<div dir="ltr"><div dir="ltr">On Tue, Mar 19, 2019 at 6:59 AM Y. Shidi via petsc-users <<a href="mailto:petsc-users@mcs.anl.gov">petsc-users@mcs.anl.gov</a>> wrote:<br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hello Barry,<br>
<br>
Thank you for your reply.<br>
<br>
I reduced the tolerances and get desired solution.<br>
<br>
I am solving a multiphase incompressible n-s problems and currently<br>
we are using augmented lagrangina technique with uzawa iteration.<br>
Because the problems are getting larger, we are also looking for some<br>
other methods for solving the linear system.<br>
I follow pcfieldsplit tutorial from:<br>
<a href="https://www.mcs.anl.gov/petsc/documentation/tutorials/MSITutorial.pdf" rel="noreferrer" target="_blank">https://www.mcs.anl.gov/petsc/documentation/tutorials/MSITutorial.pdf</a><br>
<br>
However, it takes about 10s to finish one iteration and overall<br>
it requires like 150s to complete one time step with 100k unknowns,<br>
which is a long time compared to our current solver 10s for one<br>
time step.<br></blockquote><div><br></div><div>The first thing to do is look at how many Schur complement iterations you are doing:</div><div><br></div><div> -fieldsplit_pressure_ksp_monitor_true_residual</div><div><br></div><div>Perhaps you need a better preconditioner for it.</div><div><br></div><div> Thanks,</div><div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
I tried the following options:<br>
1).<br>
-ksp_type fgmres -pc_type fieldsplit -pc_fieldsplit_type schur<br>
-pc_fieldsplit_schur_factorization_type lower<br>
-fieldsplit_velocity_ksp_type preonly -fieldsplit_velocity_pc_type gamg<br>
-fieldsplit_pressure_ksp_type minres -fieldsplit_pressure_pc_type none<br>
2).<br>
-ksp_type fgmres -pc_type fieldsplit -pc_fieldsplit_type schur<br>
-pc_fieldsplit_schur_factorization_type diag<br>
-fieldsplit_velocity_ksp_type preonly -fieldsplit_velocity_pc_type gamg<br>
-fieldsplit_pressure_ksp_type minres -fieldsplit_pressure_pc_type none<br>
3).<br>
-ksp_type fgmres -pc_type fieldsplit -pc_fieldsplit_type schur<br>
-pc_fieldsplit_schur_factorization_type full<br>
-fieldsplit_velocity_ksp_type preonly -fieldsplit_velocity_pc_type lu<br>
-fieldsplit_pressure_ksp_rtol 1e-10 -fieldsplit_pressure_pc_type jacobi<br>
<br>
So I am wondering if there is any other options that can help improve <br>
the<br>
pcfieldsplit performance.<br>
<br>
Kind Regards,<br>
Shidi<br>
<br>
<br>
On 2019-03-17 00:05, Smith, Barry F. wrote:<br>
>> On Mar 16, 2019, at 6:50 PM, Y. Shidi via petsc-users <br>
>> <<a href="mailto:petsc-users@mcs.anl.gov" target="_blank">petsc-users@mcs.anl.gov</a>> wrote:<br>
>> <br>
>> Hello,<br>
>> <br>
>> I am trying to solve the incompressible n-s equations by<br>
>> PCFieldSplit.<br>
>> <br>
>> The large matrix and vectors are formed by MatCreateNest()<br>
>> and VecCreateNest().<br>
>> The system is solved directly by the following command:<br>
>> -ksp_type fgmres \<br>
>> -pc_type fieldsplit \<br>
>> -pc_fieldsplit_type schur \<br>
>> -pc_fieldsplit_schur_fact_type full \<br>
>> -ksp_converged_reason \<br>
>> -ksp_monitor_true_residual \<br>
>> -fieldsplit_0_ksp_type preonly \<br>
>> -fieldsplit_0_pc_type cholesky \<br>
