Cache only memory architecture (COMA) is a
computer memory
Computer memory stores information, such as data and programs, for immediate use in the computer. The term ''memory'' is often synonymous with the terms ''RAM,'' ''main memory,'' or ''primary storage.'' Archaic synonyms for main memory include ...
organization for use in
multiprocessor
Multiprocessing (MP) is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. The ...
s in which the local memories (typically
DRAM
Dram, DRAM, or drams may refer to:
Technology and engineering
* Dram (unit), a unit of mass and volume, and an informal name for a small amount of liquor, especially whisky or whiskey
* Dynamic random-access memory, a type of electronic semicondu ...
) at each node are used as cache. This is in contrast to using the local memories as actual main memory, as in
NUMA organizations.
In NUMA, each address in the global address space is typically assigned a fixed home node. When processors access some data, a copy is made in their local cache, but space remains allocated in the home node. Instead, with COMA, there is no home. An access from a remote node may cause that data to migrate. Compared to NUMA, this reduces the number of redundant copies and may allow more efficient use of the memory resources. On the other hand, it raises problems of how to find a particular data (there is no longer a home node) and what to do if a local memory fills up (migrating some data into the local memory then needs to evict some other data, which doesn't have a home to go to). Hardware
memory coherence mechanisms are typically used to implement the migration.
A huge body of research has explored these issues. Various forms of directories, policies for maintaining free space in the local memories, migration policies, and policies for read-only copies have been developed. Hybrid NUMA-COMA organizations have also been proposed, such as Reactive NUMA, which allows pages to start in NUMA mode and switch to COMA mode if appropriate and is implemented in the Sun Microsystems's WildFire. A software-based Hybrid NUMA-COMA implementation was proposed and implemented by ScaleMP,
allowing for the creation of a shared-memory multiprocessor system out of a cluster of commodity nodes.
See also
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Non-uniform memory access
Non-uniform memory access (NUMA) is a computer storage, computer memory design used in multiprocessing, where the memory access time depends on the memory location relative to the processor. Under NUMA, a processor can access its own local memory ...
*
Uniform memory access
References
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Parallel computing
Computer memory
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