in other embodiments, each entry of tag memory ways  70 A-B may include one or more additional fields of information not shown in  FIG. 4 . For example, information about how recently the cache line was replaced may also be stored in each tag of tag memory ways  70 A-B. Also, in other embodiments, tag memory ways  70 A-B may be structured in any other suitable manner. 
     Referring now to  FIG. 5 , one embodiment of a requesting agent conveying a request to a system cache is shown. Requesting agent  90  is representative of any number and type of requesting agents. Although requesting agent  90  is shown as sending request  92  directly to memory controller  110 , it is noted that one or more components (e.g., coherent point, switch) may be located between requesting agent  90  and memory controller  110 . 
     Each request sent from requesting agent  90  may include a plurality of fields. For example, in one embodiment, request  92  may include command  94 , which indicates the type of request (e.g., read, write) being sent. Request  92  may also include transaction ID  96 , which indicates the transaction ID associated with request  92 . Transaction ID  96  may uniquely identify the request for requesting agent  90 . It is noted that transaction ID  96  may also be referred to as a “request ID”. In addition, in other embodiments, request  92  may also include an agent ID to identify the requesting agent. Request  92  may also include the address  98  and data  100  fields to identify the memory address and data (for a write request), respectively. 
     Request  92  may also include a dirty status indicator  102  to indicate if the write data is dirty. Request  92  may also include a group ID  104  to identify the group ID of request  92 . Cache hint  106  may determine how request  92  is treated by system cache  112 . In other embodiments, cache hint  106  may be referred to as an “allocation hint”, “sticky hint”, “sticky flag”, “sticky bit”, or “sticky attribute”. It is noted that cache hint  106  may indicate the sticky status of request  92  and may also include other information regarding how request  92  should be treated by system cache  112 . Other attributes  108  are representative of any number and type of additional attributes (e.g., coherency, QoS attribute, size of the request, requestor ID, speculative status) which may be part of request  92 . It is noted that in other embodiments, request  92  may be structured differently, with one or more additional fields not shown in  FIG. 5  and/or one or more of the fields shown omitted. 
     Although system cache  112  is shown as a single unit, it should be understood that in other embodiments, system cache  112  may be split up into two or more separate units. For example, in another embodiment, memory controller  110  may include two channels and system cache  112  may be split up into two separate physical system caches. In this embodiment, the two separate physical system caches may be managed as one logical system cache. 
     Turning now to  FIG. 6 , one embodiment of a set of power management configuration registers for a system cache is shown. These registers may be utilized by a cache control unit (not shown) for managing the enabling and disabling of power to individual ways of a multi-way system cache. 
     Target way count register  124  may also be utilized by the cache control unit for determining how to manage the power supplied to the individual ways. The value stored in register  124  may specify a target number of active ways. Register  124  may be controlled by the cache control unit, and the cache control unit may attempt to make the current number of active ways equal to the value stored in register  124 . Current way count register  126  may store the current number of active ways. Registers  124  and  126  may include any number of bits, depending on the embodiment. 
     The cache control unit may adjust the value in target way count register  124  based on a variety of detected conditions. For example, in one embodiment, the value stored in register  124  may be increased if the replacement and allocation failure count is greater than the hit count by more than a power-up threshold. The replacement and allocation failure count includes requests that cause cache line replacements because there are no more active ways, requests that fail to allocate because all of the currently active ways are sticky, and requests that fail to allocate because there is not an active way. 
     In various embodiments, allocation fails may be tracked or otherwise monitored on a periodic basis. For example, allocation fails within a given window of time or sampling period (e.g., 1 ms, 1 μs, or otherwise) may be monitored. In various embodiments, these sampling periods are programmable. In some embodiments, rolling averages may be determined based on multiple sampling periods. In other embodiments, cache accesses themselves during a given sampling period may be used as an indication or proxy for allocation fails where such information is not directly available. Other techniques may include monitoring bandwidth more generally as an indicator for whether cache ways should or should not be disabled. While cache allocation fails may provide a better indicator, other less accurate techniques (such as bandwidth or cache accesses) may be used when cache allocation fail information is not readily available. Numerous such embodiments are possible and are contemplated. 
     The target way count value stored in register  124  may be decreased if the replacement and allocation failure count is less than a first power-down threshold. The target way count value may also be decreased if the hit count is greater than the replacement and allocation failure count by more than a second power-down threshold. 
