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

[Matthew Knepley]

On Wed, Jun 12, 2024 at 6:36 PM Yongzhong Li <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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