[Beowulf] first cluster

Christopher Samuel samuel at unimelb.edu.au
Fri Jul 16 00:09:36 EDT 2010


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On 16/07/10 11:29, Mark Hahn wrote:

> every distro I've seen leaves these at the default seting:
> vm.overcommit_memory=0.  this is basically the traditional
> setting that tells the kernel to feel free to allocate way
> too much memory, and to resolve memory crunches via OOM

Looking at the kernel code if you set it vm.overcommit_memory
to 0 (OVERCOMMIT_GUESS) then the kernel allows *each process*
to allocate up to 97% of the total of RAM+swap (the last 3% is
reserved for root, or processes with CAP_SYS_ADMIN).   The catch
is that (as highlighted) the limit is a per process one, not a
system wide one.

With it set to 1 (OVERCOMMIT_ALWAYS) there are no checks at all,
it just returns 0 (OK) so any process can allocate as much as it
wants, just that you don't know who or what will get OOM'd when
you want to use it.. ;-)

With 2 (OVERCOMMIT_NEVER) you can never specify more than
your entire RAM+swap and the limit is applied across the
system.

We enforce RLIMIT_AS for MPI and single CPU processes by
setting pvmem limits in Torque in the default queue.

That doesn't work for SMP jobs so we have an 'smp' queue
for then which sets mem= instead, this means that pbs_mom
monitors the children and kills them if they go over their
limits.

cheers!
Chris
- -- 
 Christopher Samuel - Senior Systems Administrator
 VLSCI - Victorian Life Sciences Computational Initiative
 Email: samuel at unimelb.edu.au Phone: +61 (0)3 903 55545
         http://www.vlsci.unimelb.edu.au/

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