[mpich-discuss] Need perspective on use of atomic I/O

[Ted Dunning]

I am trying to assess how well a high-performance file system that I work
on will support MPI programming for some customers we have. In particular,
I am looking at MPI I/O.

The technical aspects of the assessment are pretty simple. Collective and
direct I/O should work great, but the locking apparently required for
atomic mode I/O will be very, very expensive.

That leads to the community questions of how important these trade-offs
will be. And thus, I land here on this list.

I have a few questions that folk here probably can answer off the top of
their head:

1) how often is atomic mode I/O used? It seems that most I/O will involve
non-overlapping writes and thus atomic mode will be irrelevant. But what is
the real sense from people who actually write this kind of code?

2) do commonly used MPI libraries implement atomic mode internally as in
[1], or do they require file system support? What kind of support?

3) how to file-system people usually go wrong in approaching MPI
applications?


[1] http://www.mcs.anl.gov/~thakur/papers/atomic-mode.pdf
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