[petsc-dev] Implementing longer pipelines with VecDotBegin and VecDotEnd

[Wim Vanroose]

Thanks for the feedback.  We will explore the different options and come
back with an update.

Wim

On Fri, Mar 24, 2017 at 5:45 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Mar 23, 2017, at 11:23 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> >
> > Barry Smith <bsmith at mcs.anl.gov> writes:
> >
> >>   Jed,
> >>
> >>      Ok I buy what you are saying as a possible solution but now I am
> >>      wondering why you need the request argument. Instead, as soon as
> >>      you hit the first End or PetscCommSplitReductionBegin why don't
> >>      you start a new batch (internally) that all new Begin() calls
> >>      feed into? Then each End call just checks through the previous
> >>      (still incompleted) batches to find its begin partner
> >
> > How is the "begin partner" identified?
>
>    The current model has always been first in, first out. If you do
> anything else currently it would/should error out.
>
> >
> >>      (or we can require always first in, first out). Thus your first
> >>      VecNormEnd(Y, would match the first begin with Y posted and your
> >>      second would match the second.
> >
> > VecNormBegin(X,&norm);
> > function(X);
> > VecNormEnd(X,&norm);
> >
> > would be broken if function() uses split reductions.
>
>    Do we need to support this far more general model? I think the proposed
> use case doesn't need it. But you are right, who is to stop someone from
> writing that code.
>
>    If we match on both X and &norm then your case will work correctly but
> if you changed it to
>
> > VecNormBegin(X,&norm);
> > function(X,&norm);
> > VecNormEnd(X,&norm);
>
> and function() used split reduction with the &norm argument it would get a
> false match so yes your request argument would be required. But we could
> eliminate that possibly by checking for each new begin if its arguments
> exactly match an outstanding begin and erroring in that case. Thus
> eliminating the need for the publicly visible request? Does this still
> support all "reasonable" use cases?
>
>
>   Barry
>
>
>
>
>
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