Subject: RIMM vs SDRAM

From: Tom Kuppens <[email-masked]>
Date: Tue, 23 Oct 2001 12:07:29 +0100 (NFT)

Hi there,

We have several linux machines that run Gaussian. Now we want to add some new
power to our park i.e. a PIV - 1700MhZ with 512M RAM (SDRAM or RIMM). 

Is there someone who can provide me with some info on the Gaussian and/or 
overall performance with RIMM versus SDRAM modules.

Thanks in advance,

Tom Kuppens
Ghent University

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From: "Scott Anderson" <[email-masked]>
Date: Tue, 23 Oct 2001 08:34:14 -0600

Dear Tom:

Regarding your question about RIMM (RDRAM) v.s. SDRAM (or DDR SDRAM) for
Gaussian:

I have done a fair bit of comparing machines for Gaussian, as I am planning
to buy a bunch in the near future.  I have been using an MP2 Opt Freq job
that takes a few hours as my benchmark.  I have also run much longer Direct
Dynamics trajectory jobs on several different configurations.  The MP2 job
was deliberately chose to avoid a lot of disk i/o because I really wanted to
focus on the computational speed.  I have compared 1 and 1.4 GHz Athlons,
PIIIs, and P4s.  The conclusion is that for Gaussian, the P4 is 20-30%
faster than the Athlons for the same clock speed, and since you can get P4s
with faster clocks, the P4 ends up being substantially faster.  This speed
difference is quite surprising because the P4 floating point unit is
substantially inferior to that of the Athlon, and as far as I have been able
to determine, the rate deterimining step for these fast chips is memory
speed.  Here the RDRAM really shines.  I have also compared Athlons with
PC133 SDRAM and PC2100 DDR ram, which is theoretically substantially faster.
For Gaussian jobs, I see no difference.

One interesting thing to look at in this regard is the review in the Tom's
Hardware review of the new Athlon XP.
http://www6.tomshardware.com/cpu/01q4/011009/index.html     They compare
several different Athlons with different memory configurations, and also
2GHz P4s with both RDRAM and DDR memory.  The Athlon XP looks great on most
of the benchmarks -- equalling or exceeding the P4, even though the P4 clock
speed is much higher.   The one area where the XP fails, however, is memory
speed.  Note that the P4 with RDRAM does 50% better than the P4 with DDR,
and nearly a factor of two better than the Athlons with either PC133 or DDR
memory.

My conclusions:
1. Gaussian is probably memory-bandwidth limited, and you should spend the
extra money RDRAM.
2. DDR memory doesn't really make a huge difference -- certainly nothing
like the factor of two you might expect.
3. The ideal configuration is very application dependent.  For Gaussian, I
would go with the P4.  If you are running a mix of programs, you really have
a tough decision.

Have fun

Prof. Scott L. Anderson
Department of Chemistry
University of Utah
315 S. 1400 E. Rm 1216
Salt Lake City, UT  84112
(801)585-7289
FAX(801)581-8433
www.chem.utah.edu/chemistry/faculty/anderson/anderson.html
----- Original Message -----
From: "Tom Kuppens" <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, October 23, 2001 5:07 AM
Subject: CCL:RIMM vs SDRAM

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