     Referring now to  FIG. 7 , a block diagram of one embodiment of independently controlled power switches coupled to a system cache data memory is shown. Data memory  132  includes ways  134 A,  134 B, and  134 N, which are representative of any number of ways of data memory  132 . For example, in one embodiment, data memory  132  may include 16 ways. In other embodiments, data memory  132  may include other numbers of ways. It is also noted that in one embodiment, data memory  132  may be a static random-access memory (SRAM). In other embodiments, data memory  132  may be other types of memory. 
     In one embodiment, cache control unit  130  may track a replacement and allocation failure count and a hit count for received requests. The replacement and allocation failure count includes any requests that replace existing cache lines or any requests that fail to allocate in the system cache. The hit count includes any requests that hit in the system cache. A moving average of the replacement and allocation failure count may be calculated over a programmable period of time. Similarly, a moving average of the hit count may be calculated over a programmable period of time. 
     In various embodiments, the count values may be compared to each other and to one or more thresholds. If the replacement and allocation failure count is greater than the hit count by a power-up threshold, then the target active way count may be increased. If the replacement and allocation failure count is less than a first power-down threshold, then the target active way count may be decreased. If the hit count is greater than the replacement and allocation failure count by more than a second power-down threshold, then the target active way count may decreased. Cache control unit  130  may detect a change to the target active way count, and then unit  130  may activate or inactive one or more ways of ways  134 A-N to make the count of currently active ways match the target active way count. In one embodiment, switches  136 A,  136 B, and  136 N may control whether power (V DD ) is provided to ways  134 A,  134 B, and  134 N, respectively, and each of these switches may be independently controlled by cache control unit  130 . 
     It is noted that a tag memory (not shown) may also include multiple ways, and each way of the tag memory may be powered independently by cache control unit  130 . Each way of the tag memory may store tag entries that correspond to the cache lines stored in a way of the data memory. Therefore, when a specific way of the data memory is powered down, the corresponding way of the tag memory may also be powered down by cache control unit  130 . 
     Cache control unit  130  also includes timer  138 , which may be utilized to apply a low-pass filter to prevent the number of currently active ways from oscillating between adjacent numbers. When an inactive way is powered up, timer  138  may be started and run for a programmable amount of time until expiring. While timer  138  is running, cache control unit  130  may prevent any active way of ways  134 A-N from being powered down. Unit  130  may permit another inactive way to be powered up while timer  138  is running since timer  138  was started due to an inactive way being powered up. 
     In one embodiment, there may be a status bit (not shown) associated with timer  138  that indicates whether timer  138  is running due to an inactive way being powered up or an active way being powered down. For example, the status bit may be set to zero to indicate there was a power-up operation and one to indicate there was a power-down operation. In a similar manner, when an active way is powered down, timer  138  may be started and run for a programmable amount of time until expiring. While timer  138  is running, cache control unit  130  may prevent any inactive way of ways  134 A-N from being powered up. However, unit  130  may permit another active way to be powered down while timer  138  is running since timer  138  was started due to an active way being powered down. While only one timer  138  is shown in  FIG. 7 , it is noted that other embodiments may include more than one timer. For example, in another embodiment, there may be a first timer that is started after an inactive way is powered up and a second timer that is started after an active way is powered down. 
     Cache control unit  130  also includes hash function  140  for spacing out the way being powered up to reduce the probability of supply noise affecting an active way. Powering up way by way dynamically can introduce noise on the power supply. Therefore, in order to minimize the effect of noise on the operation of the system cache, hash function  140  may be used to space out the way being powered up to reduce the possibility of supply noise on an active way. In one embodiment, hash function  140  may hash the way ID of ways  134 A-N so as to randomize the selection of ways. By using hash function  140 , cache control unit  130  may ensure that selecting an individual way from ways  134 A-N for activation is performed in a random manner to minimize the supply noise. 
     Referring now to  FIG. 8 , one embodiment of a method  160  for using coarse-grained power management techniques in a system cache is shown. For purposes of discussion, the steps in this embodiment are shown in sequential order. It should be noted that in various embodiments of the method described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
     In one embodiment, power may be supplied to each way of a system cache independently of the other ways (block  162 ). In one embodiment, the system cache may have a multi-way set associative configuration. Also, the system cache may include a cache control unit with logic for managing power at a coarse-grained level. Next, the target active way count may be initialized (block  164 ). Also, one or more programmable threshold values associated with the system cache may be initialized (block  166 ). In one embodiment, the threshold values may include a power-up threshold and first and second power-down thresholds. In other embodiments, other numbers and types of threshold values may be utilized. It is noted that in other embodiments, block  166  may be performed prior to or simultaneously with block  164 . Also, it is noted that the threshold values may be changed at any time via software while the various steps of method  160  are being performed. 
