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Subject: Re: [OMPI users] CPU burning in Wait state
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2008-09-03 12:52:09


Jeff Squyres wrote:

> On Sep 2, 2008, at 7:25 PM, Vincent Rotival wrote:
>
>> I think I already read some comments on this issue, but I'd like to
>> know of latest versions of OpenMPI have managed to solve it. I am
>> now running 1.2.5
>>
>> If I run a MPI program with synchronization routines (e.g.
>> MPI_barrier, MPI_bcast...), all threads waiting for data are still
>> burning CPU. On the other hand when using non-blocking receives all
>> threads waiting for data are not consuming any CPU.
>>
>> Would there be a possibility to use MPI_Bcast without burning CPU
>> power ?
>
> I'm afraid not at this time. We've talked about adding a blocking
> mode for progress, but it hasn't happened yet (and is very unlikely
> to happen for the v1.3 series).

I'd like to understand this issue better.

What about the variable mpi_yield_when_idle ? Is the point that this
variable will cause a polling process to yield, but if there is no one
to yield to then the process resumes burning CPU? If so, I can imagine
this solution being sufficient in some cases but not in others.

Also, Vincent, what do you mean by waiting threads not consuming any CPU
for non-blocking receives? In what state are these threads? Are they
in an MPI call (like MPI_Wait)? Or, have they returned from an MPI call
(like MPI_Irecv) and the user application can then park these threads to
the side?