Cache only memory architecture
Encyclopedia
Cache only memory architecture (COMA) is a computer memory
organization for use in multiprocessor
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
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 WildFire.
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
organization for use in multiprocessor
Multiprocessor
Computer system having two or more processing units each sharing main memory and peripherals, in order to simultaneously process programs.Sometimes the term Multiprocessor is confused with the term Multiprocessing....
s in which the local memories (typically DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...
) at each node are used as cache. This is in contrast to using the local memories as actual main memory, as in NUMA
Non-Uniform Memory Access
Non-Uniform Memory Access is a computer memory design used in Multiprocessing, where the memory access time depends on the memory location relative to a processor...
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
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....
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 WildFire.