Hi,

I am using MPI_Reduce operation on 122880x400 matrix of doubles. The parallel job runs on 32 machines, each having different processor in terms of speed, but the architecture and OS is the same on all machines (x86_64). The task is a typical map-and-reduce, i.e. each of the processes collects some data, which is then summed (MPI_Reduce w. MPI_SUM).

Having different processors, each of the jobs comes to the MPI_Reduce in different time.

The *first problem* came when I called MPI_Reduce on the whole matrix. The system ended up with *MPI_ERR_OTHER error*, each time on different rank. I fixed this problem by chunking up the matrix into 2048 submatrices, calling MPI_Reduce in cycle.

However *second problem* arose --- MPI_Reduce hangs up... It apparently gets stuck in some kind of dead-lock or something like that. It seems that if the processors are of similar speed, the problem disappears, however I cannot provide this condition all the time.

I managed to get rid of the problem (at least after few non-problematic iterations) by sticking MPI_Barrier before the MPI_Reduce line.

The questions are:

1) is this a usual behavior???
2) is there some kind of timeout for MPI_Reduce???
3) why does MPI_Reduce die on large amount of data if the system has enough address space (64 bit compilation)

Thanx
Ondrej Glembek


--
  Ondrej Glembek, PhD student  E-mail: glem...@fit.vutbr.cz
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