[mpiwg-hybridpm] Hybrid/Accelerator WG Meeting

Junchao Zhang junchao.zhang at gmail.com
Thu Mar 11 09:53:35 CST 2021

In PETSc, we don't use non-contiguous MPI data types, so packing is not
a problem for us.  It seems if MPI can send/recv data without participation
of GPU, then it is a workable solution in limited cases. My main concern
about CUDA-graph is it is not modularizable.
1) Wtih cudaGraphAddKernelNode() etc, one has to see the whole graph at
all. If kernels are spread and deeply wrapped in CPU routines as in PETSc,
at a higher level, one doesn't know the kernels and hence nodes in the
2) With graph capture, again, a caller subroutine can not guarantee
a callee subroutine will execute the same path in next iteration as the one
when it was captured.

--Junchao Zhang

On Thu, Mar 11, 2021 at 9:07 AM Jim Dinan <james.dinan at gmail.com> wrote:

> Unfortunately, CPU callbacks are not a perfect solution on their own. CUDA
> does not allow CUDA calls from within CPU callbacks, so for example you
> would not be able to launch data packing kernels or peer-to-peer copy
> operations from within the callback. However, you can use CPU callbacks to
> signal a thread in the MPI runtime to process the operation. Another option
> in this design space is to use CUDA memops (e.g. cuStreamWriteValue64 or
> cuStreamWaitValue64) to coordinate between CUDA streams and MPI
> communication helper threads. Because memops are processed from within the
> GPU control processor that manages stream execution, I would expect these
> to have lower overheads than CPU callbacks (although I haven't measured
> this).
>  ~Jim.
> On Wed, Mar 10, 2021 at 10:08 PM Junchao Zhang <junchao.zhang at gmail.com>
> wrote:
>> Jim,
>>   Thanks for the slides.  In Stephen's presentation today, it seems
>> with existing techniques, i.e, CPU MPI callback nodes in CUDA graphs, one
>> can solve the MPI GPU problem. Is my understanding correct?
>>   Thanks.
>> --Junchao Zhang
>> On Wed, Mar 10, 2021 at 8:34 PM Jim Dinan via mpiwg-hybridpm <
>> mpiwg-hybridpm at lists.mpi-forum.org> wrote:
>>> Hi All,
>>> I've posted Stephen's slides:
>>> https://github.com/mpiwg-hybrid/hybrid-issues/tree/master/slides
>>> Best,
>>>  ~Jim.
>>> On Mon, Mar 8, 2021 at 11:21 AM Jim Dinan <james.dinan at gmail.com> wrote:
>>>> Hi All,
>>>> We have an invited speaker this week at the HACC WG:
>>>> Topic: CUDA Deep Dive For the MPI Forum HACC WG
>>>> When:  Wednesday, March 10 10-11:00am ET
>>>> Connection Info: https://github.com/mpiwg-hybrid/hybrid-issues/wiki
>>>> Speaker: Stephen Jones, NVIDIA
>>>> Stephen Jones is one of the architects of CUDA, working on defining the
>>>> language, the platform, and the hardware that it runs on, to span the needs
>>>> of parallel programming from high performance computing to artificial
>>>> intelligence. Prior to his present position, he lead the Simulation &
>>>> Analytics group at SpaceX, working on large-scale simulation of rocket
>>>> engines. He has worked in diverse other industries, including networking,
>>>> CAD/CAM, and scientific computing. He has been a part of CUDA since 2008.
>>>> Cheers,
>>>>  ~Jim.
>>>> PS - Apologies for cross posting on the main list. If you would like to
>>>> continue receiving emails relating to the Hybrid & Accelerator WG, please
>>>> sign up for the mailing list here:
>>>> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-hybridpm.
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