Abstract:
A method and apparatus for scaling frequency and operating voltage of at least one clock domain of a microprocessor. More particularly, embodiments of the invention relate to techniques to divide a microprocessor into clock domains and control the frequency and operating voltage of each clock domain independently of the others.

Description:
[0001]    This application is a continuation of U.S. patent application Ser. No. 10/999,786, filed Nov. 29, 2004 entitled “A FREQUENCY AND VOLTAGE SCALING ARCHITECTURE,” the content of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention relate to the field of microprocessor architecture. More particularly, embodiments of the invention relate to a technique to scale frequency and operating voltage of various functional units within a microprocessor. 
       BACKGROUND 
       [0003]    In order to help reduce power in microprocessors while minimizing the impact to performance, prior art techniques for reducing processor clock frequency have been developed. Among these prior art techniques are architectures that divide the processor into various clock domains. For example, one prior art technique has a separate clock domain for the integer pipeline, a separate clock domain for the floating point pipeline, and a separate clock domain for memory access logic. 
         [0004]    Using separate clock domains for each pipeline and/or memory access cluster can pose challenges to maintaining the performance of the processor due to the amount of overhead circuitry needed to control each clock domain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Embodiments and the invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0006]      FIG. 1  illustrates a clock and voltage scaling architecture according to one embodiment of the invention. 
           [0007]      FIG. 2  illustrates a front-side bus computer system in which one embodiment of the invention may be used. 
           [0008]      FIG. 3  illustrates a point-to-point computer system in which one embodiment of the invention may be used. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Embodiments of the invention relate to a frequency and voltage control architecture for a microprocessor. More particularly, embodiments of the invention relate to techniques to distribute and control a clock and operating voltage among a number of clocking domains within the microprocessor, such that the frequency and operating voltage of each domain can be controlled independently. 
         [0010]      FIG. 1  illustrates a clock and voltage domain architecture according to one embodiment of the invention. In particular,  FIG. 1  illustrates a processor architecture  100  that has been divided into three fundamental clocking domains: A front end domain  101 , having a trace cache  102 , branch predictor  103 , renaming unit  104 , decoding unit  105 , sequencer  106 , free list  107 , renaming table  108 , and a re-order buffer (ROB)  109 ; several back end domains  110 , having a memory ordering buffer (MOB)  111 , a first-level cache  112 , physical register files  113 , issue queues  114 , bus interface  116  and execution units  115 ; and a memory domain including a second level cache memory  119 . In one embodiment, the bus interface is a front-side bus interface, while in other embodiments it is a point-to-point bus interface. 
         [0011]    The front-end domain, back-end domains, and the memory domain each have at least one first-in-first-out (FIFO) queue  117  used to help synchronize the exchange of information between the various clock domains. In one embodiment of the invention, at least some of the synchronization queues are queues that provide other functionality within the processor, whereas in other embodiments, the synchronization queues are dedicated to the clock domain control architecture. In addition to clock domains, one embodiment of the invention divides the processor into voltage domains, which can be regulated independently of each other. In at least one embodiment, the clock domains and the voltage domains are the same and include the same functional units, however, in other embodiments the clock domains and voltage domains are not the same and may include different functional units. 
         [0012]    In one embodiment of the invention, each clock within the various clock domains may be synchronized to a reference clock. However, in other embodiments, each domain clock may not be synchronous in relation to other domain clocks. Furthermore, in at least one embodiment, the back-end domains may communicate between each other via signals known as “crossbars”. 
         [0013]    In order to control each of the clock and voltage domains, one embodiment of the invention attempts to minimize a product of the energy and the square of the (“delay 2 ”) of each domain by determining the energy and performance of each domain at certain time intervals. Energy and performance may be determined at two time intervals, in at least one embodiment, by calculating the energy and delay of a domain during a first time interval and estimating the energy and delay of the domain in a subsequent time interval. A frequency and voltage pair for the subsequent time interval may then be chosen by minimizing the ratio between the energy-delay 2  product of the first time interval and that of the subsequent time interval. 
         [0014]    For example, in one embodiment of the invention, the processor energy, “E”, for interval n+1 is estimated according to the following equation: 
         [0000]    
       
         
           
             
               
                 E 
                 
                   n 
                   + 
                   1 
                 
               
               
                 E 
                 n 
               
             
             = 
             
               1 
               + 
               
                 
                   
                     E 
                     
                       FE 
                       , 
                       n 
                     
                   
                   
                     E 
                     n 
                   
                 
                 × 
                 
                   ( 
                   
                     
                       
