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Subject: Re: [OMPI users] MPI_Comm_spawn, caller uses CPU while waiting for spawned processes
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-12-04 12:10:28


Nicolas Bock wrote:
On Fri, Dec 4, 2009 at 08:21, Ralph Castain <rhc@open-mpi.org> wrote:
You used it correctly. Remember, all that cpu number is telling you is the percentage of use by that process. So bottom line is: we are releasing it as much as we possibly can, but no other process wants to use the cpu, so we go ahead and use it.

If any other process wanted it, then the percentage would drop and the other proc would take some.
When you say "the other proc would take some", how much do you expect it to take? In my case above, the master process does not appear to yield a whole lot. Can I reduce the polling frequency? I know that my slave processes typically run several minutes to hours.
In my (limited) experience, the situation is a little of both.  OMPI is yielding.  Yielding makes a difference only if someone else wants the CPU.  But even if someone else wants the CPU, OMPI in yield mode will still be consuming cycles.  It's like the way I drive a car.  When I approach a stop sign, I slow down -- really, officer, I do -- and if there is cross traffic I let it go by ahead of me.  But if there is no cross traffic, I, ahem, proceed expediently.  And, even if there is cross traffic, their progress is still impacted by me -- heck, I'm all for obeying stop signs and all, but I'm no doormat.  OMPI processes can yield, but they only check to yield every now and then.  Between checks, they are not timid processes, even if other processes are waiting to run.  I once had some numbers on this.  Yield helped, but not as effectively as one might have imagined.