Thank you a lot for your help, Dave. In my old code, I used MPI_Isend to push
out messages aggressively but observed that some messages get lost strangely.
So I used a flow control mechanism in which at any time, there is only one
message on the fly. MPI_Test is used to test whether the current MPI_Isend is
finished. If it's done, the next message will be sent out. However, it seems
this approach slows down the throughput significantly. I will roll back to
the old version to see whether some bugs caused message losses.
- Guanhua
On Thursday 05 May 2005 06:38, David Cronk wrote:
> There are 2 issues you need to be aware of. First, unless you know a
> message has been copied out of the send buffer you cannot touch the send
> buffer. Touching the buffer before the data has been copied out (for
> either reading or writing) is strictly prohibited by the standard.
> There are only 2 ways to know the message has been copied out of the
> send buffer. One is by completing the Isend (with a wait or test
> operation). The other is for the receiver to report that the message
> has been received.
>
> The other issue is the request handle returned from the Isend routine.
> There are a finit number of these handles available. If you do not free
> these handles (by completing a wait, a test that returns true, or
> explicitely with MPI_Request_free) you will eventually run out of
> available handles and your app will die on the Isend. You may not run
> into this for short runs, but the limit exists and you should free
> requests.
>
> To answer your real question, there should be (as long as the
> implementation is standard compliant) no worry about buffer overflow.
> What may happen is, if the system runs out of system resources your send
> data will remain in the send buffer until enough system resources have
> been freed or the message is matched to a send. See point 1 above for
> the conseequences of this.
>
> I hope this helps.
>
> Dave.
>
> Guanhua Yan wrote:
> > Hi all,
> >
> > Sorry if this is off topic. I used MPI_Isend (to the same destination)
> > for many times without checking whether each sending operation has
> > finished. Is it possible that this will run into buffer overflow problem
> > at the receiver side? Should I do message control by myself or the MPI
> > kernell handles all these? Or is it MPI implementation dependent?
> >
> > Thanks,
> > Guanhua
> > _______________________________________________
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