[gmx-developers] Slow PME with git master and gcc 4.3.3 & 4.3.4

[Roland Schulz]

Hi,

it is caused by gcc using expf instead of exp.

By changing pme.c in the two places where exp is called this way:
-                    tmp1[kx]  = exp(tmp1[kx]);
+                    double t = tmp1[kx];
+                    tmp1[kx]  = exp(t);

gcc 4.3.4 is as fast as gcc 4.4.2.

See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35202
(The bug is not related to performance but it discusses the replacement from
exp->expf)

But what I don't understand at all is why expf is so much slower (expf 7-8x
as slow in pme.c than exp (*)). A small test program which computes exp on
an array, compiled with gcc 4.3.4 is actually faster than with 4.4.2 (30%).
Thus for the test-program the expf seams to be faster. Why expf is so slow
in pme.c is odd.

*) Got that measurement form HPCToolkit. Shows performance line-by-line.
Thus it was easy to pin-point the cause.

Roland


On Fri, May 14, 2010 at 10:43 AM, Berk Hess <hess at cbr.su.se> wrote:

> Hi,
>
> The PME code in git master is a lot faster than the current 4.0.7
> release code.
> However, there seems to be a bug in gcc version 4.3.3 and 4.3.4 (4.3.2
> and 4.4.1 are fine)
> that make the pme_solve part 5 times (!) slower than with proper
> compiler versions.
> This is between 10% and 20% of the total mdrun performance.
> We are currently trying to figure out what triggers this issue and we
> are sending
> a bug report to the gcc mailing list.
>
> I someone has a hint, please reply.
>
> Berk
>
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