Abstract:
A method for providing a memory scheme in computer architectures in an efficient and cost effective manner. A processor is configured with access to dual-L2 caches, preferably configured to cache program instructions and data in one cache and shared data in another cache. In one embodiment of the present invention, one L2 cache is accessible to networking interface devices. Optionally, the cache accessible by the networking interface devices is configured as networking buffers, providing cache for packet data being sent within a network. By use of this invention, the packet forwarding speeds in a conventional computer architecture may be increased.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to computer architectures and, more particularly, to a memory management scheme and architecture for allowing faster access to data and instructions.  
           [0003]    2. Description of the Related Art  
           [0004]    As computer and networking applications become Input/Output (I/O) intensive, the conventional computer architecture is becoming a limiting factor in obtaining higher processing speeds. Conventional computer architectures typically utilize a processor and memory architecture that creates a bottleneck in accessing data as memory accesses are slow and multiple devices and applications compete for memory access.  
           [0005]    Conventional computer architectures generally utilize a cache comprising smaller, high-speed memory in addition to larger, slower main memory, such as Dynamic Random Access Memory (DRAM), in an attempt to increase memory access speeds. The cache typically stores a subset of shared data stored in main memory, which is generally paged in and out of the cache.  
           [0006]    Cache is typically configured in multiple levels. Level 1 (L1) cache is usually designed into the processor chip and is commonly referred to as internal cache. Level 2 (L2) cache is the external memory closest to the processor chip. Additional levels are possible, but with each additional level, the access speed typically decreases.  
           [0007]    In a microprocessor-based design, when the processor performs a read instruction, the processor first attempts to read the requested data from the cache. If the requested data is found in the cache, the slower main memory is not accessed. If the requested data is not contained in the cache, however, older data is paged out of the cache and the requested data is paged into the cache. The paging of data out of cache frequently requires additional memory delays and memory accesses because the data is frequently shared by other applications and/or devices, and the data must be reread from the shared memory into the cache, resulting in slower memory access speeds.  
           [0008]    The above description is particularly troublesome in networking environments involving the movement of packet data within a networking system. The movement of shared data can be broken down into 3 basic steps: (a) exchange of buffer information or Direct Memory Access (DMA) descriptors, (b) transfer of packet data to and from main memory, and (c) process of packet header by the processor. Steps (a) and (b) involve networking interface devices accessing shared data. Steps (a) and (c) involve the processor accessing the same blocks of shared data. Therefore, shared data are generally accessed by both the networking interface devices and the processor.  
           [0009]    Conventional architectures utilizing an L2 cache, however, are designed to provide only the processor with access to the L2 cache. As a result, packet traffic cannot benefit from fast memory such as the L2 cache. Instead, packet buffers are transferred between the cache and the main memory store as they are accessed by the processor and the networking interface devices. In a conventional design, these actions will cause the shuffling of shared data back and forth between the L1/L2 caches and the main memory store.  
           [0010]    Prior attempts to fix the limitations of the conventional computer architecture discussed above are not optimal for I/O intensive network router applications. Attempts utilizing DRAM technology are inefficient due to the limited memory bandwidth of the DRAM. Other attempts, such as employing a distributed routing scheme and/or specialized hardware with Static Random Access Memory (SRAM), are generally expensive and involve a redesign of the system.  
           [0011]    Therefore, there is a need for a caching scheme to further extend the life of generic computing architecture in networking products to allow an efficient and cost-effective memory scheme.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention comprises a method for providing an efficient and cost-effective memory caching scheme. This is achieved by configuring a computer memory architecture to utilize dual-L2 caches, preferably configured as an L2 Program Cache and an L2 Packet Cache. By use of this invention, memory access speeds may be increased as paging shared data is decreased and networking interface devices are given access to fast caches.  
           [0013]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing wherein like reference numbers represent like parts of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 depicts an embodiment of a computer architecture employing the present invention;  
         [0016]    [0016]FIG. 2 depicts a memory map of one embodiment of the present invention;  
         [0017]    [0017]FIG. 3 depicts an alternative embodiment of a computer architecture employing the present invention; and  
         [0018]    [0018]FIG. 4 depicts an embodiment of the present invention in which the L2 Packet Data Cache is configured to comprise networking buffers. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring now to FIG. 1 of the drawings, reference numeral  100  illustrates a portion of a computer architecture that embodies features of the present invention. Other components, such as disk drives, device drivers, power supplies, and the like, necessary for the operation of the present invention are considered obvious to one skilled in the art, and are, therefore, not shown or discussed herein.  
