[Paraview-developers] Using OpenMP for custom plugins/filters

[Sujin Philip]

Hi Michael,

- What is the default SMP backend if I do not configure anything during
ParaView compilation (I assume “Sequential”)?
Sequential is the default backend.

- With OpenMP, will I have to call vtkSMPTools::Initialize(nThreads)
explicitly with nThreads determined by myself, or will it honor e.g.
OMP_NUM_THREADS automatically?
It is not necessary to call vtkSMPTools::Initialize. If initialize is not
called and OMP_NUM_THREADS is set, it will be used.

- Am I correct in the assumption that the backend type should not change
the results, i.e. that on hosts where I do not know which SMP backend is
configured, I can assume my algorithms “just work”?
Yes, the different backends should produce the same results. The only
noticeable difference should be in the performance. If the results are not
the same then its a bug.

Thanks
Sujin


On Tue, Mar 8, 2016 at 9:08 AM, Schlottke-Lakemper, Michael <
m.schlottke-lakemper at aia.rwth-aachen.de> wrote:

> Hi Utkarsh,
>
> Thanks for the fast answer. I have a a few follow-up questions, if I may:
>
> - What is the default SMP backend if I do not configure anything during
> ParaView compilation (I assume “Sequential”)?
> - With OpenMP, will I have to call vtkSMPTools::Initialize(nThreads)
> explicitly with nThreads determined by myself, or will it honor e.g.
> OMP_NUM_THREADS automatically?
> - If I have to call vtkSMPTools::Initialize(nThreads) explicitly, where
> should it be done in case of a plugin? In the plugin’s ctor, in the
> RequestData method, or someplace else?
> - Am I correct in the assumption that the backend type should not change
> the results, i.e. that on hosts where I do not know which SMP backend is
> configured, I can assume my algorithms “just work”?
>
> Thanks a lot in advance
>
> Michael
>
> > On 08 Mar 2016, at 14:39 , Utkarsh Ayachit <utkarsh.ayachit at kitware.com>
> wrote:
> >
> > Michael,
> >
> > The VTK-SMP approach is indeed the way to go for this. To make the
> > backend use OpenMP, you simply compile VTK/ParaView with
> > VTK_SMP_IMPLEMENTATION_TYPE set to OpenMP. The programming model
> > described in the SMP Guide PDF has been implemented for all supported
> > backends including OpenMP and TBB.
> >
> > Utkarsh
> >
> > On Tue, Mar 8, 2016 at 6:22 AM, Schlottke-Lakemper, Michael
> > <m.schlottke-lakemper at aia.rwth-aachen.de> wrote:
> >> Hi everyone,
> >>
> >> We have a custom parallel reader plugin that works well with our files
> even
> >> on large-ish (100-1000) number of cores for the pvserver. Recently we’ve
> >> added the capability to also create ghost cells with this reader such
> that
> >> MPI domain boundaries become transparent to filters that need neighbor
> data,
> >> e.g., for gradient computations. However, this ghost cell generation
> takes
> >> up quite some time, so we would like to keep the number of MPI ranks at
> a
> >> minimum and use hybrid MPI-OpenMP parallelism for our plugin.
> >>
> >> My question is thus if it is possible to use OpenMP inside a custom
> plugin?
> >> And if yes, is there a canonical way to do it (maybe an example plugin)
> or
> >> is it going to be a hack?
> >>
> >> When searching the interwebs for ParaView and OpenMP, I only came up
> with an
> >> old VTK mailing list thread from 2014
> >> (http://public.kitware.com/pipermail/vtkusers/2014-March/083399.html),
> in
> >> which David (DeMarle) refers to some initial documentation of the SMP
> >> framework (http://www.vtk.org/Wiki/images/3/3b/VTK_SMP_Guide.pdf).
> However,
> >> the linked PDF does not contain references to OpenMP.
> >>
> >> Thanks a lot in advance!
> >>
> >> Michael
> >>
> >> --
> >> Michael Schlottke-Lakemper
> >>
> >> Chair of Fluid Mechanics and Institute of Aerodynamics
> >> RWTH Aachen University
> >> Wüllnerstraße 5a
> >> 52062 Aachen
> >> Germany
> >>
> >> Phone: +49 (241) 80 95188
> >> Fax: +49 (241) 80 92257
> >> Mail: m.schlottke-lakemper at aia.rwth-aachen.de
> >> Web: http://www.aia.rwth-aachen.de
> >>
> >>
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