Cache-only memory architectures

F. Dahlgren; J. Torrellas

doi:10.1109/2.769448

 Synonyms  COMA  Definition  A Cache-Only Memory Architecture (COMA) is a  type of cache-coherent nonuniform memory access  (CC-NUMA) architecture. Unlike in a conventional  CC-NUMA architecture, in a COMA, every shared- memory module in the machine is a cache, where each  memory line has a tag with the line's address and state.  As a processor references a line, it transparently brings  it to both its private cache(s) and its nearby portion of  the NUMA shared memory (Local

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... mory) -possibly  displacing a valid line from its local memory. Effectively,  each shared-memory module acts as a huge cache mem- ory, giving the name COMA to the architecture. Since  the COMA hardware automatically replicates the data  and migrates it to the memory module of the node that  is currently accessing it, COMA increases the chances of  data being available locally. This reduces the possibility  of frequent long-latency memory accesses. Effectively,  COMA dynamically adapts the shared data layout to the  application's reference patterns.  Discussion  Basic Concepts  In a conventional CC-NUMA architecture, each node  contains one or more processors with private caches and  a memory module that is part of the NUMA shared  memory. A page allocated in the memory module of  one node can be accessed by the processors of all other  nodes. The physical page number of the page speci- fies the node where the page is allocated. Such node is  referred to as the Home Node of the page. The physi- cal address of a memory line includes the physical page  number and the offset within that page.  In large machines, fetching a line from a remote  memory module can take several times longer than  fetching it from the local memory module. Conse- quently, for an application to attain high performance,  the local memory module must satisfy a large fraction  of the cache misses. This requires a good placement of  the program pages across the different nodes. If the pro- gram's memory access patterns are too complicated for  the software to understand, individual data structures  may not end up being placed in the memory module of  the node that access them the most. In addition, when a  page contains data structures that are read and written  by different processors, it is hard to attain a good page  placement.  In a COMA, the hardware can transparently elimi- nate a certain class of remote memory accesses. COMA  does this by turning memory modules into large caches  called Attraction Memory (AM). When a processor  requests a line from a remote memory, the line is  inserted in both the processor's cache and the node's  AM. A line can be evicted from an AM if another line  needs the space. Ideally, with this support, the proces- sor dynamically attracts its working set into its local  memory module. The lines the processor is not access- ing overflow and are sent to other memories. Because a  large AM is more capable of containing a node's current  working set than a cache is, more of the cache misses are  satisfied locally within the node.  There are three issues that need to be addressed in  COMA, namely finding a line, replacing a line, and deal- ing with the memory overhead. In the rest of this article,  these issues are described first, then different COMA  designs are outlined, and finally further readings are  suggested.  David Padua (ed.), Encyclopedia of Parallel Computing, DOI ./----,

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Cache-only memory architectures

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