[Pw_forum] Energy variations in noncolin-constrain_total.in with OpenMP & MKL

[stefano de gironcoli]

what was the conv_thr in this example ?
the default is 1.d-6 which is the range of variations you are reporting.
If so, could you try with a tighter threshold ?
stefano

On 26/01/2016 17:55, Paolo Giannozzi wrote:
> Very interesting, and excellent questions, for which unfortunately I 
> have no clear answer (nor has anybody else, I am afraid).
>
> One should obtain the same numbers - within the errors due to 
> roundoff, though -  in serial, OpenMP, MPI execution, and on different 
> machines, and with different compilers and mathematical libraries. In 
> practice, there are invariably small differences, that sometimes do 
> not completely disappear even if one pushes convergence thresholds to 
> very strict limits.  In addition to noncolin-constrain_total.in 
> <http://noncolin-constrain_total.in>, another notable offender is 
> vdw-ts.in <http://vdw-ts.in>.
>
> This could signal a small bug, but in my experience, most of those 
> cases can be linked to specific optimized mathematical libraries or 
> compiler versions. As long as we can blame somebody else, not  a big 
> problem :-)
>
> Paolo
>
> On Tue, Jan 26, 2016 at 12:23 PM, Nick Wilson 
> <nw.qeforge.5211 at family-wilson.me.uk 
> <mailto:nw.qeforge.5211 at family-wilson.me.uk>> wrote:
>
>     Hi,
>
>     I’ve been testing the OpenMP build of Quantum Espresso 5.3.0  on
>     our system using the Intel compiler and MKL and have a question
>     about variation of energy with the number of OpenMP threads used.
>
>     I ran all the plane wave tests in the test-suite directory using
>     between 1 and 16 OpenMP threads and they all gave consistent
>     results apart from pw_noncolin/noncolin-constrain_total.in
>     <http://noncolin-constrain_total.in> which showed variation in 
>     between -55.54478325 Ry and -55.54478414 Ry.
>
>     I ran the test through the Intel Inspector tool but that didn’t
>     show up any threading deadlocks or data races.
>
>     I dropped the compiler optimisation to -O0 and added the
>     “-fp-model strict” and “-fp-model source” compiler flags but that
>     had no effect.
>
>     I tried using some of the relevant environment variables
>     (KMP_DETERMINISTIC_REDUCTION=1 and MKL_CBWR=COMPATIBLE) which also
>     had no effect.
>
>     Changing to use the internal BLAS library resolved the issue so it
>     looks to be MKL-related. It’s present with both the GNU and Intel
>     compilers.
>
>     I dropped back to an earlier version of MKL but the effect was
>     still present.
>
>     As it was thread-related I tried linking against the sequential
>     version of MKL but that didn’t help.
>
>     So, I guess my questions are:
>     Should the results always be invariant of the number of OpenMP
>     threads?
>     Is there anything unique about the noncolin-constrain_total.in
>     <http://noncolin-constrain_total.in> test case which would cause
>     it to behave differently to the rest of the tests?
>
>     Best regards,
>     Nick Wilson
>
>
>     System details:
>       Intel Sandy Bridge E5-2650 CPU
>       CentOS Linux release 7.2.1511
>       MKL from Intel compiler 16.0.0
>      GNU compiler version 4.8.5
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>
>
>
> -- 
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
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