[petsc-users] SNES_DIVERGED_LS_FAILURE

[Juha Jäykkä]

Hi all!

I am having nasty problems with SNES. As you probably guess already, my result 
is non-converging line search. I have a good candidate reason for this, but I 
do not know how to fix it.

Jacobian should be fine: -snes_ls_type test gives nice small numbers (not 
quite as small as I would like: around 1e-7).

And even if Jacobian was wrong, -snes_fd behaves in exactly the same way.

When snes gives up with its line search, the values in the residual vector 
are:

0.00000000e+00,   1.02404374e+00,   2.28558501e-01,
1.94195641e-02,  -4.82401091e-02,  -7.07456591e-02,
-7.64790651e-02,  -7.55281623e-02,  -7.19176452e-02

before they stabilise to interval (1.e-5, 8e-2). Now, my candidate is the huge 
(compared to others) value of the rhs at the second lattice point and slightly 
smaller, but still large at the third. The diverged solution vector grows very 
fast in the beginning and then settles to an almost constant value after about 
one tenth of the lattice, so my guess is that the big change needed at i=1 
causes precision to be lost at i>>1 and therefore line search fails.

Do you have any ideas how to get around this or if my theory is even correct?

Trying trust region search does not really help: it suffers from the same 
deficiency - except that it thinks it converges even though the function norm 
is of the order of 0.5. (It does fare slightly better, though, its rhs starts 
like 

0.00000000e+00,   3.93745063e-01,   1.79187741e-01,
1.03100385e-01,   6.76686117e-02,   4.82967347e-02,
3.65243377e-02,   2.88059666e-02,   2.34452314e-02,

which is better, but still not good enough.)

Cheers,
Juha

-- 
		 -----------------------------------------------
		| Juha Jäykkä, juhaj at iki.fi			|
		| http://www.maths.leeds.ac.uk/~juhaj		|
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