[petsc-users] Communication during MatAssemblyEnd

[Zhang, Junchao]

On Fri, Jun 21, 2019 at 8:07 AM Ale Foggia <amfoggia at gmail.com<mailto:amfoggia at gmail.com>> wrote:
Thanks both of you for your answers,

El jue., 20 jun. 2019 a las 22:20, Smith, Barry F. (<bsmith at mcs.anl.gov<mailto: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<mailto: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.
It is not reasonable. Could you send log view of that test with 180 seconds MatAssembly?

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<mailto: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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