MOLPRO >2GB RAM on IA32/Linux?

Gershom (Jan M.L.) Martin comartin at
Sun Jan 25 21:00:28 GMT 2004


SHORT VERSUON: Can MOLPRO use more than 2GB of RAM on a 
P4 or Xeon machine running Linux?


We are running MOLPRO on a cluster of dual Xeon machines running RedHat 8
with updated kernels.

Even on the 4GB machines, the most memory I can get with a stock kernel
is -m 253700000 (1936 MB) for a single-CPU job, and
-m 194600000 (1485 MB) for a dual-CPU job.

Upon downloading kernel 2.4.24 from and applying Andrea
Arcangeli's "3.5GB user space" patch                                               

I can now get -m 268200000 (2046 MB) in both single-and dual-CPU jobs.
The largest continguous memory segment available is a bit more than 2.5 GB
(due to the obnoxious Linux 7.3/8/9 habit of loading shared object libraries
at the 1 GB mark --- you can see them by 'cat /proc/<processid>/maps) ---
this can be increased a bit further by going to kernel 2.6 with Andrew
Morton's "mm" patches (which include a separate 4GB/4GB user/kernel 
space patch), and with a lot more doing (somehow forcing the shared object
libraries to load at lower addresses --- ideas anybody?) this can probably
be pushed over the 3 GB mark.

My question is now: is the P4/Linux version of MOLPRO capable at all of 
handling memory spaces larger than 2GB? Or is this intrinsically off the

Many thanks in advance
Jan Martin
Prof. Gershom (Jan M.L.) Martin      Computational Quantum Chemistry
    Department of Organic Chemistry / Kimmelman Building, Room 252
     Weizmann Institute of Science / IL-76100 Rechovot / ISRAEL
Email:       comartin at      Phone: +972(8)9342533
Web site:       FAX: +972(8)9344142

More information about the Molpro-user mailing list