[vtk-developers] New SMP implementation for image filters

[Andrew Maclean]

David,
   This is going to be really useful for tuning applications.
   I am trying it out on several different machines all using TBB.

  Thank You for all your efforts in writing this.

Regards
   Andrew


---------- Forwarded message ----------
> From: David Gobbi <david.gobbi at gmail.com>
> To: VTK Developers <vtk-developers at vtk.org>
> Cc:
> Date: Thu, 10 Mar 2016 06:19:22 -0700
> Subject: [vtk-developers] New SMP implementation for image filters
> Hi All,
>
> As of today, the VTK imaging pipeline has a new implementation based
> on vtkSMPTools.  A big chunk of this work is from the "Shared Memory
> Parallelism" Google Summer of Code project for 2015.
>
> Many of you will not see much difference, because the the imaging
> pipeline has been multithreaded for many, many years.  The advantages
> of the new implementation are that it provides more performance tuning,
> and it will be easier to maintain as VTK moves forward.  The new SMP
> code will only be used if you set VTK_SMP_IMPLEMENTATION_TYPE
> to OpenMP or TBB in cmake.  Otherwise, the old threading code will be
> used.
>
> The main difference with the new implementation is the load balancing,
> Previously, the data was divided evenly (or roughly so) among the threads.
> So if there were 10 slices and 8 threads, then 6 of the threads would get
> one slice, and 2 of the threads would get 2 slices.  Now, things follow
> a different paradigm: the data is divided into a large number of pieces
> that are queued for a thread pool.  Pieces are assigned to threads based
> on thread availability, and overall CPU utilization is improved because
> the load balancing is done dynamically.
>
>
> So, what performance tuning is available and what gains can you expect
> to see?  Let's look at the tuning first.  The following new methods are
> available for image filters derived from vtkThreadedImageAlgorithm:
>
> // Enable or disable the new behavior for all filters.
> static void SetGlobalDefaultEnableSMP(bool);
>
> // Enable or disable the new behavior for just one filter.
> void SetEnableSMP(bool);
>
> // Set the size of the image pieces that will be sent to the
> // threads for execution. The ideal size will depend on the
> // memory use pattern of the image filter that is being used,
> // but the default size of 65536 bytes works well for most.
> void SetDesiredBytesPerPiece(vtkIdType bytes);
>
> // Set the minimum size of piece to send to a thread.  Obviously
> // giving the threads one voxel at a time would be inefficient.
> // A default minimum size of [16,1,1] ensures some contiguity.
> void SetMinimumPieceSize(const int size[3]);
>
> // Use pieces that are roughly square in shape (or cubic for 3D
> // images).  This provides best results for filters that operate
> // on a neighborhood around each output voxel.
> void SetSplitModeToBlock();
>
> // Use slab-shaped pieces.  This provides best results for filters
> // that perform simple operations on the scalars, such as color mapping.
> void SetSplitModeToSlab();
>
> // Use thin rod-shaped pieces.  This also provides good results
> // filters like color mapping.  I haven't yet found any algorithms
> // for which this splitting method is the best to use.
> void SetSplitModeToBeam();
>
>
> The performance improvements to be gained by tweaking these parameters
> are modest, usually less than 20%, but sometimes much higher.  As part
> of this patch, I have added a new example to VTK called "ImageBenchmark"
> that makes it easy to run a filter under different conditions in order to
> optimize
> the settings.  I'll create a wiki page in the future, but for now, you can
> run "ImageBenchmark --help" to get a comprehensive description of all the
> options (assuming that you built VTK with BUILD_EXAMPLES=ON).
>
> Cheers,
>   - David
>
>
>
-- 
___________________________________________
Andrew J. P. Maclean

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