[mpich-discuss] MPI_Irecv does block

[Robert Kubrick]

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?
>