Read/write Conflicts
Read/write conflicts in accessing the same shared memory location simultaneously are resolved by one of the following strategies:
- Exclusive read exclusive write (EREW)—every memory cell can be read or written to by only one processor at a time
- Concurrent read exclusive write (CREW)—multiple processors can read a memory cell but only one can write at a time
- Exclusive read concurrent write (ERCW)—never considered
- Concurrent read concurrent write (CRCW)—multiple processors can read and write. A CRCW PRAM is sometimes called a concurrent random-access machine.
Here, E and C stand for 'exclusive' and 'concurrent' respectively. The read causes no discrepancies while the concurrent write is further defined as:
-
- Common—all processors write the same value; otherwise is illegal
- Arbitrary—only one arbitrary attempt is successful, others retire
- Priority—processor rank indicates who gets to write
- Another kind of array reduction operation like SUM, Logical AND or MAX.
Several simplifying assumptions are made while considering the development of algorithms for PRAM. They are:
- There is no limit on the number of processors in the machine.
- Any memory location is uniformly accessible from any processor.
- There is no limit on the amount of shared memory in the system.
- Resource contention is absent.
- 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.
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.
Read more about this topic: Parallel Random-access Machine
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