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Subject: Re: [OMPI users] nonblocking MPI_File_iwrite() does block?
From: Rob Latham (robl_at_[hidden])
Date: 2009-11-20 12:23:33


On Mon, Nov 16, 2009 at 11:20:44AM +0100, Christoph Rackwitz wrote:
> It's been ten days now. I'd like to resurrect this, in case someone
> can help and just missed it.

Hi. I only check in on the OpenMPI list periodically. Sorry for the
delay.

The standard in no way requires any overlap for either the nonblocking
communication or I/O routines. There are long and heated discussions
about "strict" or "weak" interpretation of the progress rule and which
one is "better".

If you want asynchronous nonblocking I/O, you might have to roll all
the way back to LAM or MPICH-1.2.7, when ROMIO used its own request
objects and test/wait routines on top of the aio routines.

In order to have standard request objects and use the standard
test/wait routines, ROMIO switched to generalized requests. However,
it's difficult to make progress on generalized requests without using
threads, so we do all the work when the job is posted and as you
observe, MPI_Wait() discovers immediately that the job is complete.

I proposed an extension to MPI generalized requests a few years ago
that would make them more amenable to libraries like ROMIO.

Today systems have a ton of cores. Spawning an I/O thread is not such
an onerous burden. But we don't spawn such a thread in ROMIO, and so
nonblocking I/O is not asynchronous.

What if you moved your MPI_File_write call into a thread? There are
several ways to do this: you could use standard generalized reqeusts
and make progress with a thread -- the
application writer has a lot more knowledge about the systems and how
best to allocate threads.

If I may ask a slightly different question: you've got periods of I/O
and periods of computation. Have you evaluated collective I/O? I
know you are eager to hide I/O in the background -- to get it for free
-- but there's no such thing as a free lunch. Background I/O might
still perturb your computation phase, unless you make zero MPI calls
in your computational phase. Collective I/O can bring some fairly
powerful optimizations to the table and reduce your overall I/O costs,
perhaps even reducing them enough that you no longer miss true
asynchronous I/O ?

==rob

-- 
Rob Latham
Mathematics and Computer Science Division
Argonne National Lab, IL USA