Shared memory

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[edit] In Hardware

Diagram of a typical Shared memory system. Three processors are connected to the same memory module through a bus or crossbar switch

In computer hardware, shared memory refers to a (typically) large block of random access memory that can be accessed by several different central processing units (CPUs) in a multiple-processor computer system.

A shared memory system is relatively easy to program since all processors share a single view of data and the communication between processors can be as fast as memory accesses to a same location.

The issue with shared memory systems is that many CPUs need fast access to memory and will likely cache memory,which has two complications:

CPU-to-memory connection becomes bottleneck. Shared memory computers can not scale very well. Most of them have only ten processors.
Cache coherence: Whenever one cache is updated with information that may be used by other processors, the change needs to be reflected to the other processors, otherwise the different processors will be working with incoherent data (see cache coherence and memory coherence). Such coherence protocols can, when they work well, provide extremely high performance access to shared information between multiple processors. On the other hand they can sometimes become overloaded and become a bottleneck to performance.

The alternatives to shared memory are distributed memory and distributed shared memory, with another, similar set of issues. See also Non-Uniform Memory Access.

[edit] See also

Shared Memory Architecture, where the graphics chip uses main system RAM

[edit] External links

Shared Memory Allocation
Shared Memory Interface
Shared Memory Library FAQ by Márcio Serolli Pinho
Article "IPC:Shared Memory" by Dave Marshall
shared memory facility from the Single UNIX® Specification
shm_open - POSIX
shmop - documentation from SunOS 5.9
CreateSharedMemory function from Win32-SDK
Functions in PHP-API
Paper "A C++ Pooled, Shared Memory Allocator For The Standard Template Library" by Marc Ronell
Citations from CiteSeer
Linux Shared Memory Allocation with example

Topics in Parallel Computing

v • d • e

General	High-performance computing
Parallelism	Data parallelism • Task parallelism
Theory	Speedup • Amdahl's law • Flynn's Taxonomy • Cost efficiency • Gustafson's Law • Karp-Flatt Metric
Elements	Process • Thread • Fiber • Parallel Random Access Machine
Coordination	Multiprocessing • Multitasking • Memory coherency • Cache coherency • Barrier • Synchronization • Distributed computing • Grid computing
Programming	Programming model • Implicit parallelism • Explicit parallelism
Hardware	Computer cluster • Beowulf • Symmetric multiprocessing • Non-Uniform Memory Access • Cache only memory architecture • Asymmetric multiprocessing • Simultaneous multithreading • Shared memory • Distributed memory • Massively parallel processing • Superscalar processing • Vector processing • Supercomputer
Software	Distributed shared memory • Application checkpointing
APIs	Pthreads • OpenMP • Message Passing Interface (MPI)
Problems	Embarrassingly parallel • Grand Challenge