                         V 
                         
                           n 
                           + 
                           1 
                         
                         2 
                       
                       
                         V 
                         n 
                         2 
                       
                     
                     - 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0015]    In the above equation, “E FE,n ” is the energy of the front-end domain at time interval “n”, where as “E n+1 ” is the energy of the front-end at time interval n+1 and “V n+1 ” is the operating voltage of the front-end domain at time interval n+1, and “V n ” is the operating voltage of the front-end domain at time interval n. 
         [0016]    Performance of the processor as a function of the frequency of the front-end domain can be estimated by using the clock frequency of the front-end domain for a given time interval, the rate at which instructions are fetched by the front-end, and the rate at which micro-operations (decoded instructions) are delivered to subsequent pipeline stages. In one embodiment, the performance estimation, “T n+1 ”, of an interval, n+1, is estimated according to the equation: 
         [0000]    
       
         
           
             
               
                 T 
                 
                   n 
                   + 
                   1 
                 
               
               
                 T 
                 n 
               
             
             = 
             
               1 
               + 
               
                 
                   ( 
                   
                     
                       
                         f 
                         n 
                       
                       
                         f 
                         
                           n 
                           + 
                           1 
                         
                       
                     
                     - 
                     1 
                   
                   ) 
                 
                 × 
                 
                   
                     1 
                     - 
                     
                       p 
                       n 
                     
                   
                   
                     1 
                     + 
                     b 
                   
                 
               
             
           
         
       
     
         [0017]    In the above equation, “p n ” is the average number of entries in the front-end queue for the n-th interval, and “b” is the branch misprediction rate. The value, “1+b”, is an indicator of the rate at which the fetch queue may be loaded and “1−pn” is an indicator of average number of entries in the queue. “T n ” is the performance of front-end at interval “n”, “fn” is the frequency of the front-end domain at interval n, and “f n+1 ” is the frequency of the front-end domain at the following time interval. 
         [0018]    Once the energy and performance of the processor has been calculated according to the above equations, in one embodiment, the front-end domain frequency and voltage can be adjusted for the next time interval, n+1, at the end of each time interval, n. In one embodiment, the selection of frequency and voltage is made according to the ratio: 
         [0000]    
       
         
           
             
               R 
                
               
                 ( 
                 
                   〈 
                   
                     f 
                     , 
                     V 
                   
                   〉 
                 
                 ) 
               
             
             = 
             
               
                 
                   E 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   E 
                   n 
                 
               
               × 
               
                 
                   T 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   T 
                   n 
                 
               
               × 
               
                 
                   T 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   T 
                   n 
                 
               
             
           
         
       
     
         [0019]    The frequency and voltage selected for the interval n+1 are those that minimize the above ratio. If two or more pairs are found that result in the same value, R, then the pair with the minimum frequency is chosen, in one embodiment. The frequency and operating voltage of the front-end domain may then be set to the appropriate values for the interval n+1 and the process repeated for the next interval. 
         [0020]    Each back-end frequency and operating voltage may be estimated in a similar manner to the front-end, by estimating the energy and performance of the processor as a function of the operating voltage and frequency of each back-end domain and choosing a frequency and operating voltage that minimizes the ratio between the energy performance product between interval n+1 and interval n. In one embodiment, the processor energy, “E n ”, as a function of the back-end domain energy, “E BE,n ” is estimated according to the equation: 
         [0000]    
       
         
           
             
               
                 E 
                 
                   n 
                   + 
                   1 
                 
               
               
                 E 
                 n 
               
             
             = 
             
               1 
               + 
               
                 
                   
                     E 
                     
                       BE 
                       , 
                       n 
                     
                   
                   
                     E 
                     n 
                   
                 
                 × 
                 
                   ( 
                   
                     
                       
                         V 
                         
                           n 
                           + 
                           1 
                         
                         2 
                       
                       
                         V 
                         n 
                         2 
                       
                     
                     - 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0021]    Performance of the processor as a function of the frequency of each back-end domain can be calculated at each interval, n+1, according to the equation: 
         [0000]    
       
         
           
             
               
                 
                   T 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   T 
                   n 
                 
               
               = 
               
                 1 
                 + 
                 
                   S 
                   × 
                   
                     
                       ( 
                       
                         1 
                         - 
                         
                           2 
                            
                           
                             m 
                             n 
                           
                         
                       
                       ) 
                     
                     2 
                   
                   × 
                   p 
                 
               
             
             , 
             
               
 
             
              
             where 
           
         
       
       
         
           
             p 
             = 
             
               
                 
                   - 
                   
                     L 
                     
                       q 
                       , 
                       n 
                     
                   
                 
                 + 
                 
                   
                     
                       L 
                       
                         q 
                         , 
                         n 
                       
                       2 
                     
                     + 
                     
                       4 
                        
                       
                         L 
                         
                           q 
                           , 
                           n 
                         
                       
                     
                   
                 
               
               2 
             
           
         
       
       
         
           and 
         
       
       
         
           
