[vtkusers] vtkMultiThreader question

[Sujin Philip]

Hi Laurent,

If you are using a recent version of VTK you may want to give vtkSMPTools a
try. It is a new, simplified framework for doing parallelism in VTK and it
can target TBB or OpenMP as backends. It has support for a "parallel_for"
and maybe a simpler way to implement your code. Please refer to this blog:
http://www.kitware.com/blog/home/post/915. Let me know if you have any
questions about this framework.

Thanks
Sujin


On Sun, Oct 11, 2015 at 12:08 PM, Dan Lipsa <dan.lipsa at kitware.com> wrote:

> Laurent,
> I think a good function to look at is
>
>
> http://www.vtk.org/gitweb?p=VTK.git;a=blob;f=Common/Core/Testing/Cxx/TestConditionVariable.cxx
>
> When you do SingleMethodExecute, the method you set is run an all
> processors.
> Then, by looking at threadId you can execute different parts of the method
> on different processors.
>
> So for your 2 for loops you can set different range indexes to run on
> different processors - if there are no dependencies between different index
> computations.
>
> This would be the same as spliting a 2D array into chunks and executing a
> chunk on each processor.
> However, you'll have to do this explicitly, VTK does not do it
> automatically.
>
> Dan
>
>
>
>
>
> On Fri, Oct 9, 2015 at 6:22 PM, Chauvin, Laurent,Ph.D. <
> lchauvin at bwh.harvard.edu> wrote:
>
>> Thank you for your answer.
>>
>> I actually looked at it, but I’m still not sure what happen in this
>> example.
>> Is each function run in one separated thread, or each function run in
>> multiple threads ?
>>
>> Here is what I have:
>>
>> myMethod(void* arg)
>> {
>> For loop
>>               For loop
>>                     Computation()
>> }
>>
>> And then I use vtkMultiThreader to execute this method like this:
>>
>> Threader->SetSingleMethod(myMethod,NULL);
>> Threader->SingleMethodExecute()
>>
>> What I was expecting was that thread detect the function could be split
>> in multiple thread (because of the loops) and run each iteration of
>> Computation() in a different thread.
>>
>> But I think I got it wrong. I think I have to do something like that:
>>
>> For Loop
>>      For Loop
>>          Threader->SetSingleMethod(myMethod,data[i][j])
>>          Threader->SingleMethodExecute()
>>
>> Is it correct ?
>>
>> Thank you.
>> -Laurent
>>
>> From: Dan Lipsa <dan.lipsa at kitware.com>
>> Date: Friday, October 9, 2015 at 11:18 PM
>> To: "Chauvin, Laurent,Ph.D." <lchauvin at bwh.harvard.edu>
>> Cc: Franz Neubert via vtkusers <vtkusers at vtk.org>
>> Subject: Re: [vtkusers] vtkMultiThreader question
>>
>> Laurent,
>> Did you look at the tests that come with this class. Do they work?
>>
>>
>> http://www.vtk.org/doc/nightly/html/c2_vtk_t_12.html#c2_vtk_t_vtkMultiThreader
>>
>> Dan
>>
>>
>> On Fri, Oct 9, 2015 at 5:00 PM, Chauvin, Laurent,Ph.D. <
>> lchauvin at bwh.harvard.edu> wrote:
>>
>>> Hello,
>>>
>>> I’m trying to use a vtkMultiThreader to compute several thing in
>>> parallel.
>>> However I’m not sure how it works.
>>>
>>> Here is what I did.
>>> I created a single method that calculate some metric pairwise. So I have
>>> 2 for loops in this method.
>>> What I want to do, is to have one thread per iteration (kind of), so I
>>> can calculate all metrics simultaneously.
>>>
>>> So I created a vtkMultiThreader and set the single method, which
>>> contains the loops, and executed it. However when I do that, I can only see
>>> one core working. I thought SingleMethod was using multiple thread to run
>>> this method. Is it true ? Why then it is not using all my other cores ?
>>>
>>> Should I put the loops out of the single method then, and spawn a thread
>>> for each iteration, with different data as input ?
>>>
>>> Thank you very much.
>>> -Laurent
>>>
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