[petsc-users] inconsistent SNES/KSP behavior

[Gideon Simpson]

So I may be imagining this, but I like to think that the following happened.  I submitted the same job script to my cluster, twice, and I got different results.    This is a SNES problem where I am using the MUMPS linear solver.  Here is what I mean:

Trial 1:

Generating initial guess with a0 = 1.95, b0 = 2.09, sigma = 2, xmax = 100, nx = 2560000

2560001
10240006
Writing to disk as guess.bin

 Solving sigma = 2
  0 SNES Function norm 1.691829175711e-01 
    0 KSP preconditioned resid norm 6.098266966934e-04 true resid norm 1.691829175711e-01 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 1.698748932236e-12 true resid norm 5.163872517578e-10 ||r(i)||/||b|| 3.052242266367e-09
  1 SNES Function norm 1.395434035402e-04 
    0 KSP preconditioned resid norm 7.922333994913e-07 true resid norm 1.395434035402e-04 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 1.839375186797e-14 true resid norm 3.034980160221e-13 ||r(i)||/||b|| 2.174936316030e-09
  2 SNES Function norm 1.394208676802e-04 
 sigma = 2, a = 1.94758, b = 2.09002, reason =4

Trial 2:

Generating initial guess with a0 = 1.95, b0 = 2.09, sigma = 2, xmax = 100, nx = 2560000

2560001
10240006
Writing to disk as guess.bin

 Solving sigma = 2
  0 SNES Function norm 1.691829175711e-01 
    0 KSP preconditioned resid norm 6.098266972456e-04 true resid norm 1.691829175711e-01 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 1.497022256767e-11 true resid norm 1.953323112042e-09 ||r(i)||/||b|| 1.154562848357e-08
  1 SNES Function norm 1.396785292629e-04 
    0 KSP preconditioned resid norm 1.430883443301e-05 true resid norm 1.396785292629e-04 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 5.226190977119e-14 true resid norm 3.918710626146e-12 ||r(i)||/||b|| 2.805521111100e-08
  2 SNES Function norm 1.396634713981e-04 
    0 KSP preconditioned resid norm 1.693020976611e-05 true resid norm 1.396634713981e-04 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 6.991045787498e-14 true resid norm 4.339991163168e-12 ||r(i)||/||b|| 3.107463332911e-08
  3 SNES Function norm 1.395099855381e-04 
    0 KSP preconditioned resid norm 8.023057268371e-06 true resid norm 1.395099855381e-04 ||r(i)||/||b|| 1.000000000000e+00
    1 KSP preconditioned resid norm 3.476126449145e-14 true resid norm 2.800497344039e-12 ||r(i)||/||b|| 2.007381287610e-08
  4 SNES Function norm 1.394785126062e-04 
 sigma = 2, a = 1.94758, b = 2.09002, reason =4

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