The Multiphase Shared Arrays (MSA) library provides a specialized shared memory abstraction in Charm++ that provides automatic memory management. Explicitly shared memory provides the convenience of shared memory programming while exposing the performance issues to programmers and the "intelligent" Adaptive Runtime System (aRTS).

Each MSA is accessed in one specific mode during each phase of execution: read-only mode, in which any thread can read any element of the array; write-once mode, in which each element of the array is written to (possibly multiple times) by at most one worker thread, and no reads are allowed and accumulate mode, in which any threads can add values to any array element, and no reads or writes are permitted. A sync call is used to denote the end of a phase.

We permit multiple copies of a page of data on different processors and provide automatic fetching and caching of remote data. For example, initially an array might be put in write-once mode while it is populated from a file. This determines the cache behavior and the permitted operations on the array during this phase. write-once means every thread can write to a different element of the array. The user is responsible for ensuring that two threads do not write to the same element; the system helps by detecting violations. From the cache maintenance viewpoint, each page of the data can be over-written on it's owning processor without worrying about transferring ownership or maintaining coherence. At the sync, the data is simply merged. Subsequently, the array may be read-only for a while, thereafter data might be accumulate'd into it, followed by it returning to read-only mode. In the accumulate phase, each local copy of the page on each processor could have its accumulations tracked independently without maintaining page coherence, and the results combined at the end of the phase. The accumulate operations also include set-theoretic union operations, i.e. appending items to a set of objects would also be a valid accumulate operation. User-level or compiler-inserted explicit prefetch calls can be used to improve performance.
 

• Chao Huang

• 04-10    Jayant DeSouza and Laxmikant V. Kale,  MSA: Multiphase Specifically Shared Arrays,  Proceedings of the 17th International Workshop on Languages and Compilers for Parallel Computing, West Lafayette, Indiana, USA, September 22-25 2004.

• Global Arrays