[petsc-users] Big discrepancy between machines

[Timothée Nicolas]

Hum, OK. I use FORTRAN by the way. Is your comment still valid ? I'll check
anyway, but I thought I had been careful about this sort of things.

Also, I thought the problem on Mac OS X may have been due to the fact I
used the version with debugging on, so I rerun configure with
--with-debugging=no, which did not change anything.

Thx

Timothee


2015-12-14 17:04 GMT+09:00 Dave May <dave.mayhem23 at gmail.com>:

> One suggestion is you have some uninitialized variables in your pcshell.
> Despite your arch being called "debug", your configure options indicate you
> have turned debugging off.
>
> C standard doesn't prescribe how uninit variables should be treated - the
> behavior is labelled as undefined. As a result, different compilers on
> different archs with the same optimization flags can and will treat uninit
> variables differently. I find OSX c compilers tend to set them to zero.
>
> I suggest compiling a debug build on both machines and trying your
> test again. Also, consider running the debug builds through valgrind.
>
> Thanks,
>   Dave
>
> On Monday, 14 December 2015, Timothée Nicolas <timothee.nicolas at gmail.com>
> wrote:
>
>> Hi,
>>
>> I have noticed I have a VERY big difference in behaviour between two
>> machines in my problem, solved with SNES. I can't explain it, because I
>> have tested my operators which give the same result. I also checked that
>> the vectors fed to the SNES are the same. The problem happens only with my
>> shell preconditioner. When I don't use it, and simply solve using -snes_mf,
>> I don't see anymore than the usual 3-4 changing digits at the end of the
>> residuals. However, when I use my pcshell, the results are completely
>> different between the two machines.
>>
>> I have attached output_SuperComputer.txt and output_DesktopComputer.txt,
>> which correspond to the output from the exact same code and options (and of
>> course same input data file !). More precisely
>>
>> output_SuperComputer.txt : output on a supercomputer called Helios, sorry
>> I don't know the exact specs.
>> In this case, the SNES norms are reduced successively:
>> 0 SNES Function norm 4.867111712420e-03
>> 1 SNES Function norm 5.632325929998e-08
>> 2 SNES Function norm 7.427800084502e-15
>>
>> output_DesktopComputer.txt : output on a Mac OS X Yosemite 3.4 GHz Intel
>> Core i5 16GB 1600 MHz DDr3. (the same happens on an other laptop with Mac
>> OS X Mavericks).
>> In this case, I obtain the following for the SNES norms,
>> while in the other, I obtain
>> 0 SNES Function norm 4.867111713544e-03
>> 1 SNES Function norm 1.560094052222e-03
>> 2 SNES Function norm 1.552118650943e-03
>> 3 SNES Function norm 1.552106297094e-03
>> 4 SNES Function norm 1.552106277949e-03
>> which I can't explain, because otherwise the KSP residual (with the same
>> operator, which I checked) behave well.
>>
>> As you can see, the first time the preconditioner is applied (DB_, DP_,
>> Drho_ and PS_ solves), the two outputs coincide (except for the few last
>> digits, up to 9 actually, which is more than I would expect), and
>> everything starts to diverge at the first print of the main KSP (the one
>> stemming from the SNES) residual norms.
>>
>> Do you have an idea what may cause such a strange behaviour ?
>>
>> Best
>>
>> Timothee
>>
>
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