[petsc-users] Unstructured mesh

[Matthew Knepley]

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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