On Sat, Oct 2, 2010 at 12:51 PM, Manuel Prinz <[email protected]> wrote: > > I did some reading and it seems the Open MPI indeed does support two modes > of waiting: aggressive and degraded. The default behavior is "aggressive", > but you can switch them by setting the mpi_yield_when_idle MCA parameter. > See the following FAQ entries (and links therein): > > http://www.open-mpi.org/faq/?category=running#force-aggressive-degraded > http://www.open-mpi.org/faq/?category=running#oversubscribing > > I guess this is basically the behaviour you want. It would be great if you > could give it a try and report back if it works for you. If it doesn't do > what you (and I) expect, I'll forward this issue upstream.
I wrote a test MPI program that just calls MPI_Probe() once - this should block forever, since there are no sends happening. When run with $ mpirun -np 2 ./a.out MPI_Probe never returns and the processes spin through poll(), which is what I originally reported. So far so good. If I change the invocation to $ mpirun -np 2 --mca mpi_yield_when_idle 1 ./a.out the behavior is the same, except that the processes alternate between poll() and sched_yield(). This doesn't help anything; the scheduler is still being thrashed, and the CPU is not allowed to go idle. [In fact, my understanding of the Linux scheduler is that a zero-timeout poll() counts as a yield, so "Aggressive" mode isn't even doing anything constructive!] The desired behavior is for an idle cluster's processes to BLOCK in poll(). So mpi_yield_when_idle does not do what I want. Also, putting "mpi_yield_when_idle = 1" into ~/.openmpi/mca-params.conf has no effect, contra the documentation -- this perhaps ought to be its own bug. (I can set MCA parameters for R with environment variables, but that's not nearly as convenient as the host file.) zw -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]
