[petsc-users] Unstructured mesh

[Mark Adams]

On Mon, Dec 13, 2021 at 8:35 AM Karthikeyan Chockalingam - STFC UKRI <
karthikeyan.chockalingam at stfc.ac.uk> wrote:

> Thanks Matt. Couple of weeks back you mentioned
>
> “There are many unstructured grid examples, e.g. SNES ex13, ex17, ex56.
> The solver can run on the GPU, but the vector/matrix FEM assembly does not.
> I am working on that now.”
>
>
>
> I am able to run other examples in ksp/tutorials on gpus. I complied ex56
> in snes/tutorials no differently. The only difference being I didn’t
> specify _dm_vec_type and _dm_vec_type (as you mentioned they are not
> assembled on gpus anyways plus I am working on an unstructured grid thought
> _dm is not right type for this problem). I was hoping to see gpu flops
> recorded for KSPSolve, which I didn’t.
>
>
>
> Okay, I will wait for Mark to comment.
>

This (DM) example works like any other, with a prefix, as far as GPU:
-ex56_dm_vec_type cuda and -ex56_dm_mat_type cusparse, or aijkokkos/kokkos,
etc.
Run with -options_left to verify that these are used.


>
>
> Kind regards,
>
> Karthik.
>
>
>
> *From: *Matthew Knepley <knepley at gmail.com>
> *Date: *Monday, 13 December 2021 at 13:17
> *To: *"Chockalingam, Karthikeyan (STFC,DL,HC)" <
> karthikeyan.chockalingam at stfc.ac.uk>
> *Cc: *Mark Adams <mfadams at lbl.gov>, "petsc-users at mcs.anl.gov" <
> petsc-users at mcs.anl.gov>
> *Subject: *Re: [petsc-users] Unstructured mesh
>
>
>
> On Mon, Dec 13, 2021 at 7:15 AM Karthikeyan Chockalingam - STFC UKRI <
> karthikeyan.chockalingam at stfc.ac.uk> wrote:
>
> Thank you. I was able to confirm both the below options produced the same
> mesh
>
>
>
>  ./ex56 -cells 2,2,1 -max_conv_its 2
>
> ./ex56 -cells 4,4,2 -max_conv_its 1
>
> Good
>
> But I didn’t get how is -cells i,j,k <1,1,1> is related to the number of
> MPI processes.
>
> It is not. The number of processes is specified independently using
> 'mpiexec -n <p>' or when using the test system NP=<p>.
>
> (i)     Say I start with -cells 1,1,1 -max_conv its 7; that would
> eventually leave all refinement on level 7 running on 1 MPI process?
>
> (ii)    Say I start with -cells 2,2,1 -max_conv its n; is it recommended
> to run on 4 MPI processes?
>
> No, those options do not influence the number of processes.
>
>
>
> I am running ex56 on gpu; I am looking at KSPSolve (or any other event)
> but no gpu flops are recorded in the -log_view?
>
>
>
> I do not think you are running on the GPU then. Mark can comment, but we
> usually specify GPU execution using the Vec and Mat types
>
> through -dm_vec_type and -dm_mat_type.
>
>
>
>   Thanks,
>
>
>
>      Matt
>
>
>
> For your reference I used the below flags:
>
> ./ex56 -cells 1,1,1 -max_conv_its 3 -lx 1. -alpha .01 -petscspace_degree 1
> -ksp_type cg -ksp_monitor -ksp_rtol 1.e-8 -pc_type asm -snes_monitor
> -use_mat_nearnullspace true -snes_rtol 1.e-10 -ex56_dm_view -log_view
>
>
>
> Kind regards,
>
> Karthik.
>
>
>
>
>
> *From: *Mark Adams <mfadams at lbl.gov>
> *Date: *Sunday, 12 December 2021 at 23:00
> *To: *"Chockalingam, Karthikeyan (STFC,DL,HC)" <
> karthikeyan.chockalingam at stfc.ac.uk>
> *Cc: *Matthew Knepley <knepley at gmail.com>, "petsc-users at mcs.anl.gov" <
> petsc-users at mcs.anl.gov>
> *Subject: *Re: [petsc-users] Unstructured mesh
>
>
>
>
>
>
>
> On Sun, Dec 12, 2021 at 3:19 PM Karthikeyan Chockalingam - STFC UKRI <
> karthikeyan.chockalingam at stfc.ac.uk> wrote:
>
> Thank for your response that was helpful. I have a couple of questions:
>
>
>
> (i)                  How can I control the level of refinement? I tried
> to pass the flag “-ex56_dm_refine 0” but that didn’t stop the refinement
> from 8 giving 32 cubes.
>
>
>
> I answered this question recently but ex56 clobbers ex56_dm_refine in the
> convergence loop. I have an MR that prints a warning if you provide a
> ex56_dm_refine.
>
>
>
> * snes/ex56 runs a convergence study and confusingly sets the options
> manually, thus erasing your -ex56_dm_refine.
>
>
>
> * To refine, use -max_conv_its N <3>, this sets the number of steps of
> refinement. That is, the length of the convergence study
>
>
>
> * You can adjust where it starts from with -cells i,j,k <1,1,1>
>
> You do want to set this if you have multiple MPI processes so that the
> size of this mesh is the number of processes. That way it starts with one
> cell per process and refines from there.
>
>
>
> (ii)                What does -cell 2,2,1 correspond to?
>
>
>
> The initial mesh or mesh_0. The convergence test uniformly refines this
> mesh. So if you want to refine this twice you could use -cells 8,8,4
>
>
>
> How can I determine the total number of dofs?
>
> Unfortunately, that is not printed but you can calculate from the initial
> cell grid, the order of the element and the refinement in each iteration of
> the convergence tests.
>
>
>
> So that I can perform a scaling study by changing the input of the flag
> -cells.
>
>
>
>
>
> You can and the convergence test gives you data for a strong speedup study
> in one run. Each solve is put in its own "stage" of the output and you want
> to look at KSPSolve lines in the log_view output.
>
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>
> --
>
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
>
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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