[petsc-dev] PETSc programming model for multi-core systems

[Barry Smith]

On Nov 11, 2010, at 6:58 PM, Matthew Knepley wrote:

> On Fri, Nov 12, 2010 at 11:54 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
> On Nov 11, 2010, at 6:15 PM, Matthew Knepley wrote:
> 
> > On Fri, Nov 12, 2010 at 9:52 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> >   What should we use a for programming model for PETSc on multi-core systems? Currently for conventional multicore we have only have one MPI process per core and for GPU we have subclasses of Vec and Mat with custom CUDA code.
> >
> >   Should we introduce subclasses of Vec and Mat built on pthreads (this is what Bill G recommends, and not to use OpenMP)?
> >
> > pthreads are a nightmare and do not do vectorization right, which is necessary here.
> 
>   What do you mean do not vectorize?
> 
> I mean its easy to tell a thread to do something, but I was not aware that pthreads had nice support
> for telling all threads to do something at the same time. On a multicore, you want vector instructions,

   Why do you want vector instructions on multicore? Since each core has a full instruction stream what do you get by vectorization? Is it because you think that is the way to best stream the data from main memory or something else?

   Barry

> and on a GPU you basically have no choice.
>  
> > I am for OpenCL/CUDA. Eventually
> > I think OpenCL will take its head out of its ass and be as nice as CUDA.
> 
>   That's fine for GPUs but what about 8 core or 12 core Intel processors? CUDA for that also?
> 
> OpenCL already runs on these, but the interface is too low-level. It should look more like CUDA. In CUDA,
> you have constructs for vectorization, but you can also branch as Jed indicated. This is slow on a GPU,
> but would be fine on a multicore.
> 
> Jed, do you see obvious shortcomings of CUDA for these multicore machines?
> 
>    Matt
>  
> 
>   Barry
> 
> >
> >   Matt
> >
> >   Is there a way to have some kind of consistent model between conventional multicore and GPU multi-core? If not the same code. What about this MCUDA stuff?
> >
> >   Barry
> > --
> > 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