[petsc-users] Question about the performance of KSP solver

Sun Feb 27 09:23:13 CST 2022

First, you probably want -ksp_type cg
ILU sucks, you can try 'gamg' and I would configure it with hypre and try
that also.

Now, you are getting very little increase in flop rate on MatMult (8K
-->8.6K). That is the problem
* You have a "bad" network
* Your problem is too small (with respect to your network) to get speedup
after 16 porcs
* Bad partitioning
* You have a fair amount of load imbalance (MatMult              671 1.0
4.7602e+01 *2.0* ), and it gets worse at 32 procs. That can be from bad
partitioning or a bad network.

With a decent network you probably want to keep at least 25K equations per
processor. This depends on these other issues, but it is  a start. With a
good network you should be able to get down to 10K/proc

It is best to start with a simple model problem like a cube, with cube
shaped subdomains is ideal, and isolate issues.

Mark

On Sun, Feb 27, 2022 at 9:51 AM Gong Yujie <yc17470 at connect.um.edu.mo>
wrote:

> Hi,
>
> I'm using the GMRES with ASM preconditioner with sub-domain solver ILU(2)
> to solve an elasticity problem. First, I use 16 cores to test the
> computation time, then use 32 cores to run the same code with the same
> parameters.  But I just get about 10% speed up. From the log file I found
> that the computation time of KSPSolve() and MatSolve() just decrease a
> little bit. My PETSc version is 3.16.0 and use --with-debugging=0 when
> configure it. The matrix size is about 7*10^6. Some detail of the log is
> shown below:
>
> 16-cores:
> ------------------------------------------------------------------------------------------------------------------------
>
> Event                Count      Time (sec)     Flop
>        --- Global ---  --- Stage ----  Total
>                    Max Ratio  Max     Ratio   Max  Ratio  Mess   AvgLen
>  Reduct  %T %F %M %L %R  %T %F %M %L %R Mflop/s
>
> ------------------------------------------------------------------------------------------------------------------------
> MatMult              664 1.0 5.0794e+01 1.6 2.70e+10 1.1 7.1e+04 4.8e+04
> 1.0e+00  7 13 49 20  0   7 13 49 20  0  8010
> MatSolve             663 1.0 1.9868e+02 1.1 1.43e+11 1.1 0.0e+00 0.0e+00
> 0.0e+00 33 70  0  0  0  33 70  0  0  0 10932
> MatLUFactorNum         1 1.0 6.1501e+00 1.1 1.40e+10 1.1 0.0e+00 0.0e+00
> 0.0e+00  1  7  0  0  0   1  7  0  0  0 35056
> MatILUFactorSym        1 1.0 1.5566e+01 1.2 0.00e+00 0.0 0.0e+00 0.0e+00
> 0.0e+00  2  0  0  0  0   2  0  0  0  0     0
> KSPSetUp               2 1.0 5.9627e-03 1.9 0.00e+00 0.0 0.0e+00 0.0e+00
> 2.0e+00  0  0  0  0  0   0  0  0  0  0     0
> KSPSolve               1 1.0 2.5168e+02 1.0 1.90e+11 1.1 1.4e+05 4.8e+04
> 1.3e+03 44 93 98 40 89  44 93 98 40 90 11437
> KSPGMRESOrthog       641 1.0 1.8980e+01 1.7 1.82e+10 1.1 0.0e+00 0.0e+00
> 6.4e+02  3  9  0  0 43   3  9  0  0 44 14578
> PCSetUp                2 1.0 2.2480e+01 1.1 1.40e+10 1.1 5.3e+02 6.5e+05
> 7.0e+00  4  7  0  2  0   4  7  0  2  0  9591
> PCSetUpOnBlocks        1 1.0 2.1555e+01 1.1 1.40e+10 1.1 0.0e+00 0.0e+00
> 0.0e+00  3  7  0  0  0   3  7  0  0  0 10002
> PCApply              663 1.0 2.0296e+02 1.1 1.43e+11 1.1 7.0e+04 4.8e+04
> 1.0e+00 33 70 49 20  0  33 70 49 20  0 10701
> PCApplyOnBlocks      663 1.0 1.9908e+02 1.1 1.43e+11 1.1 0.0e+00 0.0e+00
> 0.0e+00 33 70  0  0  0  33 70  0  0  0 10910
>
> 32-cores:
> ------------------------------------------------------------------------------------------------------------------------
>
> Event                Count      Time (sec)     Flop
>        --- Global ---  --- Stage ----  Total
>                    Max Ratio  Max     Ratio   Max  Ratio  Mess   AvgLen
>  Reduct  %T %F %M %L %R  %T %F %M %L %R Mflop/s
>
> ------------------------------------------------------------------------------------------------------------------------
> MatMult              671 1.0 4.7602e+01 2.0 1.39e+10 1.1 1.7e+05 2.8e+04
> 1.0e+00  7 13 49 23  0   7 13 49 23  0  8637
> MatSolve             670 1.0 1.7800e+02 1.1 7.56e+10 1.1 0.0e+00 0.0e+00
> 0.0e+00 33 71  0  0  0  33 71  0  0  0 12544
> MatLUFactorNum         1 1.0 3.5714e+00 1.1 7.16e+09 1.1 0.0e+00 0.0e+00
> 0.0e+00  1  7  0  0  0   1  7  0  0  0 60743
> MatILUFactorSym        1 1.0 8.4088e+00 1.2 0.00e+00 0.0 0.0e+00 0.0e+00
> 0.0e+00  2  0  0  0  0   2  0  0  0  0     0
> KSPSetUp               2 1.0 3.8060e-03 1.5 0.00e+00 0.0 0.0e+00 0.0e+00
> 2.0e+00  0  0  0  0  0   0  0  0  0  0     0
> KSPSolve               1 1.0 2.1680e+02 1.0 9.95e+10 1.1 3.5e+05 2.8e+04
> 1.3e+03 44 93 98 47 89  44 93 98 47 90 13592
> KSPGMRESOrthog       648 1.0 1.6999e+01 2.0 9.39e+09 1.1 0.0e+00 0.0e+00
> 6.5e+02  2  9  0  0 43   2  9  0  0 44 16450
> PCSetUp                2 1.0 1.2439e+01 1.1 7.16e+09 1.1 1.3e+03 3.7e+05
> 7.0e+00  2  7  0  2  0   2  7  0  2  0 17440
> PCSetUpOnBlocks        1 1.0 1.1876e+01 1.1 7.16e+09 1.1 0.0e+00 0.0e+00
> 0.0e+00  2  7  0  0  0   2  7  0  0  0 18267
> PCApply              670 1.0 1.8235e+02 1.1 7.56e+10 1.1 1.7e+05 2.7e+04
> 1.0e+00 34 71 49 23  0  34 71 49 23  0 12245
> PCApplyOnBlocks      670 1.0 1.7838e+02 1.1 7.56e+10 1.1 0.0e+00 0.0e+00
> 0.0e+00 33 71  0  0  0  33 71  0  0  0 12517
>
> Hope you can help me!
>
> Best Regards,
> Yujie
>
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