Array memory allocation when using MPI

I am using C and MPI. How is memory allocated for arrays if the program runs on multiple processors on the same machine? Is that array being sha开发者_如何转开发red between all participating tasks or every task has it's own copy?

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Every rank has its own copy of the data. They are generally run in separate processes, and so do not share a virtual address space.

Implementations like Adaptive MPI and Phoenix put multiple ranks on threads in a common process, but they take steps to isolate each rank so that thinks it's running as a separate process.

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MPI is a distributed memory framework. It has only a limited notion of shared-memory processing. If shared memory is of importance to your program's performance, then I suggest you look into OpenMP; both can be combined in the same application. (I.e., each node would run, ideally, a single OpenMP-based process that would communicate with other instances through MPI.)