             S 
             = 
             
               
                 ( 
                 
                   
                     
                       f 
                       n 
                     
                     
                       f 
                       
                         n 
                         + 
                         1 
                       
                     
                   
                   - 
                   1 
                 
                 ) 
               
               × 
               
                 
                   
                      
                     
                       
                         f 
                         
                           n 
                           + 
                           1 
                         
                       
                       - 
                       
                         f 
                         n 
                       
                     
                      
                   
                   
                     
                       f 
                       max 
                     
                     - 
                     
                       f 
                       min 
                     
                   
                 
               
             
           
         
       
     
         [0022]    In the above equation, m n  is the number of second level cache misses divided by the number of committed micro-operations for the interval, n, and L q,n  is the average utilization of all micro-operation issue queues for all back-end domains containing execution units. Once the energy and performance of the processor has been calculated according to the above equations, in one embodiment, the back-end domain frequency and voltage can be adjusted for the next time interval, n+1, at the and of each time interval, n. In one embodiment, the selection of frequency and voltage is made according to the ratio: 
         [0000]    
       
         
           
             
               R 
                
               
                 ( 
                 
                   
                     f 
                     
                       n 
                       + 
                       1 
                     
                   
                   , 
                   
                     V 
                     
                       n 
                       + 
                       1 
                     
                   
                 
                 ) 
               
             
             = 
             
               
                 
                   E 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   E 
                   n 
                 
               
               × 
               
                 
                   T 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   T 
                   n 
                 
               
               × 
               
                 
                   T 
                   
                     n 
                     + 
                     1 
                   
                 
                 
                   T 
                   n 
                 
               
             
           
         
       
     
         [0023]    The frequency and voltage selected for the interval n+1 are those that minimize the above ratio. If two or more pairs are found that result in the same value, R, then the pair with the minimum frequency is chosen, in one embodiment. The frequency and operating voltage of the back-end domain may then be set to the appropriate values for the interval n+1 and the process repeated for the next interval. 
         [0024]      FIG. 2  illustrates a front-side-bus (FSB) computer system in which one embodiment of the invention may be used. A processor  205  accesses data from a level one (L1) cache memory  210  and main memory  215 . In other embodiments of the invention, the cache memory may be a level two (L2) cache or other memory within a computer system memory hierarchy. Furthermore, in some embodiments, the computer system of  FIG. 2  may contain both a L1 cache and an L2 cache, which comprise an inclusive cache hierarchy in which coherency data is shared between the L1 and L2 caches. 
         [0025]    Illustrated within the processor of  FIG. 2  is one embodiment of the invention  206 . Other embodiments of the invention, however, may be implemented within other devices within the system, such as a separate bus agent, or distributed throughout the system in hardware, software, or some combination thereof. 
         [0026]    The main memory may be implemented in various memory sources, such as dynamic random-access memory (DRAM), a hard disk drive (HDD)  220 , or a memory source located remotely from the computer system via network interface  230  containing various storage devices and technologies. The cache memory may be located either within the processor or in close proximity to the processor, such as on the processor&#39;s local bus  207 . Furthermore, the cache memory may contain relatively fast memory cells, such as a six-transistor (6T) cell, or other memory cell of approximately equal or faster access speed. 
         [0027]    The computer system of  FIG. 2  may be a point-to-point (PtP) network of bus agents, such as microprocessors, that communicate via bus signals dedicated to each agent on the PtP network. Within, or at least associated with, each bus agent is at least one embodiment of invention  206 , such that store operations can be facilitated in an expeditious manner between the bus agents. 
         [0028]      FIG. 3  illustrates a computer system that is arranged in a point-to-point (PtP) configuration. In particular,  FIG. 3  shows a system where processors, memory, and input/output devices are interconnected by a number of point-to-point interfaces. 
         [0029]    The system of  FIG. 3  may also include several processors, of which only two, processors  370 ,  380  are shown for clarity. Processors  370 ,  380  may each include a local memory controller hub (MCH)  372 ,  382  to connect with memory  22 ,  24 . Processors  370 ,  380  may exchange data via a point-to-point (PtP) interface  350  using PtP interface circuits  378 ,  388 . Processors  370 ,  380  may each exchange data with a chipset  390  via individual PtP interfaces  352 ,  354  using point to point interface circuits  376 ,  394 ,  386 ,  398 . Chipset  390  may also exchange data with a high-performance graphics circuit  338  via a high-performance graphics interface  339 . 
         [0030]    At least one embodiment of the invention may be located within the PtP interface circuits within each of the PtP bus agents of  FIG. 3 . Other embodiments of the invention, however, may exist in other circuits, logic units, or devices within the system of  FIG. 3 . Furthermore, other embodiments of the invention may be distributed throughout several circuits, logic units, or devices illustrated in  FIG. 3 . 
         [0031]    While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.