         [0020]    The computer architecture employs dual-L2 caches  102  and  104 , each of which preferably comprises SRAM. Both of the L2 caches  102  and  104  are connected to a processor  106  via a processor bus  107  and a Shared Memory  108  via a memory bus  110 . The processor  106  may be a central processing unit (CPU) such as an Intel Pentium-class processor or a Reduced Instruction Set Computer processor, and the like. Preferably, at least one of the dual-L2  102  and  104  caches is accessible by an I/O Bridge  112  via a Cache Memory Bus  118 . The I/O Bridge  112  provides access to the L2 Cache  104  to one or more Networking Interface Devices  114  via an I/O Bus  116 .  
         [0021]    Preferably, the L2 cache  102  is configured as an L2 Program Cache  102 , and the L2 cache  104  is configured as an L2 Packet Cache  104 . The L2 Program Cache  102  stores program instructions and memory used exclusively by the processor  106 , such as application code, program data, routing tables, and the like. The L2 Packet Cache  104 , on the other hand, stores memory data that are shared by and transferred between the processor  106  and networking interface devices  114 , such as DMA descriptors, packet data, control messages, and the like.  
         [0022]    In operation, the L2 caches  102  and  104  store a subset of the data contained in the slower shared memory  108 . The processor  106  and the networking interface devices  114  first attempt to retrieve the required data from the L2 caches  102  and  104 . If the data is stored in either of the L2 caches  102  or  104 , shared memory  108  is not accessed. If the data is not stored in either of the L2 caches  102  or  104 , however, the data is retrieved from the shared memory  108 . Furthermore, by configuring the L2 caches as an L2 Program Cache  102  and an L2 Packet Cache  104 , the processor  106  and the networking interface devices  114  attempt to retrieve shared data from the L2 Packet Cache  104 , and the processor  106  retrieves exclusive data from the L2 Program Cache  102 , thereby increasing memory access speeds.  
         [0023]    The enhanced shared-memory architecture utilizing dual-L2 caches in accordance with the preset invention overcomes the speed limitation of systems utilizing a single L2 cache by providing a separate cache for shared data and limiting the possibility of access conflicts and delays caused by the processor accessing program data simultaneously as a networking interface device is attempting to access shared control data and descriptors and packet data. Additionally, in the case of a system without a dual-L2 cache, the processor frequently accesses the main memory, generally comprising DRAM, where factors like precharge, dead time between read and write cycles, and other timing constraints limit the maximum throughput of a DRAM by affecting sequential random accesses and collision wait times. For instance, a typical cache access cycle in a 66 MHz design is 2-1-1-1 at the cache interface while that of a DRAM is 5-1-1 on the memory bus. As a result, this architecture provides a cost-effective alternative to a single large cache of expensive, faster SRAM, which is commonly used to provide faster access than the main memory.  
         [0024]    [0024]FIG. 2 illustrates a preferred memory map of one embodiment of the present invention particularly useful in the embodiment illustrated in FIG. 1. The processor  106  generally contains L1 caches, which are generally configured as an L1 Instruction Cache  210  and an L1 Data Cache  212 . The processor  106  contains additional components, such as an Arithmetic Logic Unit (ALU), read only memory (ROM), a clock, I/O circuitry, and the like, not shown in FIG. 2, but are considered to be well-known to one skilled in the art, and, therefore, will not be described herein. The L1 Instruction Cache  210  generally provides cache to shared Program Code Space  220 . The L1 Data Cache  212  generally provides cache to exclusive Program Data Space and Stack  222 , shared Packet Control Data and Descriptors  224 , and shared Packet Data Buffer  226 . The I/O Space  228  is non-cacheable and, therefore, is not mapped to the L1 caches.  
