Subject: Re: CCL:"%mem" trouble of Gaussian

From: Toomas Tamm <tt-ccl!at!kky.ttu.ee>
Date: Tue, 25 Nov 2003 18:35:31 +0200

On Tue, Nov 25, 2003 at 03:55:44PM +0100, VITORGE Pierre 094605 wrote:
> I had similar problems. I run tests with different values for %mem, it seems
> there is an optimum %mem value to obtain the shortest CPU time, this optimum
> value is usually far from the maximum reasonable value (i.e the amount of
> physical memory minus 150to300Mb for the system). This probably depends
> (among other reasons) on the size of the exchange file memory, and whether
> this file is at the begining of the disk. I asked help to Gaussian, but did
> not get any answer.

There exists a hypothesis that with today's fast CPU's it is better to
re-calculate some integrals in the CPU rather than store and read them
> from memory. Integrals are accessed rather randomly and the CPU cannot
cache them effectively. A cache miss can cost several tens of CPU
cycles while the value is fetched from RAM. It may be faster to
re-calculate a simple integral. Thus the calculation is faster if
there is less memory available, since this forces the program to
re-calculate integrals rather than store them.

Can someone with intricate knowledge of modern CPUs and integral
calculation algorithms tell if this is true?

-- 
Toomas Tamm                                 e-mail: tt-ccl!at!kky.ttu.ee
Chair of Inorganic Chemistry                voice:  INT+372-620-2810
Tallinn Technical University                fax:    INT+372-620-2020
Ehitajate tee 5, EE-19086 Tallinn, Estonia  http://www.kk.ttu.ee/toomas/