[petsc-users] [petsc-maint] Assistance Needed with PETSc KSPSolve Performance Issue

[Yongzhong Li]

Thanks, I have attached the results without using any KSPGuess. At low frequency, the iteration steps are quite close to the one with KSPGuess, specifically

  KSPGuess Object: 1 MPI process
    type: fischer
    Model 1, size 200

However, I found at higher frequency, the # of iteration steps are  significant higher than the one with KSPGuess, I have attahced both of the results for your reference.

Moreover, could I ask why the one without the KSPGuess options can be used for a baseline comparsion? What are we comparing here? How does it relate to the performance issue/bottleneck I found? “I have noticed that the time taken by KSPSolve is almost two times greater than the CPU time for matrix-vector product multiplied by the number of iteration”

Thank you!
Yongzhong

From: Barry Smith <bsmith at petsc.dev>
Date: Thursday, June 13, 2024 at 2:14 PM
To: Yongzhong Li <yongzhong.li at mail.utoronto.ca>
Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>, petsc-maint at mcs.anl.gov <petsc-maint at mcs.anl.gov>, Piero Triverio <piero.triverio at utoronto.ca>
Subject: Re: [petsc-maint] Assistance Needed with PETSc KSPSolve Performance Issue

  Can you please run the same thing without the  KSPGuess option(s) for a baseline comparison?

   Thanks

   Barry


On Jun 13, 2024, at 1:27 PM, Yongzhong Li <yongzhong.li at mail.utoronto.ca> wrote:

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Hi Matt,

I have rerun the program with the keys you provided. The system output when performing ksp solve and the final petsc log output were stored in a .txt file attached for your reference.

Thanks!
Yongzhong

From: Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>>
Date: Wednesday, June 12, 2024 at 6:46 PM
To: Yongzhong Li <yongzhong.li at mail.utoronto.ca<mailto:yongzhong.li at mail.utoronto.ca>>
Cc: petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>, petsc-maint at mcs.anl.gov<mailto:petsc-maint at mcs.anl.gov> <petsc-maint at mcs.anl.gov<mailto:petsc-maint at mcs.anl.gov>>, Piero Triverio <piero.triverio at utoronto.ca<mailto:piero.triverio at utoronto.ca>>
Subject: Re: [petsc-maint] Assistance Needed with PETSc KSPSolve Performance Issue
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On Wed, Jun 12, 2024 at 6:36 PM Yongzhong Li <yongzhong.li at mail.utoronto.ca<mailto:yongzhong.li at mail.utoronto.ca>> wrote:
Dear PETSc’s developers, I hope this email finds you well. I am currently working on a project using PETSc and have encountered a performance issue with the KSPSolve function. Specifically, I have noticed that the time taken by KSPSolve is
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Dear PETSc’s developers,
I hope this email finds you well.
I am currently working on a project using PETSc and have encountered a performance issue with the KSPSolve function. Specifically, I have noticed that the time taken by KSPSolve is almost two times greater than the CPU time for matrix-vector product multiplied by the number of iteration steps. I use C++ chrono to record CPU time.
For context, I am using a shell system matrix A. Despite my efforts to parallelize the matrix-vector product (Ax), the overall solve time remains higher than the matrix vector product per iteration indicates when multiple threads were used. Here are a few details of my setup:

  *   Matrix Type: Shell system matrix
  *   Preconditioner: Shell PC
  *   Parallel Environment: Using Intel MKL as PETSc’s BLAS/LAPACK library, multithreading is enabled
I have considered several potential reasons, such as preconditioner setup, additional solver operations, and the inherent overhead of using a shell system matrix. However, since KSPSolve is a high-level API, I have been unable to pinpoint the exact cause of the increased solve time.
Have you observed the same issue? Could you please provide some experience on how to diagnose and address this performance discrepancy? Any insights or recommendations you could offer would be greatly appreciated.

For any performance question like this, we need to see the output of your code run with

  -ksp_view -ksp_monitor_true_residual -ksp_converged_reason -log_view

  Thanks,

     Matt

Thank you for your time and assistance.
Best regards,
Yongzhong
-----------------------------------------------------------
Yongzhong Li
PhD student | Electromagnetics Group
Department of Electrical & Computer Engineering
University of Toronto
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<ksp_petsc_log.txt>

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