Hi all,
I was trying to compile and run the ex20.c example code in the tutorial section of SNES. Although it does not explicitly specify that -snes_mf option can be used, my understanding is that as long as a nonlinear residual function is written correctly, PETSc will calculate via finite difference the action of the Jacobian on a given vector. Is that correct ? Now if that is the case, then please observe the discrepancy in the number of linear iterations taken with an analytical Jacobian and matrix-free option. What puzzles me is that the SNES function norm are quite close for both the methods but the linear iterations differ by a factor of 3. Why exactly is this ? Here's the output to make this clearer. vijay :mpirun -np 1 ex20 -ksp_type gmres -snes_monitor 0 SNES Function norm 2.271442542876e-01 1 SNES Function norm 6.881516100891e-02 2 SNES Function norm 1.813939751552e-02 3 SNES Function norm 2.354176462207e-03 4 SNES Function norm 3.063728077362e-05 5 SNES Function norm 3.106106268946e-08 6 SNES Function norm 5.344742712545e-12 0 SNES Function norm 2.271442542876e-01 1 SNES Function norm 6.881516100891e-02 2 SNES Function norm 1.813939751552e-02 3 SNES Function norm 2.354176462207e-03 4 SNES Function norm 3.063728077362e-05 5 SNES Function norm 3.106106268946e-08 6 SNES Function norm 5.344742712545e-12 Number of Newton iterations = 6 Number of Linear iterations = 18 Average Linear its / Newton = 3.000000e+00 vijay :mpirun -np 1 ex20 -ksp_type gmres -snes_monitor -snes_mf 0 SNES Function norm 2.271442542876e-01 1 SNES Function norm 6.870629867542e-02 2 SNES Function norm 1.804335379848e-02 3 SNES Function norm 2.290074339682e-03 4 SNES Function norm 3.082384186373e-05 5 SNES Function norm 3.926396277038e-09 6 SNES Function norm 3.754922566585e-16 0 SNES Function norm 2.271442542876e-01 1 SNES Function norm 6.870629867542e-02 2 SNES Function norm 1.804335379848e-02 3 SNES Function norm 2.290074339682e-03 4 SNES Function norm 3.082384186373e-05 5 SNES Function norm 3.926396277038e-09 6 SNES Function norm 3.754922566585e-16 Number of Newton iterations = 6 Number of Linear iterations = 54 Average Linear its / Newton = 9.000000e+00 Thanks, Vijay -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20071229/2b6050ee/attachment.htm>
