Unless there is a significant performance penalty in each boost::mpi::communicator holding onto a shadow version of itself, I think the headaches this would save would be worth it.  Simply put, the current behavior is just very annoying to debug, and it is a somewhat common use case (especially in applications that know they are receiving exactly N messages, and want to wait on them simultaneously).

Thanks,
Walt


On Sun, Feb 23, 2014 at 8:18 AM, Matthias Troyer <troyer@phys.ethz.ch> wrote:
Indeed, that is the problem. If we don't want to reserve certain tags for internal use of Boost.MPI then the only secure way of solving this problem is to create a copy of the communicator, and send the actual message using a unique tag in this shadow communicator. We so far hesitated to implement this procedure, thinking it to be very unlikely that a user would send a second message with the same tag before the first one is received. if this should turn out to be a common usage case then we can consider the solution I outlined. Does anyone see problems with that solution?

Matthias



On 23 Feb 2014, at 04:39, Walter Woods <woodswalben@gmail.com> wrote:

> seems to indicate that MPI guarantees that sends and recvs are kept ordered on a single-threaded process not using MPI_ANY_SOURCE. If that is the case then boost::mpi should as well.

Right, so they are ordered, and that's the problem.

boost::mpi needs to know exactly the size of data that it's receiving.  So, if you If you're sending / receiving a non-native type, boost::mpi needs to transmit how big that data is going to be.  Then, it sends the data.  So one send becomes two sends to MPI - these are ordered.

Receiving is the opposite - it uses one receive to get the size, and then after it has the size, issues another receive to get the data.  If you issue one irecv command before another has gotten its length (and thus issued its data irecv command internally), then because of message ordering, the first irecv will get the length, as expected, but then the second irecv will get the first's data, mistaking it for a length submission.

Hopefully that makes sense.  It's an interleaving problem - because everything is ordered, but irecvs turn into two underlying MPI irecvs, the two boost::mpi irecvs interleave, causing the problem.


On Fri, Feb 21, 2014 at 5:52 PM, Roy Hashimoto <roy.hashimoto@gmail.com> wrote:
On Fri, Feb 21, 2014 at 11:49 AM, Walter Woods <woodswalben@gmail.com> wrote:
In Roy's case, especially the test file, the problem is having multiple irecv's happening.  Lookat the underlying request::handle_serialized_irecv implementation in boost/mpi/communicator.hpp - one recv is accomplished through several MPI_IRecv requests issued in sequence.  If you have several irecvs running at once, then one is likely to get the other's data as its length.

Thanks for your reply and looking at the boost::mpi source - I haven't got that far. I understand what you're saying, but the first few paragraphs of this page:


seems to indicate that MPI guarantees that sends and recvs are kept ordered on a single-threaded process not using MPI_ANY_SOURCE. If that is the case then boost::mpi should as well.
 
In other words, if you want to receive multiple messages in the same tag, be sure to only have one IRecv() with that tag running at a time.  Data may only be transferred serially (not in parallel) over a single tag anyhow.

I did change my development code to do this.

Hope that helps, 
Walt

It does, thanks!

Roy 

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