         [0025]    As described above, the L2 caches  102  and  104  are preferably configured as an L2 Program Cache and an L2 Packet Cache, respectively. In a preferred embodiment, the L2 Program Cache  102  is mapped to the Shared Program Code Space  220  and the Exclusive Program Data Space and Stack  222 , and the L2 Packet Cache  104  is mapped to the Shared Packet Control Data and Descriptors  224  and the Shared Packet Data Buffer  226 . The I/O Space  228  remains non-cacheable.  
         [0026]    Caching the shared data as described above with respect to FIG. 2 has been found to allow an efficient use of the caches. First, the dual-L2 caches generally split the instruction cache and data caches similar to the caches internal to the processor  106 , thereby preventing required instruction data from being paged out for the transient shared data. Second, the L2 Packet Cache  232  provides the Networking Interface Devices  114  with direct access to an L2 cache via the I/O Bridge  112 , reducing processor overhead and increasing shared memory access speed.  
         [0027]    [0027]FIG. 3 illustrates an alternative embodiment of the present invention in which one of the dual-L2 caches is available on an interface chip. Accordingly, an interface chip  310 , such as the IBM Talos chip, provides a PCI-PCI Bridge  312  and an embedded L2 cache  314 . In this instance, it is preferable to configure the L2 cache  314  on the interface chip  310  as an L2 Packet Cache. In this manner, the processor  316 , such as an IBM PPC750, and PCI Masters  318  have access to the L2 Packet Cache  314 .  
         [0028]    This embodiment, however, generally prohibits the L2 Program Cache  320  from being configured to store only the Shared Program Code Space  220  and the Exclusive Program Data Space  222 , as illustrated in FIG. 2, because restrictions inherent in the processor  106  allow a user to only enable or disable the L2 cache, not map the memory. Nevertheless, this architecture provides the benefit of providing a Networking Interface Device, such as the PCI Masters, access to the L2 Packet Cache  314 .  
         [0029]    As a result, the L2 Program Cache  320  stores information required exclusively by the processor  316  as well as information that may also be required by the PCI Masters  318 . The processor  316  first attempts to retrieve information from the L2 Program Cache  320 , and, if the information is not available, attempts to retrieve the data from the L2 Packet Cache  314 . If the information is not available in the L2 Packet Cache  314 , the data is retrieved from the shared memory  108 . The PCI Masters  318 , on the other hand, first attempt to retrieve packet information from the L2 Packet Cache  314  and only retrieves packet information from the shared memory  108  if the information is not stored in the L2 Packet Cache  314 .  
         [0030]    [0030]FIG. 4 depicts an enhancement to the embodiment illustrated in FIG. 1, namely, that the L2 Packet Caches  104  is configured as networking buffers  412  to improve packet routing. Likewise, L2 Packet Cache  314  of FIG. 3 may be similarly configured. Preferably, the networking buffers  412  are of sufficient size to store a packet header, approximately 64 bytes for the typical Ethernet application.  
         [0031]    When a data packet is stored, the networking buffer  412  is used to cache the data. If the data size exceeds the size of the networking buffer  412 , the data overflows into the Shared Memory. In this embodiment, it is preferable to utilize fast SRAM for the L2 cache and less expensive Synchronous DRAM (SDRAM) for the Shared Memory.  
         [0032]    This embodiment allows the processor  106  and networking interface devices  114  to transfer short packets into the high-speed cache and allows designers to implement large buffers with less expensive SDRAM while maintaining the performance advantage of fast cache memory. Additionally, since the processor  106  generally only requires access to the packet header, the processor is able to retrieve the packet header from the fast L2 cache instead of the slower Shared Memory.  
         [0033]    Moreover, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. For example, an L2 Packet Cache remains advantageous when used with a shared memory comprising Double-Data-Rate (DDR) DRAM, which operates in burst mode with a fixed transfer size. Even though the DDR DRAM is faster than its predecessor DRAM, the DMA descriptors are generally manipulated at the word-size level, causing needless Read-Modify-Write burst cycles. Therefore, an L2 Packet Cache is advantageous to cache the DMA descriptors, avoiding the burst cycles. Additionally, an L2 Packet Cache continues to provide faster access to packet data by overcoming the first access latency inherent in DDR DRAM.  
         [0034]    It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.