I see: only the first function evaluation of each Newton solve appears
by itself in the flamegraph and all the subsequent ones appear inside
the linesearch routine. It's all clear now.
Thanks
Matteo
On 08/02/2023 23:00, Barry Smith wrote:
Yes, the "basic" line search updates the solution vector and
evaluates the function at the new solution point; not much else.
There are no duplicate or extra function evaluations in PETSc,
(most of them occur in the linesearch function). When the code returns
from the line search routine it "knows" the function evaluation has
already been done (at the newest solution) so it does not need to do
it again.
The line search does not perform the Newton step, it takes the
step computed by the linear solve and decides how to use that step
(you will see that KSPSolve time is not part of the line search time).
Barry
On Feb 8, 2023, at 7:56 AM, Matteo Semplice
<[email protected]> wrote:
Dear all,
I am trying to optimize the nonlinear solvers in a code of mine,
but I am having a hard time at interpreting the profiling data from
the SNES. In particular, if I run with -snesCorr_snes_lag_jacobian 5
-snesCorr_snes_linesearch_monitor -snesCorr_snes_monitor
-snesCorr_snes_linesearch_type basic -snesCorr_snes_view I get, for
all timesteps an output like
0 SNES Function norm 2.204257292307e+00
1 SNES Function norm 5.156376709750e-03
2 SNES Function norm 9.399026338316e-05
3 SNES Function norm 1.700505246874e-06
4 SNES Function norm 2.938127043559e-08
SNES Object: snesCorr (snesCorr_) 1 MPI process
type: newtonls
maximum iterations=50, maximum function evaluations=10000
tolerances: relative=1e-08, absolute=1e-50, solution=1e-08
total number of linear solver iterations=4
total number of function evaluations=5
norm schedule ALWAYS
Jacobian is rebuilt every 5 SNES iterations
SNESLineSearch Object: (snesCorr_) 1 MPI process
type: basic
maxstep=1.000000e+08, minlambda=1.000000e-12
tolerances: relative=1.000000e-08, absolute=1.000000e-15,
lambda=1.000000e-08
maximum iterations=40
KSP Object: (snesCorr_) 1 MPI process
type: gmres
restart=30, using Classical (unmodified) Gram-Schmidt
Orthogonalization with no iterative refinement
happy breakdown tolerance 1e-30
maximum iterations=10000, initial guess is zero
tolerances: relative=1e-05, absolute=1e-50, divergence=10000.
left preconditioning
using PRECONDITIONED norm type for convergence test
PC Object: (snesCorr_) 1 MPI process
type: ilu
out-of-place factorization
0 levels of fill
tolerance for zero pivot 2.22045e-14
matrix ordering: natural
factor fill ratio given 1., needed 1.
Factored matrix follows:
Mat Object: (snesCorr_) 1 MPI process
type: seqaij
rows=1200, cols=1200
package used to perform factorization: petsc
total: nonzeros=17946, allocated nonzeros=17946
using I-node routines: found 400 nodes, limit used is 5
linear system matrix = precond matrix:
Mat Object: 1 MPI process
type: seqaij
rows=1200, cols=1200
total: nonzeros=17946, allocated nonzeros=17946
total number of mallocs used during MatSetValues calls=0
using I-node routines: found 400 nodes, limit used is 5
I guess that this means that no linesearch is performed and the full
Newton step is always performed (I did not report the full output,
but all timesteps are alike). Also, with the default (bt) LineSearch,
the total CPU time does not change, which seems in line with this.
However, I'd have expected that the time spent in SNESLineSearch
would be negligible, but the flamegraph is showing that about 38% of
the time spent by SNESSolve is actually spent in SNESLineSearch.
Furthermore, SNESLineSearch seems to cause more SNESFunction
evaluations (in terms of CPU time) than the SNESSolve itself. The
flamegraph is attached.
Could some expert help me in understanding these data? Is the
LineSearch actually performing the newton step? Given that the full
step is always taken, can the SNESFunction evaluations from the
LineSearch be skipped?
Thanks a lot!
Matteo
<flame.svg>
--
Prof. Matteo Semplice
Università degli Studi dell’Insubria
Dipartimento di Scienza e Alta Tecnologia – DiSAT
Professore Associato
Via Valleggio, 11 – 22100 Como (CO) – Italia
tel.: +39 031 2386316
