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

[Sujin Philip]

Hi Michael

It will be the same as a regular OpenMP program without the OMP_NUM_THREADS
environment variable.

-Sujin


On Tue, Mar 8, 2016 at 9:39 AM, Schlottke-Lakemper, Michael <
m.schlottke-lakemper at aia.rwth-aachen.de> wrote:

> Hi Sujin,
>
> Thanks again for the fast and helpful answer! One last question: if I do
> not call vtkSMPTools::Initialize() and OMP_NUM_THREADS is not set, which
> number of threads will be used? Will it be the same as if running a normal
> OpenMP program with OMP_NUM_THREADS not set or will a different value be
> used?
>
> Michael
>
> On 08 Mar 2016, at 15:22 , Sujin Philip <sujin.philip at kitware.com> wrote:
>
> 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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