[petsc-dev] Why no SpGEMM support in AIJCUSPARSE and AIJVIENNACL?

[Karl Rupp]

Hi Richard,

CPU spGEMM is about twice as fast even on the GPU-friendly case of a 
single rank: http://viennacl.sourceforge.net/viennacl-benchmarks-spmm.html

I agree that it would be good to have a GPU-MatMatMult for the sake of 
experiments. Under these performance constraints it's not top priority, 
though.

Best regards,
Karli


On 10/3/19 12:00 AM, Mills, Richard Tran via petsc-dev wrote:
> Fellow PETSc developers,
> 
> I am wondering why the AIJCUSPARSE and AIJVIENNACL matrix types do not 
> support the sparse matrix-matrix multiplication (SpGEMM, or MatMatMult() 
> in PETSc parlance) routines provided by cuSPARSE and ViennaCL, 
> respectively. Is there a good reason that I shouldn't add those? My 
> guess is that support was not added because SpGEMM is hard to do well on 
> a GPU compared to many CPUs (it is hard to compete with, say, Intel Xeon 
> CPUs with their huge caches) and it has been the case that one would 
> generally be better off doing these operations on the CPU. Since the 
> trend at the big supercomputing centers seems to be to put more and more 
> of the computational power into GPUs, I'm thinking that I should add the 
> option to use the GPU library routines for SpGEMM, though. Is there some 
> good reason to *not* do this that I am not aware of? (Maybe the CPUs are 
> better for this even on a machine like Summit, but I think we're at the 
> point that we should at least be able to experimentally verify this.)
> 
> --Richard