[petsc-users] EXTERNAL: Re: F90 to petsc 3.4.3 interface - loss of accuracy

[Barry Smith]

> On Jan 28, 2015, at 12:54 PM, <Carol.Brickley at awe.co.uk> <Carol.Brickley at awe.co.uk> wrote:
> 
> Hi,
> 
> I have a F90 suite that uses the petsc and runs for structured and unstructured matrices. For the unstructured we do not see the change at tiny decimal places but in the structured we do.

   What data is the change in? A solution? Some derived quantity of the solution? Or in a residual?  Depending on what the difference is in generally that seems a very small difference to me.

> The same solver is being used - GMRES. The tolerance should be 1e-10 in both cases but I can check in case there is a mistake.

   You can try using KSP and SNES tolerances of 1e-12 to see if you get closer match -ksp_rtol 1.e-12 -snes_rtol 1.e-12
> 
> Carol
> 
> Dr Carol Brickley 
> BSc,PhD,ARCS,DIC,MBCS
> 
> Senior Software Engineer
> Applied Computer Science
> DS+T,
> AWE
> Aldermaston
> Reading
> Berkshire
> RG7 4PR
> 
> Direct: 0118 9855035
> 
> -----Original Message-----
> From: Barry Smith [mailto:bsmith at mcs.anl.gov] 
> Sent: 28 January 2015 18:52
> To: Brickley Carol AWE
> Cc: petsc-users at mcs.anl.gov
> Subject: EXTERNAL: Re: [petsc-users] F90 to petsc 3.4.3 interface - loss of accuracy
> 
> 
>> On Jan 28, 2015, at 12:39 PM, <Carol.Brickley at awe.co.uk> <Carol.Brickley at awe.co.uk> wrote:
>> 
>> Hi,
>> 
>> Does anyone find loss of accuracy between F90 code and Petsc 3.4.3 C codes? I am noticing that some of my data is returning 9.99999997e-13 instead of 1.00000000e-12 for example.
> 
>  I am not sure exactly what your question is. Do you have two codes a F90 code without PETSc and a PETSc based C code? What is the data that is different? Is it solutions, residual values, ...?  This kind of small difference can often be caused by different tolerances used during the iterative solvers. We'll need more information in order to make any suggestions.
> 
>   Barry
> 
>> 
>> Carol
>> 
>> Dr Carol Brickley 
>> BSc,PhD,ARCS,DIC,MBCS
>> 
>> Senior Software Engineer
>> Applied Computer Science
>> DS+T,
>> AWE
>> Aldermaston
>> Reading
>> Berkshire
>> RG7 4PR
>> 
>> Direct: 0118 9855035
>> 
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> 
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