     Next, the system cache may attempt to keep the number of ways of the system cache that are active equal to the target active way count (block  168 ). In one embodiment, this may entail adjusting the number of active ways to match the value of a target active way count value. In one embodiment, a current way count register may be maintained by the cache control unit, and the current way count register may be read to determine how many ways are currently active. In this embodiment, the cache control unit may compare the value of the current way count register to the value of a target active way count register. If the number of active ways does not equal the target active way count value, then one or more active ways may be powered down or one or more inactive ways may be powered up to make the values match. 
     Next, the cache control unit may monitor the performance of the system cache (block  170 ). While monitoring the performance of the system cache, the cache control unit may calculate one or more metrics based on the performance. In one embodiment, these metrics may include a replacement and allocation failure count and a hit count. The replacement and allocation failure count tracks the number of replacement and allocation failures for received requests over a given length of time. The given length of time may be programmable and may vary depending on the embodiment. In other words, a running average of the replacement and allocation failure count may be maintained. The hit count tracks the number of hits to the system cache over a given length of time. In other embodiments, other metrics relevant to the operation of the system cache may be tracked by the cache control unit. 
     Next, multiple comparisons may be made between the metrics and the thresholds (block  172 ). The number and type of comparisons that are made may be dependent on the number of metrics and thresholds and may vary from embodiment to embodiment. In one embodiment, one of the comparisons may include determining if the replacement and allocation failure count (RAFC) is greater than the hit count (HC) by more than a power-up threshold (conditional block  174 ). If the replacement and allocation failure count is greater than the hit count by more than a power-up threshold (conditional block  174 , “yes” leg), then the target active way count may be increased (block  176 ). After block  176 , method  160  may return to block  168  and adjust the number of active ways to match the target active way count, if these two values are not equal. Alternatively, in another embodiment, method  160  may go to conditional block  178  after block  176  to continue checking the results of the other comparisons. If the replacement and allocation failure count is not greater than the hit count by more than a power-up threshold (conditional block  174 , “no” leg), then the cache control unit may determine if the replacement and allocation failure count is greater than a first power-down threshold (conditional block  178 ). 
     If the replacement and allocation failure count is greater than a first power-down threshold (conditional block  178 , “yes” leg), then the target active way count may be decreased (block  180 ). After block  180 , method  160  may return to block  168  and adjust the number of active ways to match the target active way count, if these two values are not equal. If the replacement and allocation failure count is not greater than a first power-down threshold (conditional block  178 , “no” leg), then the cache control unit may determine if the hit count is greater than the replacement and allocation failure count by more than a second power-down threshold (conditional block  182 ). 
     If the hit count is greater than the replacement and allocation failure count by more than a second power-down threshold (conditional block  182 , “yes” leg), then the target active way count may be decreased (block  180 ). If the replacement and allocation failure count is not greater than a first power-down threshold (conditional block  178 , “no” leg), then method  160  may return to block  170  and calculate one or more metrics based on the performance of the system cache. 
     It is noted that although conditional blocks  174 ,  178 , and  182  are shown as being steps within method  160 , one or two of these steps may be excluded from other methods. For example, in another embodiment, only conditional blocks  174  and  178  may be utilized for determining whether to increase or decrease the target number of active ways, respectively. Alternatively, in a further embodiment, one or more of conditional blocks  174 ,  178 , and  182  may be used with one or more other determining conditions not shown in  FIG. 8 . Other variations of numbers and types of determining conditions for increasing or decreasing the target number of active ways are possible and are contemplated. 
