[petsc-dev] Memory problem with OpenMP and Fieldsplit sub solvers

[Matthew Knepley]

On Thu, Jan 21, 2021 at 10:34 AM Mark Adams <mfadams at lbl.gov> wrote:

> It looks like PETSc is just too clever for me. I am trying to get a
> different MPI_Comm into each block, but PETSc is thwarting me:
>

It looks like you are using SELF. Is that what you want? Do you want a
bunch of comms with the same group, but independent somehow? I am confused.

   Matt


>   if (jac->use_openmp) {
>     ierr          = KSPCreate(MPI_COMM_SELF,&ilink->ksp);CHKERRQ(ierr);
> PetscPrintf(PETSC_COMM_SELF,"In PCFieldSplitSetFields_FieldSplit with
> -------------- link: %p. Comms %p
> %p\n",ilink,PetscObjectComm((PetscObject)pc),PetscObjectComm((PetscObject)ilink->ksp));
>   } else {
>     ierr          =
> KSPCreate(PetscObjectComm((PetscObject)pc),&ilink->ksp);CHKERRQ(ierr);
>   }
>
> produces:
>
> In PCFieldSplitSetFields_FieldSplit with -------------- link: 0x7e9cb4f0.
> Comms 0x660c6ad0 0x660c6ad0
> In PCFieldSplitSetFields_FieldSplit with -------------- link: 0x7e88f7d0.
> Comms 0x660c6ad0 0x660c6ad0
>
> How can I work around this?
>
>
> On Thu, Jan 21, 2021 at 7:41 AM Mark Adams <mfadams at lbl.gov> wrote:
>
>>
>>
>> On Wed, Jan 20, 2021 at 6:21 PM Barry Smith <bsmith at petsc.dev> wrote:
>>
>>>
>>>
>>> On Jan 20, 2021, at 3:09 PM, Mark Adams <mfadams at lbl.gov> wrote:
>>>
>>> So I put in a temporary hack to get the first Fieldsplit apply to NOT
>>> use OMP and it sort of works.
>>>
>>> Preonly/lu is fine. GMRES calls vector creates/dups in every solve so
>>> that is a big problem.
>>>
>>>
>>>   It should definitely not be creating vectors "in every" solve. But it
>>> does do lazy allocation of needed restarted vectors which may make it look
>>> like it is creating "every" vectors in every solve.  You can
>>> use -ksp_gmres_preallocate to force it to create all the restart vectors up
>>> front at KSPSetUp().
>>>
>>
>> Well, I run the first solve w/o OMP and I see Vec dups in cuSparse Vecs
>> in the 2nd solve.
>>
>>
>>>
>>>   Why is creating vectors "at every solve" a problem? It is not thread
>>> safe I guess?
>>>
>>
>> It dies when it looks at the options database, in a Free in the
>> get-options method to be exact (see stacks).
>>
>> ======= Backtrace: =========
>> /lib64/libc.so.6(cfree+0x4a0)[0x200021839be0]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(PetscFreeAlign+0x4c)[0x2000002a368c]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(PetscOptionsEnd_Private+0xf4)[0x2000002e53f0]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(+0x7c6c28)[0x2000008b6c28]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecCreate_SeqCUDA+0x11c)[0x20000052c510]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecSetType+0x670)[0x200000549664]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecCreateSeqCUDA+0x150)[0x20000052c0b0]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(+0x43c198)[0x20000052c198]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecDuplicate+0x44)[0x200000542168]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecDuplicateVecs_Default+0x148)[0x200000543820]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(VecDuplicateVecs+0x54)[0x2000005425f4]
>>
>> /gpfs/alpine/csc314/scratch/adams/petsc/arch-summit-opt-gnu-cuda-omp/lib/libpetsc.so.3.014(KSPCreateVecs+0x4b4)[0x2000016f0aec]
>>
>>
>>
>>>
>>> Richardson works except the convergence test gets confused, presumably
>>> because MPI reductions with PETSC_COMM_SELF is not threadsafe.
>>>
>>>
>>>
>>> One fix for the norms might be to create each subdomain solver with a
>>> different communicator.
>>>
>>>
>>>    Yes you could do that. It might actually be the correct thing to do
>>> also, if you have multiple threads call MPI reductions on the same
>>> communicator that would be a problem. Each KSP should get a new MPI_Comm.
>>>
>>
>> OK. I will only do this.
>>
>>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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