[petsc-dev] Slow ViennaCL performance on KSP ex12
Matthew Knepley
knepley at gmail.com
Mon Oct 12 18:28:48 CDT 2015
On Mon, Oct 12, 2015 at 3:25 PM, Mani Chandra <mc0710 at gmail.com> wrote:
> Here is the code: http://github.com/afd-illinois/grim, branch:opencl.
>
> Now using
>
> Kernel Builder for OpenCL API - compiler command line, version 1.4.0.134
> Copyright (C) 2014 Intel Corporation. All rights reserved.
>
>
> manic at bh27:~/grim_opencl/grim> ioc64 -input=computeresidual.cl
> -bo='-DOPENCL'
> -device='cpu'
>
> No command specified, using 'build' as default
>
Sorry if I am being obtuse, but I cannot find that source file in the repo
above. Can you give the direct link to the file?
> Using build options: -DOPENCL
> Setting target instruction set architecture to: Default (Advanced Vector
> Extension (AVX))
> OpenCL Intel CPU device was found!
> Device name: Intel(R) Xeon(R) CPU E5-2420 v2 @ 2.20GHz
> Device version: OpenCL 1.2 (Build 44)
> Device vendor: Intel(R) Corporation
> Device profile: FULL_PROFILE
> Compilation started
> Compilation done
> Linking started
> Linking done
> Device build started
> Device build done
> Kernel <ComputeResidual> was successfully vectorized
> Done.
> Build succeeded!
>
It definitely says it vectorized, but what code did it generate. Can you
post the object file since I do not have the compiler. I
have seen that message with really bad code before.
Thanks,
Matt
> Cheers,
> Mani
>
> On Mon, Oct 12, 2015 at 12:52 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Mon, Oct 12, 2015 at 2:44 PM, Mani Chandra <mc0710 at gmail.com> wrote:
>>>
>>> On Mon, Oct 12, 2015 at 12:36 PM, Barry Smith <bsmith at mcs.anl.gov>
>>> wrote:
>>>
>>>>
>>>> > On Oct 12, 2015, at 2:29 PM, Matthew Knepley <knepley at gmail.com>
>>>> wrote:
>>>> >
>>>> > On Mon, Oct 12, 2015 at 2:13 PM, Mani Chandra <mc0710 at gmail.com>
>>>> wrote:
>>>> > Hi Karl,
>>>> >
>>>> > My motivation was to avoid duplicating code for the CPU and the GPU.
>>>> This is important considering that it takes a long time to test and make
>>>> sure the code produces the right results.
>>>> >
>>>> > I guess, I can add a switch in my code with something like:
>>>> >
>>>> > if (usingCPU) use VecGetArray()
>>>> >
>>>> > else if (usingGPU) use VecViennaCLGetArray()
>>>> >
>>>> > and then wrap the pointers that the above functions return with
>>>> OpenCL buffers with the appropriate memory flags (CL_USE_HOST_PTR for CPU
>>>> and CL_ALLOC_.. for GPU)
>>>> >
>>>> > Hopefully, this will avoid unnecessary data transfers.
>>>> >
>>>> > I do not understand this comment at all. This looks crazy to me. The
>>>> whole point of having Vec
>>>> > is so that no one ever ever ever ever does anything like this. I saw
>>>> nothing in the thread that would
>>>> > compel you to do this. What are you trying to accomplish with this
>>>> switch?
>>>>
>>>>
>>> I'm trying to assemble the residual needed for SNES using an OpenCL
>>> kernel. The kernel operates on OpenCL buffers which can either live on the
>>> CPU or the GPU.
>>>
>>> I think it is useful to use OpenCL on the CPU basically because of
>>> vectorization and vector data types. If I had to write usual C code, I'd
>>> have to use all sorts of pragmas in icc to get the code to vectorize and
>>> even then its pretty hard.
>>>
>>
>> I would completely agree with you, if I thought the compiler actually
>> vectorized that code. I do not think that
>> is the case. Is there an example you have where you get vectorized
>> assembly?
>>
>> Thanks,
>>
>> Matt
>>
>>
>>> Mani
>>>
>>>
>>>> Matt,
>>>>
>>>> The current OpenCL code in PETSc is hardwired for GPU usage. So
>>>> the correct fix, I believe, is to add to the VecViennaCL wrappers support
>>>> for either using the GPU or the CPU.
