Cache only memory architecture (COMA) is a
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
organization for use in
multiprocessor
Multiprocessing 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. There ar ...
s in which the local memories (typically
DRAM) at each node are used as cache. This is in contrast to using the local memories as actual main memory, as in
NUMA
Nuclear mitotic apparatus protein 1 is a protein that in humans is encoded by the ''NUMA1'' gene.
Interactions
Nuclear mitotic apparatus protein 1 has been shown to interact with PIM1, Band 4.1, GPSM2
G-protein-signaling modulator 2, also call ...
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 Memory coherence is an issue that affects the design of computer systems in which two or more processors or cores share a common area of memory.
In a uniprocessor system (whereby, in today's terms, there exists only one core), there is only one p ...
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 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 faster than non ...
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Uniform memory access
References
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Parallel computing
Computer memory
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