In computer science, a parallel external memory (PEM) model is a cache-aware, external-memory abstract machine. It is the parallel-computing analogy to the single-processor external memory (EM) model. In a similar way, it is the cache-aware analogy to the parallel random-access machine (PRAM). The PEM model consists of a number of processors, together with their respective private caches and a shared main memory.
In the PEM model, there is no direct communication network between the P processors. The processors have to communicate indirectly over the main memory. If multiple processors try to access the same block in main memory concurrently read/write conflicts occur. Like in the PRAM model, three different variations of this problem are considered:
- Concurrent Read Concurrent Write (CRCW): The same block in main memory can be read and written by multiple processors concurrently.
- Concurrent Read Exclusive Write (CREW): The same block in main memory can be read by multiple processors concurrently. Only one processor can write to a block at a time.
- Exclusive Read Exclusive Write (EREW): The same block in main memory cannot be read or written by multiple processors concurrently. Only one processor can access a block at a time.
Comparison to other models
|Random-access machine (RAM)||No||No|
|Parallel random-access machine (PRAM)||Yes||No|
|External memory (EM)||No||Yes|
|Parallel external memory (PEM)||Yes||Yes|
^ Jump up to:a b c d e f g h i j k l Arge, Lars; Goodrich, Michael T.; Nelson, Michael; Sitchinava, Nodari (2008). “Fundamental parallel algorithms for private-cache chip multiprocessors”. Proceedings of the Twentieth Annual Symposium on Parallelism in Algorithms and Architectures – SPAA ’08. New York, New York, USA: ACM Press: 197. doi:10.1145/1378533.1378573. ISBN 9781595939739.
- ^ Jump up to:ab c d Arge, Lars; Goodrich, Michael T.; Sitchinava, Nodari (2010). “Parallel external memory graph algorithms”. 2010 IEEE International Symposium on Parallel & Distributed Processing (IPDPS). IEEE: 1–11. doi:10.1109/ipdps.2010.5470440. ISBN 9781424464425.
Ofer Abarbanel is a 25 year securities lending broker and expert who has advised many Israeli regulators, among them the Israel Tax Authority, with respect to stock loans, repurchase agreements and credit derivatives. Founder TBIL.co STATX Fund.