[petsc-users] PETSc with modern C++

[Filippo Leonardi]

You are in fact right, it is the same speedup  of approximatively 2.5x
(with 2 ranks), my brain rounded up to 3. (This was just a test done in 10
min on my Workstation, so no pretence to be definite, I just wanted to have
an indication).

As you say, I am using OpenBLAS, so I wouldn't be surprised of those
results. If/when I use MKL (or something similar), I really do not expect
such an improvement).

Since you seem interested (if you are interested, I can give you all the
details): the comparison I make, is with "petscxx" which is my template
code (which uses a single loop) using AVX (I force PETSc to align the
memory to 32 bit boundary and then I use packets of 4 doubles). Also notice
that I use vectors with nice lengths, so there is no need to "peel" the end
of the loop. The "PETSc" simulation is using PETSc's VecMAXPY.

On Tue, 4 Apr 2017 at 19:12 Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   MAXPY isn't really a BLAS 1 since it can reuse some data in certain
> vectors.
>
>
> > On Apr 4, 2017, at 10:25 AM, Filippo Leonardi <filippo.leon at gmail.com>
> wrote:
> >
> > I really appreciate the feedback. Thanks.
> >
> > That of deadlock, when the order of destruction is not preserved, is a
> point I hadn't thought of. Maybe it can be cleverly addressed.
> >
> > PS: If you are interested, I ran some benchmark on BLAS1 stuff and, for
> a single processor, I obtain:
> >
> > Example for MAXPY, with expression templates:
> > BM_Vector_petscxx_MAXPY/8 38 ns 38 ns 18369805
> > BM_Vector_petscxx_MAXPY/64 622 ns 622 ns 1364335
> > BM_Vector_petscxx_MAXPY/512 281 ns 281 ns 2477718
> > BM_Vector_petscxx_MAXPY/4096 2046 ns 2046 ns 349954
> > BM_Vector_petscxx_MAXPY/32768 18012 ns 18012 ns 38788
> > BM_Vector_petscxx_MAXPY_BigO 0.55 N 0.55 N
> > BM_Vector_petscxx_MAXPY_RMS 7 % 7 %
> > Direct call to MAXPY:
> > BM_Vector_PETSc_MAXPY/8 33 ns 33 ns 20973674
> > BM_Vector_PETSc_MAXPY/64 116 ns 116 ns 5992878
> > BM_Vector_PETSc_MAXPY/512 731 ns 731 ns 963340
> > BM_Vector_PETSc_MAXPY/4096 5739 ns 5739 ns 122414
> > BM_Vector_PETSc_MAXPY/32768 46346 ns 46346 ns 15312
> > BM_Vector_PETSc_MAXPY_BigO 1.41 N 1.41 N
> > BM_Vector_PETSc_MAXPY_RMS 0 % 0 %
> >
> > And 3x speedup on 2 MPI ranks (not much communication here, anyway). I
> am now convinced that this warrants some further investigation/testing.
> >
> >
> > On Tue, 4 Apr 2017 at 01:08 Jed Brown <jed at jedbrown.org> wrote:
> > Matthew Knepley <knepley at gmail.com> writes:
> >
> > >> BLAS. (Here a interesting point opens: I assume an efficient BLAS
> > >>
> > >> implementation, but I am not so sure about how the different BLAS do
> > >> things
> > >>
> > >> internally. I work from the assumption that we have a very well tuned
> BLAS
> > >>
> > >> implementation at our disposal).
> > >>
> > >
> > > The speed improvement comes from pulling vectors through memory fewer
> > > times by merging operations (kernel fusion).
> >
> > Typical examples are VecMAXPY and VecMDot, but note that these are not
> > xGEMV because the vectors are independent arrays rather than single
> > arrays with a constant leading dimension.
> >
> > >> call VecGetArray. However I will inevitably foget to return the array
> to
> > >>
> > >> PETSc. I could have my new VecArray returning an object that restores
> the
> > >>
> > >> array
> > >>
> > >> when it goes out of scope. I can also flag the function with
> [[nodiscard]]
> > >> to
> > >>
> > >> prevent the user to destroy the returned object from the start.
> > >>
> > >
> > > Jed claims that this pattern is no longer preferred, but I have
> forgotten
> > > his argument.
> > > Jed?
> >
> > Destruction order matters and needs to be collective.  If an error
> > condition causes destruction to occur in a different order on different
> > processes, you can get deadlock.  I would much rather have an error
> > leave some resources (for the OS to collect) than escalate into
> > deadlock.
> >
> > > We have had this discussion for years on this list. Having separate
> names
> > > for each type
> > > is really ugly and does not achieve what we want. We want smooth
> > > interoperability between
> > > objects with different backing types, but it is still not clear how to
> do
> > > this.
> >
> > Hide it internally and implicitly promote.  Only the *GetArray functions
> > need to be parametrized on numeric type.  But it's a lot of work on the
> > backend.
>
>
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