Subject: Quick question on G98

From: Douglas_Stack/CAS/UNO/[email-masked]
Date: Mon, 8 Jan 2001 15:28:34 -0600

Dear CCl'ers,

In Gaussian when one assigns the amount of memory and the number of processor in
the link0 input area, is the memory requested for each processor?  In other
works if I assign %mem=16 and %NProc=4 will the job use a total of 16X4=48 MW,
or just 16 MW total?

Thanks in advance,

Douglas E. Stack
Assistant Professor
Department of Chemistry
University of Nebraska at Omaha
Omaha, NE 68182-0109
(402) 554-3647
(402) 554-3888 (fax)
[email-masked]

---

From: "Phillip D. Matz" <[email-masked]>
Date: Mon, 8 Jan 2001 16:12:39 -0800

Hello Dr. Stack,

The answer is that the %mem link0 command specifies the total non-coherent
memory size accessible virtually or physically.  For a Unified Memory
Architecture (UMA) system the system's memory is coherent and each processor
will have access to all of the system memory.  Thus, 16MW on a 4-way system
with UMA will access a total of 16MW.

However, for computer systems that employ Non-Unified Memory Architectures
(NUMA) the system's memory is not coherent.  In this case each processor is
assigned a portion of the total system memory and no other processor can
access the information stored in that memory (thus the designation NUMA).
NUMA systems can be logically or physically partitioned.  A cluster of
workstations or Beowulfs typically have NUMA as the memory is stored on each
motherboard and is accessible only by the cpus physically connected by a
northbridge (in x86 land) to the memory.  In this case specifying 16MW will
result in the use of 48MW for four processors, but since it is NUMA the job
itself will appear to be using only 16MW.

So the answer is that the gaussian job will believe it is occupying a memory
footprint that is the size specified by the Link0 %mem command (16MW in your
example), but in the real world it may or may not be tying up more physical
memory depending on the UMA vs NUMA for your particular system.  A rule of
thumb is that physically separated processors access memory in NUMA style
(COWS, NOWS, and Beowulfs), while integrated workstations (SP2s, CRAYs,
Alphas, Sparcs, etc...) access in UMA style.

As a personal example, my cluster has roughly 1.5GB total, but it is NUMA
and my G98 jobs are ran using only about 70MB.  If it was UMA I could run
G98 jobs employing all 1.5GB (great for in-core jobs!)...

Respectfully,
Phillip Matz
[email-masked]

----- Original Message -----
From: <Douglas_Stack/CAS/UNO/[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Sent: Monday, January 08, 2001 1:28 PM
Subject: CCL:Quick question on G98

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