[gmx-users] Question about benchmarks

[Susan Chacko]

Thanks. I'm just a sysadmin, and am building it for a user
who thinks she needs double-precision. But when she sees the
difference in time, she may have second thoughts :-)

Susan.

On Tue, 11 Jun 2002, Erik Lindahl wrote:

> Susan Chacko wrote:
> > Hi folks,
> >
> > Are the benchmarks on the www.gromacs.org site for single-precision
> > builds of Gromacs, and would one expect a large difference between
> > single-precision and double-precision times?
> >
> > I just built Gromacs in double-precision mode, but am wondering
> > whether I should go back and rebuild in single-p as well, as the
> > benchmark times I'm seeing are quite different from those on the
> > Gromacs website.
> >
> > I'm running the DPPC benchmark on a Linux cluster, dual-processor Athlon
> > 1.4GHz nodes, RedHat 7, MPICH.
> >
>
> Hi Susan,
>
> The benchmarks are done in single precision, since that is accurate
> enough for almost everything except normal mode analysis.
>
> For an athlon you will probably gain a factor of 2 in speed by using
> single precision; we have written assembly loops that use multimedia
> instructions, but Athlon & Pentium 3 CPUs only support this in single
> precision.
>
> (On Pentium 4 systems we can do double precision multimedia
> instructions, but that is still 25-40% slower than single).
>
> The same thing applies for Apple machines since we can use
> single-precision Altivec instructions, but for the rest (IBM, etc), the
> difference is much smaller.
>
> In general, I'd recommend to stick to single precision unless you are
> really sure you need double.
>
> Cheers,
>
> Erik
>