Parallel Random Access Machine

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A Parallel Random Access Machine (PRAM) is an abstract machine for designing the algorithms applicable to parallel computers. It eliminates the focus on miscellaneous issues such as synchronization and communication, but lets designer think explicitly about the exploitation of concurrency. In terms of Flynn's taxonomy, PRAMs are multiple instruction multiple data (MIMD) computers.

The operation of a synchronous PRAM can result in simultaneous access by multiple processors to the same location in shared memory. There are several variants of our PRAM model, depending on whether such simultaneous access is permitted or prohibited. As accesses can be reads or writes, there are four possibilities:

1. Exclusive Read Exclusive Write (EREW) - every memory cell can be read or written to by only one processor at a time
2. Concurrent Read Exclusive Write (CREW) - multiple processors can read a memory cell but only one can write at a time
3. Exclusive Read Concurrent Write (ERCW) - never considered
4. Concurrent Read Concurrent Write (CRCW) - multiple processors can read and write

Common CRCW - if processors writing the same value, OK; otherwise, it is an illegal operation

Arbitrary CRCW - one attempt is successful (we don't know which in advance), others retire

Priority CRCW - rank indicates who gets to write

Several simplifying assumptions are made while considering the development of algorithms for PRAM. They are :

1. There is no limit on the number of processors in the machine.
2. Any memory location is uniformly accessible from any processor.
3. There is no limit on the amount of shared memory in the system.
4. Resource contention is absent.
5. The programs written on these machines are, in general, of type MIMD. Certain special cases such as SIMD may also be handled in such a framework.

Algorithms are written in pseudo-code as there is no actual implementation of a PRAM, (except University of Maryland A. James Clark School of Engineering's ParaLeap prototype 64-core supercomputer). However, these kinds of algorithms are useful for understanding the exploitation of concurrency, dividing the original problem into similar sub-problems and solving them in parallel.

[edit] See also

• Flynn's taxonomy
• Lock-free and wait-free algorithms
• Random access machine

[edit] External links

• University Of Maryland's prototype PRAM

[edit] References

Keller, Jörg; Christoph Keßler, Jesper Träff (2001). Practical PRAM Programming. John Wiley and Sons. 0471353515. 

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