[petsc-users] GAMG setup scalability

[John Fettig]

GAMG without any options is using MatTransposeMatMult (in petsc-dev)
during setup at line 1031 of agg.c.  What I find is that this
operation takes up a majority of the total setup time, and doesn't
scale well.  Is there anything that can be done about this?

I am a little surprised that it is taking significantly more time than
the RAP construction of the coarse grid operators done by MatPtAP.  On
1 processor, for example, it takes 5% of the setup time, and on 8
processors it takes ~4%.  The MatTransposeMatMult, on the other hand,
takes 67% of setup on 1 processor and 71% of setup on 8.

I've attached the log_summary for 1 processor and 8 processors.  You
can also see that the solve time is completely eclipsed by the setup
time.

Regards,
John
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