On Tue, Nov 29, 2011 at 2:38 AM, Fredrik Heffer Valdmanis <fredva at ifi.uio.no > wrote:
> 2011/10/28 Matthew Knepley <knepley at gmail.com> > >> On Fri, Oct 28, 2011 at 10:24 AM, Fredrik Heffer Valdmanis < >> fredva at ifi.uio.no> wrote: >> >>> Hi, >>> >>> I am working on integrating the new GPU based vectors and matrices into >>> FEniCS. Now, I'm looking at the possibility for getting some speedup during >>> finite element assembly, specifically when inserting the local element >>> matrix into the global element matrix. In that regard, I have a few >>> questions I hope you can help me out with: >>> >>> - When calling MatSetValues with a MATSEQAIJCUSP matrix as parameter, >>> what exactly is it that happens? As far as I can see, MatSetValues is not >>> implemented for GPU based matrices, neither is the mat->ops->setvalues set >>> to point at any function for this Mat type. >>> >> >> Yes, MatSetValues always operates on the CPU side. It would not make >> sense to do individual operations on the GPU. >> >> I have written batched of assembly for element matrices that are all the >> same size: >> >> >> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatSetValuesBatch.html >> >> >>> - Is it such that matrices are assembled in their entirety on the CPU, >>> and then copied over to the GPU (after calling MatAssemblyBegin)? Or are >>> values copied over to the GPU each time you call MatSetValues? >>> >> >> That function assembles the matrix on the GPU and then copies to the CPU. >> The only time you do not want this copy is when >> you are running in serial and never touch the matrix afterwards, so I >> left it in. >> >> >>> - Can we expect to see any speedup from using MatSetValuesBatch over >>> MatSetValues, or is the batch version simply a utility function? This >>> question goes for both CPU- and GPU-based matrices. >>> >> >> CPU: no >> >> GPU: yes, I see about the memory bandwidth ratio >> >> >> Hi, > > I have now integrated MatSetValuesBatch in our existing PETSc wrapper > layer. I have tested matrix assembly with Poisson's equation on different > meshes with elements of varying order. I have timed the single call to > MatSetValuesBatch and compared that to the total time consumed by the > repeated calls to MatSetValues in the old implementation. I have the > following results: > > Poisson on 1000x1000 unit square, 1st order Lagrange elements: > MatSetValuesBatch: 0.88576 s > repeated calls to MatSetValues: 0.76654 s > > Poisson on 500x500 unit square, 2nd order Lagrange elements: > MatSetValuesBatch: 0.9324 s > repeated calls to MatSetValues: 0.81644 s > > Poisson on 300x300 unit square, 3rd order Lagrange elements: > MatSetValuesBatch: 0.93988 s > repeated calls to MatSetValues: 1.03884 s > > As you can see, the two methods take almost the same amount of time. > What behavior and performance should we expect? Is there any way to > optimize the performance of batched assembly? > Almost certainly it is not dispatching to the CUDA version. The regular version just calls MatSetValues() in a loop. Are you using a SEQAIJCUSP matrix? > I also have a problem with Thrust throwing std::bad_alloc on some calls to > MatSetValuesBatch. The exception originates in thrust::device_ptr<void> > thrust::detail::device::cuda::malloc<0u>(unsigned long). It seems to be > thrown when the number of double values I send to MatSetValuesBatch > approaches 30 million. I am testing this on a laptop with 4 GB RAM and a > GeForce 540 M (1 GB memory), so the 30 million doubles are far away from > exhausting my memory, both on the host and device side. Any clues on what > causes this problem and how to avoid it? > It uses more memory that just the values. I would have to look at the specific case, but I assume that the memory is exhausted. Matt > Thanks, > > Fredrik > > -- 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20111129/3a466361/attachment-0001.htm>