>>>>
>>>> Barry
>>>>
>>>> >
>>>> > Matt
>>>> >
>>>> > Cheers,
>>>> > Mani
>>>> >
>>>> > On Sun, Oct 11, 2015 at 1:14 PM, Karl Rupp <rupp at iue.tuwien.ac.at>
>>>> wrote:
>>>> > Hi Mani,
>>>> >
>>>> > > Following
>>>> http://www.mcs.anl.gov/petsc/petsc-20/conference/Rupp_K.pdf
>>>> > (page 16), I ran KSP ex12 for two cases:
>>>> >
>>>> > 1) time ./ex12 -m 100 -n 100 -log_summary > log_summary_no_viennacl
>>>> >
>>>> > real 0m0.213s
>>>> > user 0m0.206s
>>>> > sys 0m0.004s
>>>> >
>>>> > 2) ./ex12 -m 100 -n 100 -vec_type viennacl -mat_type aijviennacl
>>>> > -log_summary > log_summary_with_viennacl
>>>> >
>>>> > real 0m20.296s
>>>> > user 0m46.025s
>>>> > sys 0m1.435s
>>>> >
>>>> > The runs have been performed on a CPU : AMD A10-5800K, with OpenCL
>>>> from
>>>> > AMD-APP-SDK-v3.0.
>>>> >
>>>> > there are a couple of things to note here:
>>>> >
>>>> > a) The total execution time contains the OpenCL kernel compilation
>>>> time, which is on the order of one or two seconds. Thus, you need much
>>>> larger problem sizes to get a good comparison.
>>>> >
>>>> > b) Most of the execution time is spent on VecMDot, which is optimized
>>>> for GPUs (CPUs are not an optimization goal in ViennaCL's OpenCL backend
>>>> because one can use just plain C/C++/whatever).
>>>> >
>>>> > c) My experiences with this AMD APU are quite mixed, as I've never
>>>> found a way to get more than 45% of STREAM bandwidth with OpenCL on the CPU
>>>> part. The integrated GPU, however, reached 80% without much effort. This is
>>>> particularly remarkable as both CPU and GPU share the same DDR3 memory
>>>> link. Thus, it is more than unlikely that you will ever beat the
>>>> performance of PETSc's native types.
>>>> >
>>>> >
>>>> >
>>>> > Attached are:
>>>> > 1) configure.log for the petsc build
>>>> > 2) log summary without viennacl
>>>> > 3) log summary with viennacl
>>>> > 4) OpenCL info for the system on which the runs were performed
>>>> >
>>>> > Perhaps the reason for the slow performance are superfluous copies
>>>> being
>>>> > performed, which need not occur when running ViennaCL on the CPU.
>>>> > Looking at
>>>> >
>>>> http://www.mcs.anl.gov/petsc/petsc-dev/src/vec/vec/impls/seq/seqviennacl/vecviennacl.cxx
>>>> :
>>>> >
>>>> > /* Copies a vector from the CPU to the GPU unless we already have an
>>>> up-to-date copy on the GPU */
>>>> > PetscErrorCode VecViennaCLCopyToGPU(Vec v)
>>>> > {
>>>> > PetscErrorCode ierr;
>>>> >
>>>> > PetscFunctionBegin;
>>>> > ierr = VecViennaCLAllocateCheck(v);CHKERRQ(ierr);
>>>> > if (v->map->n > 0) {
>>>> > if (v->valid_GPU_array == PETSC_VIENNACL_CPU) {
>>>> > ierr =
>>>> PetscLogEventBegin(VEC_ViennaCLCopyToGPU,v,0,0,0);CHKERRQ(ierr);
>>>> > try {
>>>> > ViennaCLVector *vec = ((Vec_ViennaCL*)v->spptr)->GPUarray;
>>>> > viennacl::fast_copy(*(PetscScalar**)v->data,
>>>> *(PetscScalar**)v->data + v->map->n, vec->begin());
>>>> > ViennaCLWaitForGPU();
>>>> > } catch(std::exception const & ex) {
>>>> > SETERRQ1(PETSC_COMM_SELF,PETSC_ERR_LIB,"ViennaCL error: %s",
>>>> ex.what());
>>>> > }
>>>> > ierr =
>>>> PetscLogEventEnd(VEC_ViennaCLCopyToGPU,v,0,0,0);CHKERRQ(ierr);
>>>> > v->valid_GPU_array = PETSC_VIENNACL_BOTH;
>>>> > }
>>>> > }
>>>> > PetscFunctionReturn(0);
>>>> > }
>>>> >
>>>> > When running ViennaCL with OpenCL on the CPU, the above function
>>>> should
>>>> > maybe be modified?
>>>> >
>>>> > Unfortunately that is quite hard: OpenCL manages its own memory
>>>> handles, so 'injecting' memory into an OpenCL kernel that is not allocated
>>>> by the OpenCL runtime is not recommended, fairly tricky, and still involves
>>>> some overhead. As I see no reason to run OpenCL on a CPU, I refrained from
>>>> adding this extra code complexity.
>>>> >
>>>> > Overall, I recommend rerunning the benchmark on more powerful
>>>> discrete GPUs with GDDR5 (or on-chip memory). Otherwise you won't see any
>>>> performance benefits.
>>>> >
>>>> > Hope this sheds some light on things :-)
>>>> >
>>>> > Best regards,
>>>> > Karli
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> > --
>>>> > What most experimenters take for granted before they begin their
>>>> experiments is infinitely more interesting than any results to which their
>>>> experiments lead.
>>>> > -- Norbert Wiener
>>>>
>>>>
>>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>
>
--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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