[petsc-users] Profile a matrix-free solver.

[Barry Smith]

   So 

    KSPSolve is 96 % and MatMult is 70 % + PCApply 24 % = 94 % so this makes sense; the solver time is essentially the 
multiply time plus the PCApply time.

compute_rhs         1823 1.0 4.2119e+02 1.0 0.00e+00 0.0 4.4e+04 5.4e+04 1.1e+04 71  0100100 39  71  0100100 39     0
LU-SGS              1647 1.0 1.3590e+02 1.0 0.00e+00 0.0 0.0e+00 0.0e+00 0.0e+00 23  0  0  0  0  23  0  0  0  0     0
SURFINT             1823 1.0 1.0647e+02 1.1 0.00e+00 0.0 0.0e+00 0.0e+00 0.0e+00 17  0  0  0  0  17  0  0  0  0     0
VOLINT              1823 1.0 2.2373e+02 1.1 0.00e+00 0.0 0.0e+00 0.0e+00 0.0e+00 35  0  0  0  0  35  0  0  0  0     0

   Depending on the "quality" of the preconditioner (if it is really good) one expects the preconditioner time to be larger than the MatMult(). Only for simple preconditioners (like Jacobi) does one see it being much less than the MatMult().  For matrix based solvers the amount of work in  SGS is as large as the amount of work in the MatMult() if not more, so I would expect the time of the preconditioner to be higher than the time of the multiply. 

  So based on knowing almost nothing I think the MatMult_ is taking more time then it should unless you are ignoring (skipping) a lot of the terms in your matrix-free SGS; then it is probably reasonable.

  Barry



> On Jan 14, 2016, at 3:01 PM, Song Gao <song.gao2 at mail.mcgill.ca> wrote:
> 
> Hello,
> 
> I am profiling a finite element Navier-Stokes solver. It uses the Jacobian-free Newton Krylov method and a custom preconditoner LU-SGS (a matrix-free version of Symmetic Gauss-Seidel ). The log summary is attached. Four events are registered.  compute_rhs is compute rhs (used by MatMult_MFFD). SURFINT and VOLINT are parts of compute_rhs. LU-SGS is the custom preconditioner. I didn't call PetscLogFlops so these flops are zeros. 
> 
>  I'm wondering, is the percent time of the events reasonable in the table? I see 69% time is spent on  matmult_mffd. Is it expected in matrix-free method? What might be a good starting point of profiling this solver? Thank you in advance.
> 
> 
> Song Gao 
> <log_summary>