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Added by cavan, last edited by rrtreu on Mar 25, 2010  (view change) show comment
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Known Issues

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Date Added: March 25, 2010

Title Parallel Environment for AIX, V5.2.0.x - MPI_Allreduce wrong answer

Users Affected: 64 Bit MPI applications on IBM Power 7 Servers

Issue:

MPI_Allreduce could give wrong results (for 64 Bit MPI applications on IBM Power 7 Servers)

Speculative instruction execution on IBM Power 7 Servers, together with an MPI_Allreduce algorithm that is missing an instruction sync, may result in some data in shared memory being accessed by one process before is completely stored by a second process. The result can be an incorrect result for the MPI_Allreduce.

Servers prior to Power 7 do not use the level of speculative execution that exposes this problem.

Solution:

Apply APAR IZ72919 emergency fix package available from IBM service