[petsc-users] sources of floating point randomness in JFNK in serial

[Mark Lohry]

>
> Do they start very similarly and then slowly drift further apart?


Yes, this. I take it this sounds familiar?

See these two examples with 20 fixed iterations pasted at the end. The
difference for one solve is slight (final SNES norm is identical to 5
digits), but in the context I'm using it in (repeated applications to solve
a steady state multigrid problem, though here just one level) the
differences add up such that I might reach global convergence in 35
iterations or 38. It's not the end of the world, but I was expecting that
with -np 1 these would be identical and I'm not sure where the root cause
would be.


  0 SNES Function norm 2.801842107848e+04
    0 KSP Residual norm 4.045639499595e+01
    1 KSP Residual norm 1.917999809040e+01
    2 KSP Residual norm 1.616048521958e+01
[...]
   19 KSP Residual norm 8.788043518111e-01
   20 KSP Residual norm 6.570851270214e-01
  Linear solve converged due to CONVERGED_ITS iterations 20
  1 SNES Function norm 1.801309983345e+03
Nonlinear solve converged due to CONVERGED_ITS iterations 1


Same system, identical initial 0 SNES norm, 0 KSP is slightly different

  0 SNES Function norm 2.801842107848e+04
    0 KSP Residual norm 4.045639473002e+01
    1 KSP Residual norm 1.917999883034e+01
    2 KSP Residual norm 1.616048572016e+01
[...]
   19 KSP Residual norm 8.788046348957e-01
   20 KSP Residual norm 6.570859588610e-01
  Linear solve converged due to CONVERGED_ITS iterations 20
  1 SNES Function norm 1.801311320322e+03
Nonlinear solve converged due to CONVERGED_ITS iterations 1

On Wed, May 3, 2023 at 11:05 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>   Do they start very similarly and then slowly drift further apart? That
> is the first couple of KSP iterations they are almost identical but then
> for each iteration get a bit further. Similar for the SNES iterations,
> starting close and then for more iterations and more solves they start
> moving apart. Or do they suddenly jump to be very different? You can run
> with -snes_monitor -ksp_monitor
>
> On May 3, 2023, at 9:07 PM, Mark Lohry <mlohry at gmail.com> wrote:
>
> This is on a single MPI rank. I haven't checked the coloring, was just
> guessing there. But the solutions/residuals are slightly different from run
> to run.
>
> Fair to say that for serial JFNK/asm ilu0/gmres we should expect bitwise
> identical results?
>
>
> On Wed, May 3, 2023, 8:50 PM Barry Smith <bsmith at petsc.dev> wrote:
>
>>
>>   No, the coloring should be identical every time. Do you see differences
>> with 1 MPI rank? (Or much smaller ones?).
>>
>>
>>
>> > On May 3, 2023, at 8:42 PM, Mark Lohry <mlohry at gmail.com> wrote:
>> >
>> > I'm running multiple iterations of newtonls with an MFFD/JFNK nonlinear
>> solver where I give it the sparsity. PC asm, KSP gmres, with
>> SNESSetLagJacobian -2 (compute once and then frozen jacobian).
>> >
>> > I'm seeing slight (<1%) but nonzero differences in residuals from run
>> to run. I'm wondering where randomness might enter here -- does the
>> jacobian coloring use a random seed?
>>
>>
>
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