[mpich-discuss] MPI_Irecv does block

[William Gropp]

You're right; it would be better for MPI_Irecv to error with a class  
of MPI_ERR_TRUNCATE .

Bill

On Aug 11, 2008, at 10:38 PM, Robert Kubrick wrote:

> This is the stack of one of the receiving process. The last MPI call
> is MPI_Waitany(), as I am using a number of MPI_Irecv() requests.
> If I change the size of buf in the receiver to handle all the
> elements sent by the master, the program runs with no problems.
>
> Attaching to process 8743.
> Reading symbols for shared libraries . done
> Reading symbols for shared libraries ....... done
> 0x900bc40c in poll ()
> (gdb) where
> #0  0x900bc40c in poll ()
> #1  0x000ba702 in MPIDU_Sock_wait () at /usr/include/c++/4.0.0/
> iostream:76
> #2  0x0006b799 in MPIDI_CH3I_Progress () at /usr/include/c++/4.0.0/
> iostream:76
> #3  0x0002d49e in MPI_Waitany () at /usr/include/c++/4.0.0/iostream:76
> #4  0x001344cd in MPI::Request::Waitany (v1=3, v2=0xbffff860,
> v4=@0xbffff848) at /Users/rk/Development/tst/Worker.cpp:965
> #5  0x000060cc in MDS::Worker::run (this=0xbffff8e8) at /Users/rb/
> Development/tst/Worker.cpp:57
> #6  0x00003645 in main (argc=2, argv=0xbffff9fc) at /Users/rb/
> Development/tst/demo.cpp:61
>
> On Aug 11, 2008, at 10:22 PM, Robert Kubrick wrote:
>
>> I want to send a number of elements through MPI_Send. However the
>> receiving process does not know the maximum number of elements in
>> the array (it might be very large). The code will look something  
>> like:
>>
>> MASTER:
>> char buf[500];
>> ...
>> MPI_Send(&buf, 500, SomeDerivedType, 1, ...);
>>
>> ==============
>>
>> WORKER:
>> char buf[100];
>> const int Master = 0;
>> MPI_Irecv(&buf, 100, SomeDerivedType, Master, ...);
>>
>> I noticed that here the worker process blocks (despite the
>> MPI_Irecv non-blocking call) because it doesn't have enough room in
>> the buffer to receive all the elements. I know one way to solve the
>> problem is to use MPI_Probe, but is this the expected behavior if
>> the buffer is too small? Shouldn't MPI_Irecv return an error to
>> avoid blocking?
>>
>

William Gropp
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign


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