[petsc-users] Reaching limit number of communicator with Spectrum MPI

[Junchao Zhang]

Feimi, if it is easy to reproduce, could you give instructions on how to
reproduce that?

PS: Spectrum MPI is based on OpenMPI.  I don't understand why it has the
problem but OpenMPI does not.  It could be a bug in petsc or user's code.
For reference counting on MPI_Comm, we already have petsc inner comm. I
think we can reuse that.

--Junchao Zhang


On Fri, Aug 20, 2021 at 12:33 AM Barry Smith <bsmith at petsc.dev> wrote:

>
>   It sounds like maybe the Spectrum MPI_Comm_free() is not returning the
> comm to the "pool" as available for future use; a very buggy MPI
> implementation. This can easily be checked in a tiny standalone MPI program
> that simply comm dups and frees thousands of times in a loop. Could even be
> a configure test (that requires running an MPI program). I do not remember
> if we ever tested this possibility; maybe and I forgot.
>
>   If this is the problem we can provide a "work around" that attributes
> the new comm (to be passed to hypre) to the old comm with a reference count
> value also in the attribute. When the hypre matrix is created that count is
> (with the new comm) is set to 1, when the hypre matrix is freed that count
> is set to zero (but the comm is not freed), in the next call to create the
> hypre matrix when the attribute is found, the count is zero so PETSc knows
> it can pass the same comm again to the new hypre matrix.
>
> This will only allow one simultaneous hypre matrix to be created from the
> original comm. To allow multiply simultaneous hypre matrix one could have
> multiple comms and counts in the attribute and just check them until one
> finds an available one to reuse (or creates yet another one if all the
> current ones are busy with hypre matrices). So it is the same model as
> DMGetXXVector() where vectors are checked out and then checked in to be
> available later. This would solve the currently reported problem (if it is
> a buggy MPI that does not properly free comms), but not solve the MOOSE
> problem where 10,000 comms are needed at the same time.
>
>   Barry
>
>
>
>
>
> On Aug 19, 2021, at 3:29 PM, Junchao Zhang <junchao.zhang at gmail.com>
> wrote:
>
>
>
>
> On Thu, Aug 19, 2021 at 2:08 PM Feimi Yu <yuf2 at rpi.edu> wrote:
>
>> Hi Jed,
>>
>> In my case, I only have 2 hypre preconditioners at the same time, and
>> they do not solve simultaneously, so it might not be case 1.
>>
>> I checked the stack for all the calls of MPI_Comm_dup/MPI_Comm_free on
>> my own machine (with OpenMPI), all the communicators are freed from my
>> observation. I could not test it with Spectrum MPI on the clusters
>> immediately because all the dependencies were built in release mode.
>> However, as I mentioned, I haven't had this problem with OpenMPI before,
>> so I'm not sure if this is really an MPI implementation problem, or just
>> because Spectrum MPI has less limit for the number of communicators,
>> and/or this also depends on how many MPI ranks are used, as only 2 out
>> of 40 ranks reported the error.
>>
> You can add printf around MPI_Comm_dup/MPI_Comm_free sites on the two
> ranks, e.g., if (myrank == 38) printf(...), to see if the dup/free are
> paired.
>
>  As a workaround, I replaced the MPI_Comm_dup() at
>
>> petsc/src/mat/impls/hypre/mhypre.c:2120 with a copy assignment, and also
>> removed the MPI_Comm_free() in the hypre destroyer. My code runs fine
>> with Spectrum MPI now, but I don't think this is a long-term solution.
>>
>> Thanks!
>>
>> Feimi
>>
>> On 8/19/21 9:01 AM, Jed Brown wrote:
>> > Junchao Zhang <junchao.zhang at gmail.com> writes:
>> >
>> >> Hi, Feimi,
>> >>    I need to consult Jed (cc'ed).
>> >>    Jed, is this an example of
>> >>
>> https://lists.mcs.anl.gov/mailman/htdig/petsc-dev/2018-April/thread.html#22663
>> ?
>> >> If Feimi really can not free matrices, then we just need to attach a
>> >> hypre-comm to a petsc inner comm, and pass that to hypre.
>> > Are there a bunch of solves as in that case?
>> >
>> > My understanding is that one should be able to
>> MPI_Comm_dup/MPI_Comm_free as many times as you like, but the
>> implementation has limits on how many communicators can co-exist at any one
>> time. The many-at-once is what we encountered in that 2018 thread.
>> >
>> > One way to check would be to use a debugger or tracer to examine the
>> stack every time (P)MPI_Comm_dup and (P)MPI_Comm_free are called.
>> >
>> > case 1: we'll find lots of dups without frees (until the end) because
>> the user really wants lots of these existing at the same time.
>> >
>> > case 2: dups are unfreed because of reference counting
>> issue/inessential references
>> >
>> >
>> > In case 1, I think the solution is as outlined in the thread, PETSc can
>> create an inner-comm for Hypre. I think I'd prefer to attach it to the
>> outer comm instead of the PETSc inner comm, but perhaps a case could be
>> made either way.
>>
>
>
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