[petsc-dev] Hybrid MPI/OpenMP reflections

[Jed Brown]

Karl Rupp <rupp at mcs.anl.gov> writes:

> Hi,
>
>  >> When using good preconditioners, spMV is essentially never the
>>> bottleneck and hence I don't think a separate communication thread
>>> should be implemented in PETSc. Instead, such a fallback should be part
>>> of a good MPI implementation.
>>
>> SpMV is an important part of most of those scalable preconditioners.  In
>> multigrid, those are grid transfer operators, residuals, and Chebyshev
>> or Krylov-accelerated smoothers.
>
>  From the context I was referring to SpMV for a 'full system matrix' as 
> part of the outer Krylov solver as it was the topic in the paper (cf. 
> third paragraph). In preconditioners you may use different storage 
> formats, not communicate across nodes, etc.

It's exactly the same operation in the Chebyshev smoother, for example.
There is no reason to use a different format (it doesn't) and "not
communicating across nodes" in the preconditioner is an egregious crime
that prevents a scalable method.  So I would say that fine-grid SpMV
performance is of similarly high performance with a "scalable
preconditioner" as with Jacobi.
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