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<div class="moz-cite-prefix">what was the conv_thr in this example ?<br>
the default is 1.d-6 which is the range of variations you are
reporting.<br>
If so, could you try with a tighter threshold ?<br>
stefano<br>
<br>
On 26/01/2016 17:55, Paolo Giannozzi wrote:<br>
</div>
<blockquote
cite="mid:CAPMgbCuHLxkM-5EFtSmf09V-7tS5_e-uZmRbCgKkOh6FkzBpzw@mail.gmail.com"
type="cite">
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<div>Very interesting, and excellent questions, for which
unfortunately I have no clear answer (nor has anybody else, I
am afraid). <br>
<br>
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 <a moz-do-not-send="true"
href="http://noncolin-constrain_total.in">noncolin-constrain_total.in</a>,
another notable offender is <a moz-do-not-send="true"
href="http://vdw-ts.in">vdw-ts.in</a>. <br>
<br>
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 :-)<br>
<br>
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<div>Paolo<br>
</div>
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<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Jan 26, 2016 at 12:23 PM, Nick
Wilson <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:nw.qeforge.5211@family-wilson.me.uk"
target="_blank">nw.qeforge.5211@family-wilson.me.uk</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<br>
<br>
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.<br>
<br>
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/<a
moz-do-not-send="true"
href="http://noncolin-constrain_total.in" rel="noreferrer"
target="_blank">noncolin-constrain_total.in</a> which
showed variation in between -55.54478325 Ry and
-55.54478414 Ry.<br>
<br>
I ran the test through the Intel Inspector tool but that
didn’t show up any threading deadlocks or data races.<br>
<br>
I dropped the compiler optimisation to -O0 and added the
“-fp-model strict” and “-fp-model source” compiler flags but
that had no effect.<br>
<br>
I tried using some of the relevant environment variables
(KMP_DETERMINISTIC_REDUCTION=1 and MKL_CBWR=COMPATIBLE)
which also had no effect.<br>
<br>
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.<br>
<br>
I dropped back to an earlier version of MKL but the effect
was still present.<br>
<br>
As it was thread-related I tried linking against the
sequential version of MKL but that didn’t help.<br>
<br>
So, I guess my questions are:<br>
Should the results always be invariant of the number of
OpenMP threads?<br>
Is there anything unique about the <a
moz-do-not-send="true"
href="http://noncolin-constrain_total.in" rel="noreferrer"
target="_blank">noncolin-constrain_total.in</a> test case
which would cause it to behave differently to the rest of
the tests?<br>
<br>
Best regards,<br>
Nick Wilson<br>
<br>
<br>
System details:<br>
Intel Sandy Bridge E5-2650 CPU<br>
CentOS Linux release 7.2.1511<br>
MKL from Intel compiler 16.0.0<br>
GNU compiler version 4.8.5<br>
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<br clear="all">
<br>
-- <br>
<div class="gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>Paolo Giannozzi, Dip. Scienze Matematiche
Informatiche e Fisiche,<br>
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br>
Phone +39-0432-558216, fax +39-0432-558222<br>
<br>
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