Abstract:
A flowchart and circuit for tracking memory page accesses in a multi-processor computing system to optimize memory bandwidth utilization.

Description:
RELATED APPLICATION  
       [0001]    This application is related to application Ser. No. ______ entitled “A Method and Apparatus for determining a dynamic random access memory page management implementation””, with inventors E. Sprangle and A. Rohillah, filed Dec. 24, 2002 and assigned to the assignee of the present application. 
     
    
     
       BACKGROUND  
         [0002]    1. Field  
           [0003]    The present disclosure pertains to the field of microprocessor systems and more specifically to microprocessor systems capable of operating with memory controller over a system bus.  
           [0004]    2. Description of Related Art  
           [0005]    Dynamic Random Access Memories (DRAMs) may have memory precharge, activate, read, and write operations. In particular, a memory controller that addresses a bank of memory must first precharge the memory bank, then the addressed page within the bank must be activated before the addressed column in that page is accessed (read or written). Accesses to an open DRAM page (a “page hit”) indicates the memory being accessed has already been precharged and activated. Data may be read to or written from the DRAM page without having to precharge or activate the memory during each memory access. When a “page miss” occurs (i.e., data is accessed from a page in memory other than from the page that is open), the currently-open page must be closed (i.e., written back to the DRAM chip from the sense amps)before the new memory page can be precharged and activated to enable accessing. Writing the old page to DRAM and precharging and activating the new DRAM pages takes time and memory command bus bandwidth, which in turn increases the access latency of the memory access, resulting in an inefficient use of the memory bus (reduced bandwidth utilization) and a loss in performance of an apparatus (e.g., a computer) employing DRAM.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]    The present invention is illustrated by way of example and not limitation in the Figures of the accompanying drawings.  
         [0007]    [0007]FIG. 1 illustrates an apparatus utilized in an embodiment.  
         [0008]    [0008]FIG. 2 illustrates an apparatus utilized in an embodiment.  
         [0009]    [0009]FIG. 3 illustrates a method utilized in an embodiment.  
         [0010]    [0010]FIG. 4 illustrates a system utilized in an embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0011]    The following description provides methods for improving CPU performance in a multi-CPU system by optimizing accesses to memory. In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art that the invention may be practiced without such specific details. Those of ordinary skill in the art, with the included descriptions, will be able to implement appropriate logic circuits without undue experimentation.  
         [0012]    Various embodiments disclosed may allow CPUs to track the state of a memory such that said memory, such as a DRAM, can be efficiently utilized in a multi-CPU system. For example, the claimed subject matter facilitates the ability for decoding incoming snoop addresses from other CPUs, comparing them to contents of a DRAM tracking register(s), and updating said tracking register(s) appropriately. Likewise, the transactions from other non-CPU bus-agents and/or bus mastering devices, such as a bus bridge, memory controller, Input/Output (I/O), and graphics could also be tracked without requiring additional pins or protocols. As previously described, the DRAM tracking logic has been disclosed in the related application from E. Sprangle and A. Rohillah.  
         [0013]    Thus, the CPU can track the status of the DRAM accesses and the particular pages that are open. Therefore, the claimed subject matter improves CPU performance by optimizing accesses to DRAM memory because the CPU will reorder transactions to efficiently utilize DRAM pages that are currently open.  
         [0014]    The term “DRAM” is used loosely in this disclosure as many modern variants of the traditional DRAM memory are now available. The techniques disclosed and hence the scope of this disclosure and claims are not strictly limited to any specific type of memory, although single transistor, dynamic capacitive memory cells may be used in some embodiments to provide a high density memory array. Various memories arrays which allow piece-wise specification of the ultimate address may benefit from certain disclosed embodiments, regardless of the exact composition of the memory cells, the sense amplifiers, any output latches, and the particular output multiplexers used.  
         [0015]    [0015]FIG. 1 illustrates an apparatus utilized in an embodiment. The apparatus depicts a high-level schematic of an N number of CPUs to be coupled as to allow for tracking of the page status of DRAM memory(ies). In one embodiment, the apparatus is a system with multiple processors  102  and  104  that are coupled to DRAM memory  108  via a memory controller hub (MCH)  106 . In the same embodiment, each processor  102  and  104  has a plurality of DRAM tracking registers.  
