Re: Re: 答复: compilation of namd

From: Axel Kohlmeyer (
Date: Thu May 12 2011 - 04:53:58 CDT

On Thu, May 12, 2011 at 3:45 AM, Bjoern Olausson
<> wrote:
> On Wednesday 11 May 2011 17:53:59 Jim Phillips wrote:
>> I meant more conservative in the sense of not breaking code, but Axel's
>> suggestion is correct. Changing -O3 to -O2 and adding -no-vec improves
>> performance on newer compilers without hurting it on the old ones. I also
>> bumped -march up from pentiumpro to pentium4 on 32-bit builds to enable
>> some explicit SSE2 code. I think ten years is long enough to wait.
> Hi, I was wondering if anyone has optimized the flags for Opteron (2378 and
> 2427) CPUs and intel compiler?
> So far I am using "-O3 -xSSSE3" and from what I read here I might add "-no-
> vec" and try to revert back to "-O2"

> But since the icc help only list pentium3 and pentium4 for -march I am unshure
> what to choose here if anything at all.

pentium3 or core are closer to opteron arch then pentium4

there is not much to be gained by arch specific optimization,
but rather by having an executable that is cache efficient and
pipelines well.

> Did omeone try to let the "-xHost" flag do the search for the highest
> optimisation?

as was mentioned before, the "highest" optimization may not result
in the fastest binary. how much impact a specific flag has is often
hard to measure and even harder to predict. this gets even more
complicated by the fact that aggressive optimization may lead to
a faster binary, but that may produce "wrong" results.
you cannot trust the vendor description, since for the most part
they have to add new features that make people buy their new
software and they have to make people feel good about having
their new compiler more than providing real performance increase
(which is really difficult). furthermore, compilers are tested and
benchmarked against a variety of workloads, while MD is just
one specific subset. what works good overall, may be bad for
a specific case.

> Cheers,
> Bjoern
> --
> Bjoern Olausson
> Martin-Luther-Universitt Halle-Wittenberg
> Fachbereich Biochemie/Biotechnologie
> Kurt-Mothes-Str. 3
> 06120 Halle/Saale
> Phone: +49-345-55-24942

Dr. Axel Kohlmeyer
Institute for Computational Molecular Science
Temple University, Philadelphia PA, USA.

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