Subject: Summary: CPU-MEM-RWF relation from G94

From: Tapas Kar <[email-masked]>
Date: Mon, 19 Oct 1998 20:40:33 -0500 (CDT)

> To check the effect of %MEM on G94 jobs, i did 4 CCSD(T) freq
> calculations using same input file but different %MEM=32, 64, 128 and
> 150MB. No other application prog. was running during these four jobs.
>
> The result is:
>   %MEM      CPU       RWF(MB)       MaxMem (from output)
>    32mb    55m 35.2s   45           4194304
>    64mb    57m 11.8s   77           8388608
>   128mb    59m 37.5s  141           16777216
>   150mb   1h 0m 25.5s  163          19660800
>
> It looks odd. Expert's comment please.
> Tapas
> PS: I am using RS/6000 Model 590 and AIX4.2
>     Total 256mb memory and 13(2+2+9)gb hard disk.
>
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AIX uses the spare RAM memory as the disk cache.  Thus when you allocate
memory to G94 with the %MEM directive you are also actually reducing the
disk cache, slowing iour I/O. If your job is disk-intensive ( like CC ) it
is reasonably possible to observe behaviors like the one you experience

just my 0.02$

Ivan

--
Dr. Ivan Rossi - Computational Chemistry Consultant
e-mail: [email-masked]    phone: (+39)-051-456177
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Though not an expert, I may have some relevant observations to share.

You point out something odd about the Gaussian programs, namely the
less-than-intuitive relationship between the amount of memory allocated
and
the ressources used.  We have recently run a series of tests on a variety
of machines to examine these relationships, but I'm afraid that in the end
we were only confused, abeit on a higher level.

We concentrated our efforts on time-consuming jobs, which in our line of
work means MP2/4 and QCISD(T) calculations (G2 and CBS-Q methods).  The
results are somewhat dependent upon the size of the system and upon the
hardware employed.
        For a relatively small open-shell system (a C3H7N radical cation)
we found that the UMP2/6-311+G(3df,2p) step of a G2(MP2) calculation on a
single node of an IBM SP2 cluster would use within reason the same amount
of cpu time, regardless of whether the job was allowed to use (%Mem) 4MW,
16MW, or 64MW of memory.  The reported maximum size of the read-write file
in the three runs was 700MB, 365MB, and 633MB; in two runs using 10MW and
100MW we observed that the read-write file was almost three times bigger
in
the 100MW run than in the 10MW run (900MB vs 350MB).
        The QCISD(T)/6-311G(d,p) step ran 30% faster when 64MW were
available (for the C3H7N+. system, the memory required for AO integral
storage is 25MW); the time used for the triples was the same, regardless
of
the %Mem setting. (see below).
Doing CBS-Q calculations on an SGI Origin200 for a C3H7O cation, we found
that the execution times would vary only little with the amount of memory
allocated (8MW, 16MW, 32MW, 48MW).  The memory required for in-memory
allocated (1.33GB, 1.39GB, 1.51GB, 1.64GB).

QCISD(T)/6-31G(d) calculations (Origin200) for a somewhat bigger system (a
C4H12N cation) indicated that the QCISD step would benefit from increased
memory (test runs with 8MW, 16MW, 32MW, 48MW; in-memory storage of AO
integrals requiring 25MW), but that, strangely enough, the triples
calculations would actually slow down.

To examine this further we did QCISD(T)/6-31G(d) calculations for C5H14N
cations on an SGI Origin200, on a single node of an IBM SP2 cluster, and
on
a Fujitsu VX1.  In all three instances we observed that the cpu-time for
the triples calculations would INCREASE quite a bit with increased memory
(data points: SGI 246 min (10MW), 321 min (60MW); IBM 97 min (16MW), 209
min (80MW); the memory required for AO integral storage was 51MW; similar
results on the Fujitsu machine).  These slow-downs were on all three
machines to some extent off-set by a more rapid execution of the QCISD
step
with increased memory.  Also here we observed that increased memory would
be accompanied by an increase in the size of the read-write file.

Since doing these calculations I have been told that also other people
have
observed that the cpu-time used by the Gaussian programs for highly
correlated calculations can show an inverse dependence on the amount of
memory allocated.  I don't know if these things have changed in
Gaussian98.

Hope this helps.

Steen
--

Steen Hammerum                                           [email-masked]
Department of Chemistry                              (+45) 35 32 02 08
University of Copenhagen, Denmark               fax: (+45) 35 32 02 12

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Tapas Kar, Ph. D                           
Asst. Scientist/Asst. Professor (Adjunct)                            
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