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

[Barry Smith]

  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 <mailto: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 <mailto: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 <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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