[Trilinos-Users] [EXTERNAL] Belos::LinearProblem::setProblem() is taking too much time

[Thornquist, Heidi K]

Hi John,

The LinearProblem::setProblem method makes sure that the linear problem is valid, that there is an operator, right-hand side, and solution vector.
After that, it computes the initial residual and preconditioned initial residual (if left preconditioning is used).  So, no, the behavior that you are seeing is not expected.

A couple questions might be helpful in understanding this better:

1)  Which solver are you using?
2)  How many iterations is this solver performing before achieving convergence (assuming 400 is your maximum number of iterations)?
3)  Are you deferring your custom preconditioner or operator construction until it is actually applied?  If you are, and you are using a left preconditioner, then it would show up in the setProblem timing.


Thanks,
Heidi

--

Heidi K. Thornquist
Electrical Models & Simulation
Sandia National Laboratories
Albuquerque, NM  87185-1323


From: John Moore <johnpmooreiv at gmail.com<mailto:johnpmooreiv at gmail.com>>
Date: Monday, February 16, 2015 12:10 PM
To: "trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>" <trilinos-users at software.sandia.gov<mailto:trilinos-users at software.sandia.gov>>
Subject: [EXTERNAL] [Trilinos-Users] Belos::LinearProblem::setProblem() is taking too much time

I am interested in solving large linear systems within Newton iterations. My matrix structure remains constant, while only the values change.

I find that most of the time is actually spent in the functions Belos::LinearProblem::setProblem. Is this to be expected? The case that I am currently running is detailed below:

TpetraCrsMatrix
# processors:        32 mpi processes
Number of rows:   802225
Number of entries:  71819925
Preconditioner:       Custom
Number of iters:     400
Solve time:            42 Seconds
setProblem time:     35 seconds

The performance only gets work as the problem gets larger.  For a slightly larger (and denser) problem, the setProblem() time is about 4 times that of the solve time. For small problems, most of the time is spent in the solve, as expected. Is there a way to get around calling setProblem() every time I need to solve the linear system, or could I be doing something wrong?

Thank you,
John
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