>> -fieldsplit_0_pc_factor_mat_solver_package mumps \<br>
>> -mat_mumps_icntl_28 2 \<br>
>> -mat_mumps_icntl_29 2 \<br>
>> -fieldsplit_1_ksp_type preonly \<br>
>> -fieldsplit_1_pc_type jacobi \<br>
>> Output:<br>
>> 0 KSP unpreconditioned resid norm 1.214252932161e+04 true resid norm <br>
>> 1.214252932161e+04 ||r(i)||/||b|| 1.000000000000e+00<br>
>> 1 KSP unpreconditioned resid norm 1.642782495109e-02 true resid norm <br>
>> 1.642782495109e-02 ||r(i)||/||b|| 1.352916226594e-06<br>
>> Linear solve converged due to CONVERGED_RTOL iterations 1<br>
>> <br>
>> The system is solved iteratively by the following command:<br>
>> -ksp_type fgmres \<br>
>> -pc_type fieldsplit \<br>
>> -pc_fieldsplit_type schur \<br>
>> -pc_fieldsplit_schur_factorization_type diag \<br>
>> -ksp_converged_reason \<br>
>> -ksp_monitor_true_residual \<br>
>> -fieldsplit_0_ksp_type preonly \<br>
>> -fieldsplit_0_pc_type gamg \<br>
>> -fieldsplit_1_ksp_type minres \<br>
>> -fieldsplit_1_pc_type none \<br>
>> Output:<br>
>> 0 KSP unpreconditioned resid norm 1.214252932161e+04 true resid norm <br>
>> 1.214252932161e+04 ||r(i)||/||b|| 1.000000000000e+00<br>
>> 1 KSP unpreconditioned resid norm 2.184037364915e+02 true resid norm <br>
>> 2.184037364915e+02 ||r(i)||/||b|| 1.798667565109e-02<br>
>> 2 KSP unpreconditioned resid norm 2.120097409539e+02 true resid norm <br>
>> 2.120097409635e+02 ||r(i)||/||b|| 1.746009709742e-02<br>
>> 3 KSP unpreconditioned resid norm 4.364091658268e+01 true resid norm <br>
>> 4.364091658575e+01 ||r(i)||/||b|| 3.594054865332e-03<br>
>> 4 KSP unpreconditioned resid norm 2.632671796885e+00 true resid norm <br>
>> 2.632671797020e+00 ||r(i)||/||b|| 2.168141189773e-04<br>
>> 5 KSP unpreconditioned resid norm 2.209213998004e+00 true resid norm <br>
>> 2.209213980361e+00 ||r(i)||/||b|| 1.819401808180e-04<br>
>> 6 KSP unpreconditioned resid norm 4.683775185840e-01 true resid norm <br>
>> 4.683775085753e-01 ||r(i)||/||b|| 3.857330677735e-05<br>
>> 7 KSP unpreconditioned resid norm 3.042503284736e-02 true resid norm <br>
>> 3.042503349258e-02 ||r(i)||/||b|| 2.505658638883e-06<br>
>> <br>
>> <br>
>> Both methods give answers, but they are different<br>
> <br>
> What do you mean the answers are different? Do you mean the<br>
> solution x from KSPSolve() is different? How are you calculating their<br>
> difference and how different are they?<br>
> <br>
> Since the solutions are only approximate; true residual norm is<br>
> around 1.642782495109e-02 and 3.042503349258e-02 for the two<br>
> different solvers there will only be a certain number of identical<br>
> digits in the two solutions (which depends on the condition number of<br>
> the original matrix). You can run both solvers with -ksp_rtol 1.e-12<br>
> and then (assuming everything is working correctly) the two solutions<br>
> will be much closer to each other.<br>
> <br>
> Barry<br>
> <br>
>> so I am wondering<br>
>> if it is possible that you can help me figure out which part I am<br>
>> doing wrong.<br>
>> <br>
>> Thank you for your time.<br>
>> <br>
>> Kind Regards,<br>
>> Shidi<br>
</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>-- Norbert Wiener</div><div><br></div><div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>