[petsc-dev] programming model for PETSc

[Matthew Knepley]

On Sat, Nov 26, 2011 at 12:07 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Fri, Nov 25, 2011 at 16:48, Matthew Knepley <knepley at gmail.com> wrote:
>
>> Synopsis of what I said before to elicit comment:
>>
>> 1) I think the only thing we can learn from Brook, CUDA, OpenCL is that
>> you identify threads by a grid ID.
>>
>> 2) Things like BLAS are so easy that you can move up to the streaming
>> model, but this does not work for
>>
>>   - FD and FEM residual evaluation (Jed has an FD example with Aron, SNES
>> ex52 is my FEM example)
>>
>>   - FD and FEM Jacobian evaluation
>>
>
> I think these are also probably too simple. Discontinuous Galerkin with
> overlapped flux computations and interior integration would be a somewhat
> better model problem. Nonlinear Gauss-Seidel in a multigrid context would
> be another.
>

I do not see DG a fundamentally different.


>
>
>>
>> 3) If you look at ex52 I do a "thread transposition" meaning threads
>> start working on different areas of
>>     memory which looks like a transpose on a 2D grid. I can do this using
>> shared memory for the vector group.
>>
>> The API is very simple. Give grid indices to the thread, and its done in
>> CUDA and OpenCL essentially the
>> same way.
>>
>
> As is, this seems to assume a flat memory model and the memory access only
> appears in how the kernel uses threadIdx to determine what memory to
> operate on. If we could say something about this up-front, then the library
> could schedule tasks relative to memory and perhaps handle some updates for
> distributed memory.
>

My first rule would be, never assume library intelligence. So we make it
possible to give a schedule for kernels, but initially there is
no scheduling code. It is user input, with a dumb default. The flat memory
is the thread shared memory, and everything else is messages.


> Can we have a way to specify the required memory access before launching
> the kernels?
>

This is exactly what Victor's thing is about

   Matt

-- 
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
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