[petsc-users] Big discrepancy between machines

[Timothée Nicolas]

OK, The problem is that I don't think I can change this easily as far as
the cluster is concerned. I obtain access to petsc by loading the petsc
module, and even if I have a few choices, I don't see any debug builds...

2015-12-14 17:26 GMT+09:00 Dave May <dave.mayhem23 at gmail.com>:

>
>
> On Monday, 14 December 2015, Timothée Nicolas <timothee.nicolas at gmail.com>
> wrote:
>
>> Hum, OK. I use FORTRAN by the way. Is your comment still valid ?
>>
>
> No. Fortran compilers init variables to zero.
> In this case, I would run a debug build on your OSX machine through
> valgrind and make sure it is clean.
>
> Other obvious thing to check what happens if use exactly the same petsc
> builds on both machines. I see 3.6.1 and 3.6.0 are being used.
>
> For all this type of checking, I would definitely use debug builds on both
> machines. Your cluster build is using the highest level of optimization...
>
>
>
>
>> 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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