[petsc-users] Big discrepancy between machines

[Timothée Nicolas]

OK, I'll try that, thx

2015-12-14 17:38 GMT+09:00 Dave May <dave.mayhem23 at gmail.com>:

> You have the configure line, so it should be relatively straight forward
> to configure / build petsc in your home directory.
>
>
> On 14 December 2015 at 09:34, Timothée Nicolas <timothee.nicolas at gmail.com
> > wrote:
>
>> 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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