[petsc-users] Communication during MatAssemblyEnd

[Ale Foggia]

Thanks both of you for your answers,

El jue., 20 jun. 2019 a las 22:20, Smith, Barry F. (<bsmith at mcs.anl.gov>)
escribió:

>
>   Note that this is a one time cost if the nonzero structure of the matrix
> stays the same. It will not happen in future MatAssemblies.
>
> > On Jun 20, 2019, at 3:16 PM, Zhang, Junchao via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
> >
> > Those messages were used to build MatMult communication pattern for the
> matrix. They were not part of the matrix entries-passing you imagined, but
> indeed happened in MatAssemblyEnd. If you want to make sure processors do
> not set remote entries, you can use
> MatSetOption(A,MAT_NO_OFF_PROC_ENTRIES,PETSC_TRUE), which will generate an
> error when an off-proc entry is set.
>

I started being concerned about this when I saw that the assembly was
taking a few hundreds of seconds in my code, like 180 seconds, which for me
it's a considerable time. Do you think (or maybe you need more information
to answer this) that this time is "reasonable" for communicating the
pattern for the matrix? I already checked that I'm not setting any remote
entries.
Also I see (in my code) that even if there are no messages being passed
during MatAssemblyBegin, it is taking time and the "ratio" is very big.

>
> >
> > --Junchao Zhang
> >
> >
> > On Thu, Jun 20, 2019 at 4:13 AM Ale Foggia via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
> > Hello all!
> >
> > During the conference I showed you a problem happening during
> MatAssemblyEnd in a particular code that I have. Now, I tried the same with
> a simple code (a symmetric problem corresponding to the Laplacian operator
> in 1D, from the SLEPc Hands-On exercises). As I understand (and please,
> correct me if I'm wrong), in this case the elements of the matrix are
> computed locally by each process so there should not be any communication
> during the assembly. However, in the log I get that there are messages
> being passed. Also, the number of messages changes with the number of
> processes used and the size of the matrix. Could you please help me
> understand this?
> >
> > I attach the code I used and the log I get for a small problem.
> >
> > Cheers,
> > Ale
> >
>
>
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