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

Thu Jan 21 12:44:32 CST 2021

On Thu, Jan 21, 2021 at 11:16 AM Mark Adams <mfadams at lbl.gov> wrote:

> Yes, the problem is that each KSP solver is running in an OMP thread (So
> at this point it only works for SELF and its Landau so it is all I need).
> It looks like MPI reductions called with a comm_self are not thread safe
> (eg, the could say, this is one proc, thus, just copy send --> recv, but
> they don't)
>

Instead of using SELF, how about Comm_dup() for each thread?

  Matt

> On Thu, Jan 21, 2021 at 10:46 AM Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> 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/>
>>
>

-- 
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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