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Subject: Re: [OMPI users] Problem with MPI_BARRIER
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2011-09-08 10:53:21


On 9/8/2011 7:42 AM, Ghislain Lartigue wrote:
> I will check that, but as I said in first email, this strange behaviour happens only in one place in my code.
Is the strange behavior on the first time, or much later on? (You seem
to imply later on, but I thought I'd ask.)

I agree the behavior is noteworthy, but it's plausible and there's not
enough information to explain it based solely on what you've said.

Here is one scenario. I don't know if it applies to you since I know
very little about what you're doing. I think with VampirTrace, you can
collect performance data into large buffers. Occasionally, the buffers
need to be flushed to disk. VampirTrace will wait for a good
opportunity to do so -- e.g., a global barrier. So, you execute lots of
barriers, but suddenly you hit one where VT wants to flush to disk.
This takes a long time and everyone in the barrier spends a long time in
the barrier. Then, execution resumes and barrier performance looks
again like what it used to look like.

Again, there are various scenarios to explain what you see. More
information would be needed to decide which applies to you.