[petsc-dev] Petsc+ViennaCL usage

[Mani Chandra]

I like to pass in "-cl-nv-verbose" for compilation on nvidia cards. Also, I
pass in parameters, for ex "-D Nx=128 -D Ny=128". I'll look at the ViennaCL
api.

Thanks,
Mani


On Tue, Jan 21, 2014 at 3:28 AM, Karl Rupp <rupp at mcs.anl.gov> wrote:

> Hi Mani,
>
>
> > I have a few questions regarding the usage of Viennacl in Petsc.
>
>>
>> 1) In the residual evaluation function:
>>
>> PetscErrorCode ComputeResidual(TS ts,
>>                                 PetscScalar t,
>>                                 Vec X, Vec dX_dt,
>>                                 Vec F, void *ptr)
>> {
>>      DM da;
>>      Vec localX;
>>      TSGetDM(ts, &da)
>>      DMGetLocalVector(da, &localX);
>>
>>      DMGlobalToLocalBegin(da, X, INSERT_VALUES, localX);
>>      DMGlobalToLocalEnd(da, X, INSERT_VALUES, localX);
>>
>>      viennacl::vector<PetscScalar> *x, *f;
>>      VecViennaCLGetArrayWrite(localX, &x);
>>      VecViennaCLGetArrayRead(F, &f);
>>
>>      viennacl::ocl::enqueue(myKernel(*x, *f));
>> //Should it be viennacl::ocl::enqueue(myKernel(x, f))?
>>
>
> It should be viennacl::ocl::enqueue(myKernel(*x, *f));
> Usually you also want to pass the sizes to the kernel. Don't forget to
> cast the sizes to the correct types (e.g. cl_uint).
>
>
>
>       VecViennaCLRestoreArrayWrite(localX, &x);
>>      VecViennaCLRestoreArrayRead(F, &f);
>>      DMRestoreLocalVector(da, &localX);
>> }
>>
>> Will the residual evaluation occur on the GPU/accelerator depending on
>> where we choose the ViennaCL array computations to occur? As I
>> understand, if we simply use VecGetArray in the residual evaluation
>> function, then the residual evaluation is still done on the CPU even
>> though the solves are done on the GPU.
>>
>
> If you use VecViennaCLGetArrayWrite(), the data will be valid on the GPU,
> so your residual evaluation should happen in the OpenCL kernel you provide.
> This is already the case in the code snippet above.
>
>
>
>  2) How does one choose on which device the ViennaCL array computations
>> will occur? I was looking for some flags like -viennacl
>> cpu/gpu/accelerator but could not find any in -help.
>>
>
> Use one out of
>  -viennacl_device_cpu
>  -viennacl_device_gpu
>  -viennacl_device_accelerator
>
>
>
>  3) How can one pass compiler flags when building OpenCL kernels in
>> ViennaCL?
>>
>
> You could do that through the ViennaCL API directly, but I'm not sure
> whether you really want to do this. Which flags do you want to set? My
> experience is that these options have little to no effect on performance,
> particularly for the memory-bandwidth-limited case. This is also the reason
> why I haven't provided a PETSc routine for this.
>
> Best regards,
> Karli
>
>
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