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From: Brian Budge (brian.budge_at_[hidden])
Date: 2006-10-17 15:24:03


Hi all -

I'm running my MPI program via open-mpi on a 4 core opteron machine.
I am trying to run 5 processes where 1 of these processes is simply a
coordinating process. It does very little work other than once every
second or two recieving and sending a short ping to the other
processes.

I've seen some of the other threads regarding oversubscription, and I
read about it in the FAQ. When I don't set my process to
yield_when_idle, naturally I get terrible performance because open-mpi
is using aggressive mode when oversubscribed.

Finally I was able to get it to sometimes act degraded by using the
yield_when_idle and by setting the number of slots=4. However, it
seems as though this works off and on. If I run top while running my
processes sometimes it looks like this:

11405 budge 39 15 957m 383m 4132 R 100 9.6 83:14.78
RenderFish
11406 budge 39 15 959m 386m 4204 R 100 9.6 71:24.22
RenderFish
11408 budge 39 15 959m 386m 4200 R 100 9.6 56:12.23
RenderFish
11407 budge 39 15 959m 386m 4204 R 50 9.6 46:45.78
RenderFish
11409 budge 39 15 959m 386m 4208 R 50 9.6 75:14.43 RenderFish

and sometimes it looks more like this:

11323 budge 39 15 959m 386m 4132 R 100 9.6 83:14.78
RenderFish
11324 budge 39 15 959m 386m 4204 R 100 9.6 71:24.22
RenderFish
11325 budge 39 15 959m 386m 4200 R 100 9.6 83:12.23
RenderFish
11326 budge 39 15 959m 386m 4204 R 99 9.6 72:45.78
RenderFish

Obviously, the second is better. I only wish it were the norm.

 I used LAM for a long time and it seems to always have run in
"degraded" mode, but I have other reasons (better infiniband support)
that I am switching to open-mpi.

Does anyone have any idea why open-mpi wouldn't think a process is
idle 99% of the time when it doesn't do anything 99% of the time? One
possible clue is that my program nices itself... does this make a
difference?

Thanks,
  Brian