     Referring now to  FIG. 9 , one embodiment of a method  190  for dynamically sizing a multi-way set associative system cache is shown. For purposes of discussion, the steps in this embodiment are shown in sequential order. It should be noted that in various embodiments of the method described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
     In one embodiment, a cache control unit of a system cache may monitor the target active way count register (block  192 ). The target active way count register may store the target active way count value, which specifies how many ways of the multi-way system cache the cache control unit should keep active. If the cache control unit detects a change to the target active way count (TAWC) register (conditional block  194 , “yes” leg), then the cache control unit may determine if the target active way count has increased or decreased (conditional block  196 ). If the cache control unit does not detect a change to the target active way count register (conditional block  194 , “yes” leg), then method  190  may return to block  192  to continue monitoring the target active way count register. 
     If the target active way count has increased (conditional block  196 , “yes” leg), then the cache control unit may select an inactive way to power up (block  198 ). If the target active way count has decreased (conditional block  196 , “no” leg), then the cache control unit may select an active way to power down (block  200 ). In one embodiment, there may be a cache access counter for each way, and the cache control unit may pick the least accessed way to power down, with the least accessed way determined by the lowest cache access counter value. Each cache access counter may count requests that hit or are allocated in the corresponding way. In one embodiment, requests that replace existing cache lines in the system cache may not be counted by the cache access counters. 
     The cache control unit may include a timer that is started after a way of the system cache is activated or deactivated. After block  198 , if the timer is running and the timer was started by an active way being powered down (conditional block  202 , “yes” leg), then method  190  may return to block  198  to wait until the timer expires. If the timer is not running or if the timer is running and was started by an inactive way being powered up (conditional block  202 , “no” leg), then the cache control unit may supply power to the way selected in block  198  (block  204 ). Next, the cache control unit may initialize and start the timer (block  210 ). The timer may run for a predetermined amount of time until it expires. While the timer is running, the cache control unit may prevent any active ways from being powered down. Also, in one embodiment, when a way is powered-up, the corresponding cache access counter may be reset. After block  210 , method  190  may return to block  192  to monitor the target active way count register. 
     After block  200 , if the timer is running and the timer was started by an inactive way being powered up (conditional block  206 , “yes” leg), then method  190  may return to block  206  to wait until the timer expires. If the timer is not running or if the timer is running and was started by an active way being powered down (conditional block  206 , “no” leg), then the cache control unit may turn off power to the way selected in block  200  (block  208 ). Next, the cache control unit may initialize and start the timer (block  210 ). After block  210 , method  190  may return to block  192  to monitor the target active way count register. 
     Referring next to  FIG. 10 , a block diagram of one embodiment of a system  220  is shown. As shown, system  220  may represent chip, circuitry, components, etc., of a desktop computer  230 , laptop computer  240 , tablet computer  250 , cell phone  260 , television  270  (or set top box configured to be coupled to a television), or otherwise. Other devices are possible and are contemplated. In the illustrated embodiment, the system  220  includes at least one instance of IC  30  (of  FIG. 2 ) coupled to an external memory  222 . 
     IC  30  is coupled to one or more peripherals  224  and the external memory  222 . A power supply  226  is also provided which supplies the supply voltages to IC  30  as well as one or more supply voltages to the memory  222  and/or the peripherals  224 . In various embodiments, power supply  226  may represent a battery (e.g., a rechargeable battery in a smart phone, laptop or tablet computer). In some embodiments, more than one instance of IC  30  may be included (and more than one external memory  222  may be included as well). 
     The memory  222  may be any type of memory, such as dynamic random access memory (DRAM), synchronous DRAM (SDRAM), double data rate (DDR, DDR2, DDR3, etc.) SDRAM (including mobile versions of the SDRAMs such as mDDR3, etc., and/or low power versions of the SDRAMs such as LPDDR2, etc.), RAMBUS DRAM (RDRAM), static RAM (SRAM), etc. One or more memory devices may be coupled onto a circuit board to form memory modules such as single inline memory modules (SIMMs), dual inline memory modules (DIMMs), etc. Alternatively, the devices may be mounted with IC  30  in a chip-on-chip configuration, a package-on-package configuration, or a multi-chip module configuration. 
     The peripherals  224  may include any desired circuitry, depending on the type of system  220 . For example, in one embodiment, peripherals  224  may include devices for various types of wireless communication, such as wifi, Bluetooth, cellular, global positioning system, etc. The peripherals  224  may also include additional storage, including RAM storage, solid state storage, or disk storage. The peripherals  224  may include user interface devices such as a display screen, including touch display screens or multitouch display screens, keyboard or other input devices, microphones, speakers, etc. 
     It should be emphasized that the above-described embodiments are only non-limiting examples of implementations. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.