A few points in addition to what has already been said:
1. You can always post a receive for size N when the actual message is
<=N. You can use this fact to pre-post a receive with size N, where N
is large enough for the header and a medium-sized message. If you
have a short message, it'll fit entirely within N and you're good.
If the message size is greater than N, you can still send the first
"eager" part of the message with the header, and then send a second
message with the remaining (size-N) after that. So for short/medium
messages, you'll still accomplish the transfer in 1 network message,
but long messages will effectively require 2. And for long messages,
you're already taking a long time to transmit the message, so adding
the overhead of a 2nd MPI message is negligible compared to the
overall time.
Note that this is pretty much what most MPI's do under the covers,
anyway (including Open MPI).
2. There is a dormant-but-will-be-resurrected proposal in front of the
MPI-3 Forum right now to do exactly what you want: "MPI receive and
allocate a buffer big enough for however big the incoming message
is." But even if that proposal passes, it'll likely be a while before
it shows up in MPI implementations. :-\
On Jun 4, 2009, at 10:27 AM, Neil Ludban wrote:
> Date: Thu, 4 Jun 2009 11:14:16 +1000
> From: Lars Andersson <lars...@gmail.com>
> Subject: [OMPI users] Receiving MPI messages of unknown size
> To: us...@open-mpi.org
>
> When using blocking message passing, I have simply solved the
problem
> by first sending a small, fixed size header containing the size of
> rest of the data, sent in the following mpi message. When using
> non-blocking message passing, this doesn't seem to be such a good
> idea, since we cant post the main data transfer until we have
received
> the message header... It seems to take away most of the advantages
on
> non-blocking io in the first place.
If enough messages are small enough, a medium sized message with
fixed header and variable data eliminates most of the second message
overhead.
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Jeff Squyres
Cisco Systems