         [0016]    However, the claimed subject matter is not limited to two processors. Rather, FIG. 1 just illustrates one example of a multiprocessor shared-bus configuration. Alternatively, one could utilize three or four microprocessors in a similar shared-bus configuration. In addition, the claimed subject matter is not limited to processors in a shared-bus configuration. One could utilize multiple processors in an independent-bus configuration, wherein a plurality of tracking registers would be incorporated into a snoop filter logic. These tracking registers would generate a dummy snoop to each processor to force the processors implementing this invention to update their tracking registers. In alternative embodiments, the processors may be utilized in other configurations, such as a switching fabric or a point-to-point.  
         [0017]    Each processor, designated as  102  and  104 , comprises one or more DRAM tracking registers to track which DRAM memory pages are open in each bank and rank. For example, the CPU decodes an address for outgoing transactions from the out-going transaction queue and the DRAM tracking logic facilitates the loading of the particular page address into an appropriate tracking register. Thus, the contents of the tracking registers indicate which pages are open in the particular DRAM bank. Furthermore, the CPU then may reorder transactions to increase the possibility of a page-hit by accessing pages that are tracked as open.  
         [0018]    [0018]FIG. 2 illustrates an apparatus in accordance with one embodiment. In one embodiment, the apparatus depicts a schematic of a logic utilized in conjunction with the DRAM tracking logic depicted in connection with FIG. 1.  
         [0019]    In one embodiment, the apparatus is incorporated within each CPU as discussed in connection with FIG. 1. From the perspective of one CPU, the apparatus receives at least one snoop address  202  from other CPUs and agents to allow for decoding the snoop address and comparing to a plurality of DRAM tracking registers  204 . The CPU will use the result of the tracking register comparison to update the contents of the tracking register. Based on the status of the open pages in the DRAM, a CPU may reorder the transactions to take advantage of the open pages and increase the likelihood of a page hit. In contrast, since a page-miss degrades performance as the CPU waits for the page-miss condition to be resolved a CPU may reorder the transactions to deprioritize page-miss transactions to decrease the likelihood of a page miss.  
         [0020]    The bank decoder  208  decodes the snoop address  202  to determine the particular DRAM bank the CPU is accessing. The multiplexer  206  selects the particular tracking register  204  based at least in part on the decoded bank information from the bank decoder and forwards the contents of the particular tracking register to a comparator  210 .  
         [0021]    A plurality of page-address bits of the snoop addresses is compared against the contents of the particular tracking register by the comparator  210 . In the event of NO MATCH, this indicates the other CPU or bus-agent is accessing a different DRAM page, which will close the page that is currently open and open a new page defined by the snoop address. Also, the CPU with the snoop address will reload the contents of the tracking register with the snoop address. Therefore, the CPU utilized the snoop address information from the other CPU and agents in conjunction with its own outgoing transactions for comparing and updating its own DRAM tracking register(s)  204 .  
         [0022]    [0022]FIG. 3 illustrates a method in accordance with an embodiment. The method incorporates a first CPU receiving a snoop address from a second CPU or agent, as depicted by a block  302 . The first CPU to decode the snoop address to determine the appropriate DRAM bank, as depicted by a block  304 . The first CPU to select a register based at least in part on the bank information. Subsequently, the first CPU to compare the bank information to the content of the selected register, as depicted by a block  306  In the event of NO MATCH, the first CPU to update the contents of the selected register with the value of the snoop address, as depicted by a block  308 . The CPU will take no actions in the event of a MATCH, as depicted by a block  310 .  
         [0023]    [0023]FIG. 4 depicts an apparatus in accordance with one embodiment. The apparatus in one embodiment is a plurality of processors  402  that incorporates a memory controller  404  that is coupled to a memory  406 . For example, the processor incorporates a memory controller by allowing the processor to perform memory controller functions, thus, the processor performs memory controller functions  
         [0024]    In contrast, in another embodiment, the processors  402  are coupled to a memory controller  404  that is coupled to a memory  406  and the processor does not perform memory controller functions. In both previous embodiments, the apparatus comprises the previous embodiments depicted in FIGS. 1-3 of the specification. Also, in one embodiment, the apparatus is a system.  
         [0025]    Also, the memory may be a synchronous DRAM (SDRAM), a Rambus DRAM (RDRAM), a double data rate DRAM (DDR DRAM), a static random access memory (SRAM), or any memory that utilizes at least two accesses to generate a complete address for the memory. For example, in one embodiment, a SRAM may be utilized with several memory chips that are routed through a single multiplexer.  
         [0026]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements, shown and described, since various other modifications may occur to those ordinarily skilled in the art upon studying this disclosure.