Re: [ViennaCL-devel] Forking multiple OpenCL contexts

[Charles D. <cde...@gm...>]

Excellent, thank you for explaining that.  One followup thing, I am trying
a trivial function to test this:

int findContext(int x){
    viennacl::context ctx(viennacl::ocl::get_context(static_cast<long>(x)))
    return x
}

but I keep getting a compiler error:

>error: variable ‘viennacl::context ctx’ has initializer but incomplete type

Did I miss something?

Thanks,
Charles


On Mon, May 9, 2016 at 4:09 PM, Karl Rupp <ru...@iu...> wrote:

> Hi Charles,
>
> setContext() is not thread-safe, so if mclapply() is executing in
> parallel, there will be a race. MPI works across processes, so globals are
> not shared (and hence setContext() has no problems).
>
> If you want to run multithreaded, you should manage the contexts
> explicitly. That is, pass a viennacl::context object to the respective
> constructors of viennacl::vector, viennacl::matrix, etc. You can find
> examples here:
>
> https://github.com/viennacl/viennacl-dev/blob/master/examples/tutorial/multithreaded.cpp
>
> https://github.com/viennacl/viennacl-dev/blob/master/examples/tutorial/multithreaded_cg.cpp
>
> Best regards,
> Karli
>
>
>
>
> On 05/09/2016 10:32 PM, Charles Determan wrote:
>
>> I am trying to use multiple GPU's in parallel using ViennaCL through my
>> gpuR package.  In essence the context is selected at the R level (one
>> context per device already initialized).  Then a matrix is created on
>> that device.  The code looks like this:
>>
>> create_matrix <- function(id){
>> # sets context
>> setContext(id)
>> # create matrix
>> mat <- vclMatrix(rnorm(16), nrow=4, ncol=4)
>> return(mat)
>> }
>>
>> # a fork over context 1 and 2
>> out <- mclapply(1:2, function(x) create_matrix(x))
>>
>> Yet, strangely this just hangs.  It doesn't return anything.  Perhaps I
>> am missing something in how OpenCL contexts are handed in parallel?  I
>> ask as I recall that PETSC had some multi GPU functionality with MPI.
>> The above makes sense to me without MPI but again I may be missing
>> something.
>>
>> The actual copying happens in this file at line 36
>> (https://github.com/cdeterman/gpuR/blob/develop/src/dynVCLMat.cpp) just
>> after "starting to copy".  It seems almost like it a problem with the
>> class objects (defined in
>>
>> https://github.com/cdeterman/gpuR/blob/develop/inst/include/gpuR/dynVCLMat.hpp
>> ).
>>
>> Any insight would be appreciated.
>>
>> Regards,
>> Charles
>>
>>
>>
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