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On Wednesday 19 April 2006 10:47 am, Peter Adrian Meyer wrote:
> I've run into a memory allocation problem with fffear I'm hoping that I
> can get some pointers on fixing.
>
> Specifically, fffear will run on a linux machine (~600MB physical memory;
> ~1GB swap) although it hits the swap space pretty hard (job size is
> ~1.5GB). When I try to run the same script on an alpha with ~1GB
> physical memory and ~3GB swap, fffear exits with a CCPALC message saying
> that it can't allocate memory.
The default swap policy on DU (Digital Unix, Tru64, ...) is
"no overcommit", which means that you will get a memory allocation
error if swap space equal to the maximum run requirements cannot
be reserved in advance.
The default swap policy on most linux setups is "overcommit", which
means that it will go ahead and try to run your program anyhow, and
only error out on memory allocation failure is in fact triggered
during the course of your run.
"No overcommit" is a safer policy in the sense that you don't want
to get 24 hours into a long run and only then die because of
inadequate swap space. Better to reserve the swap in advance and
be guaranteed that you will not run out. On the other hand,
in this mode you obviously must provide larger swap areas.
The command to change swap policy for recent linux kernels is
"no overcommit"
echo -n 1 > /proc/sys/vm/overcommit_memory
"overcommit"
echo -n 2 > /proc/sys/vm/overcommit_memory
The default is actually 0, which is some heuristic hybrid with
unpredictable behaviour.
To change swap policy for DU4/Tru64 is messier:
This is from the docs. They use "eager" to mean "no overcommit",
and "lazy" to mean "overcommit".
The default swap mode is the "eager" swap mode.
To switch to "lazy" swap mode, as root rename the file
"/bin/swapdefault" to "/sbin/swapdefault.sav".
Then reboot the system. To switch back, rename the file back
to it's original name and reboot again.
On either linux or Tru64 you can of course also addess the
problem by adding [a lot] more swap space.
--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle WA
