[petsc-dev] GPU preconditioners

[Andrea Lani]

Thanks Karl! Yes, please send your preprint, I'd like to learn more about
this.
Best regards

Andrea


On Fri, Jan 17, 2014 at 10:33 PM, Karl Rupp <rupp at mcs.anl.gov> wrote:

> Hi Andrea,
>
>
> > Well, I have 9 equations, so 9x9 I guess...
>
> Ok, this is just in the range where it is technically meaningful (register
> sizes) but getting challenging implementation-wise (explicit inversion
> formulas vs. Gauss with pivoting)
>
>
>
>  I hope the one you are mentioning was a major bug, because what I get is
>> seriously wrong: while on single GPU (KSPGMRES+PCASM) I get a residual
>> of +0.72, on 8-cores/GPU I get -1.00 at the first time step, just to
>> make an example. Can this be due to the bug you are saying or you can
>> suspect something more?
>>
>
> Yes, this was a major bug, breaking the matrix-vector product when using
> multiple MPI ranks with GPUs.
>
>
>
>  What should I do then? wait for the valgrind fix which is underway and
>> then update? Can you please notify me when this is fixed? I'm writing a
>> final report for a project and I would like to include this feature
>> fully fixed if possible.
>>
>
> I will merge the fix to master tomorrow when I'm back on my main GPU
> machine (there do not seem to be any problems in 'next' with the patch) and
> fix the valgrind complaints separately. The second issue is not directly
> related to the first, it only happens in the same module.
>
>
>  Another question, what do you exactly mean by "order the unknowns
>> properly" in this case?
>>
>
> If you build the elimination graph for the triangular factors of ILU
> preconditioners, then the ordering of the unknowns (i.e. the way you assign
> the degrees of freedoms (DOFs) on your mesh) can have a considerable
> influence on the amount of parallelism. The Cuthill-McKee algorithm for
> example is quite good for reducing the bandwidth of a sparse matrix, but it
> may also reduce the amount of parallelism for ILU0 factors compared to e.g.
> a red-black ordering of the DOFs. I can send you a preprint if you're
> interested.
>
> Best regards,
> Karli
>
>


-- 
Dr. Andrea
Lani
Senior Research Engineer, PhD
Aeronautics & Aerospace dept., CFD group
Von Karman Institute for Fluid Dynamics
Chausse de Waterloo 72,
B-1640, Rhode-Saint-Genese,  Belgium
fax  : +32-2-3599600
work : +32-2-3599769
*lani at vki.ac.be <lani at vki.ac.be>*
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