[Trilinos-Users] AztecOO conjugate gradient

[C.S. Natarajan]

My previous e-mail might be a bit misleading..I just mean that 15s doesn't
sound like too long not that it sounds right.
Also, maybe a good idea to try out AnalyzeHierarchy method of the
preconditioner...this way you might get an idea as to which smoother is
performing well.

On Thu, May 9, 2013 at 5:04 PM, C.S. Natarajan <csnataraj at gmail.com> wrote:

> Hi Bart,
>            At the face of it, 15s doesn't sound bad. I would think that
> Gigabit ethernet probably doesn't matter for now as the operations are
> memory bandwidth bound. Sorry, I am not sure I uderstand what 5594877 means
>
> If this problem is a purely isotropic poisson problem I am a little
> surprised that SGS is blowing up. SGS algorithm is essential local
> processor based, so I would think that it makes sense that the solun
> /convergence rate would depend on nproc but I dont know why it would blow
> up. Maybe one of the Trilinos developers can clarify on this issue?
>
> Chebyshev/Polynomial smoothers are definitely a solid option depending on
> the problem. In case you dont have this already, here is a paper by some of
> the main developers of ML. http://www.columbia.edu/~ma2325/adams_poly.pdf.
>
> Cheers,
> C.S.N
>  On Thu, May 9, 2013 at 2:09 PM, Bart Janssens <bart at bartjanssens.org>wrote:
>
>>  On Thu, May 9, 2013 at 6:33 PM, C.S. Natarajan <csnataraj at gmail.com>wrote:
>>
>>>                Bart I have run across the same issue. Out of curiosity,
>>> is this with periodic BC’s in any of the directions? I have had a similar
>>> problem (using both 2nd order FD and FE) when using Belos+ML (IFPACK
>>> IC) on a 3D anisotropic(from the coefficients) poisson problem. Does SGS
>>> smoothing work when using Belos+ML?
>>>
>>
>>
>> Hi,
>>
>> I poked around a little more, and with these settings I can solve the
>> system with Belos Block CG:
>>  ML ->
>>   Reuse Fine Level Smoother = 0
>>   Base Method Defaults = SA   [default]
>>   ML Settings ->
>>    default values = SA
>>    max levels = 10
>>    prec type = MGV
>>    increasing or decreasing = increasing
>>    aggregation: type = Uncoupled
>>    aggregation: damping factor = 1.333
>>    eigen-analysis: type = cg
>>    eigen-analysis: iterations = 10
>>    smoother: sweeps = 2
>>    smoother: damping factor = 1
>>    smoother: pre or post = both
>>    smoother: type = Chebyshev
>>    coarse: type = Amesos-KLU
>>    coarse: max size = 128
>>    coarse: pre or post = post
>>    coarse: sweeps = 1
>>
>> Changing to symmetric GS seems to blow things up (same error as in the
>> first mail) although it appears to depend on the number of CPUs. I'm not
>> sure if there's an optimal set of parameters for the 3D Poisson problem?
>> Right now I a system with 5594877 is taking 15s (71 CG iterations) to solve
>> on 64 CPU cores (2.2GHz Xeon, 8 cores/machine, gigabit ethernet
>> interconnect). I'm not sure if it's realistic to expect much faster?
>>
>> Kind regards,
>>
>> Bart
>>
>
>
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