On Aug 27, 2008, at 10:22 AM, Richard Treumann wrote:
Robert -
A return from a blocking send means the application send buffer is
available for reuse. If it is a BSEND, the application buffer could
be available because the message data has been copied to the
attached buffer or because the data has been delivered to the
destination. The application on the side that used BSEND only cares
that the application send buffer is available and a receive on the
destination side can get the data from somewhere.
A return from a blocking receive means the data is available in the
application receive buffer. If there were a BRECV in the standard,
what would it mean? It cannot return until the data is in the
application receive buffer
I see, not it's clear.
However from an application point of view I see an odd result here.
On the sender side I can use a buffered send to "queue" messages for
delivery and decide how many messages my buffer should contain before
the MPI_Bsend blocks if it's running out of space.
On the receiving side I have no control over the number of messages
that MPI can buffer. This is basically left to the implementation
details as you very well described. Shouldn't the user be allowed to
specify a memory space to buffer messages on the receiving side, just
like on the send?
so at the same time the BRECV call is able to tell libmpi to "use a
scratch buffer", libmpi is also being told the address of the
application buffer and would have no use for a scratch buffer.
Before the BRECV call, libmpi could not know if the message will be
matched by BRECV or some other kind of receive.
There is a hidden optimization in most MPI implementations that
will allow small sends to ship their data to the destination
eagerly. When the data shows up at the destination and there is no
matching receive, the data is buffered in space hidden within the
MPI implementation. When a receive call is made, libmpi will first
check to see if the data is already available in this hidden
scratch buffer. In this optimization, when there is a matching
receive waiting for the data at the moment it arrives, the hidden
buffering is bypassed.
There is nothing in the MPI API to let an application say a receive
or send should use this hidden buffering. That is up to libmpi.
There is a ready send defined by the MPI standard which allows the
send side to dictate that the message go immediately and libmpi
need not attempt to buffer it. If the application can be certain
the matching receive is posted before the RSEND call is made, there
is a possibility of some performance gains but pre-posting receives
with absolute reliability is not as easy as you might expect.
The danger with RSEND is that if the receive is not available the
data must be discarded. Since the MPI standard does not allow data
to be lost, the next thing the MPI library will do after discarding
the data is abort the job.
Dick Treumann - MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363
users-boun...@open-mpi.org wrote on 08/27/2008 08:41:59 AM:
> [image removed]
>
> Re: [OMPI users] MPI_Brecv vs multiple MPI_Irecv
>
> Robert Kubrick
>
> to:
>
> Open MPI Users
>
> 08/27/2008 08:51 AM
>
> Sent by:
>
> users-boun...@open-mpi.org
>
> Cc:
>
> mpich-discuss
>
> Please respond to Open MPI Users
>
> Interesting, MPI_Brecv is not in the standard (MPI_Bsend is). I
don't
> understand why a recv could not implemented with an external buffer.
> Anyway, my bad, I was assuming MPI_Brecv was on. Thanks George for
> pointing this out.
>
> On Aug 27, 2008, at 7:14 AM, George Bosilca wrote:
>
> > What is the definition of a buffered receive ?
> >
> > george.
> >
> > On Aug 26, 2008, at 10:17 PM, Robert Kubrick wrote:
> >
> >> From a performance point of view, which one is better:
> >>
> >> MPI_Battach(10*sizeof(MSG))
> >> MPI_Brecv()
> >>
> >> or
> >>
> >> MPI_recv_init()
> >> MPI_recv_init()
> >> MPI_recv_init()
> >> ... /* 10 recv handlers */
> >> MPI_Start(all recv)
> >> MPI_Waitany()
> >>
> >>
> >> I understand MPI_Brecv will require an extra message copy, from
> >> the attached buffer to the MPI_Brecv() buffer. I'd like to
know if
> >> there other differences between the two methods.
> >>
> >> Thanks,
> >> Rob
> >> _______________________________________________
> >> users mailing list
> >> us...@open-mpi.org
> >> http://www.open-mpi.org/mailman/listinfo.cgi/users
> >
> > _______________________________________________
> > users mailing list
> > us...@open-mpi.org
> > http://www.open-mpi.org/mailman/listinfo.cgi/users
>
> _______________________________________________
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