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Subject: [OMPI users] Memory manager
From: Terry Frankcombe (terry_at_[hidden])
Date: 2007-11-20 06:52:09


Hi folks

I posted this to the devel list the other day, but it raised no
responses. Maybe people will have more to say here.

Questions: How much does using the MPI wrappers influence the memory
management at runtime? What has changed in this regard from 1.2.3 to
1.2.4?

The reason I ask is that I have an f90 code that does very strange
things. The structure of the code is not all that straightforward, with
a "tree" of modules usually allocating their own storage (all with save
applied globally within the module). Compiling with OpenMPI 1.2.4
coupled to a gcc 4.3.0 prerelease and running as a single process (with
no explicit mpirun), the elements of one particular array seem to revert
to previous values between where they are set and a later part of the
code. (I'll refer to this as The Bug, and having the matrix elements
stay as set as "expected behaviour".)

The most obvious explanation would be a coding error. However,
compiling and running this with OpenMPI 1.2.3 gives me the expected
behaviour! As does compiling and running with a different MPI
implementation and compiler set. Replacing the prerelease gcc 4.3.0
with the released 4.2.2 makes no change.

The Bug is unstable. Removing calls to various routines in used modules
(that otherwise do not effect the results) returns to expected behaviour
at runtime. Removing a call to MPI_Recv that is never called returns to
expected behaviour.

Because of this I can't reduce the problem to a small testcase, and so
have not included any code at this stage.

If I run the code with mpirun -np 1 the problem goes away. So one could
presumably simply say "always run it with mpirun." But if this is
required, why does OpenMPI not detect it? And why the difference
between 1.2.3 and 1.2.4?

Does anyone care to comment?

Ciao
Terry

-- 
Dr Terry Frankcombe
Physical Chemistry, Department of Chemistry
Göteborgs Universitet
SE-412 96 Göteborg Sweden
Ph: +46 76 224 0887   Skype: terry.frankcombe
<terry_at_[hidden]>