In message from Mark Hahn <[EMAIL PROTECTED]> (Tue, 10 Jun 2008
00:58:12 -0400 (EDT)):
...
for instance, you can always avoid OOM with the vm.overcommit_memory=2
sysctl (you'll need to tune vm.overcommit_ratio and the amount of swap
to get the desired limits.) in this mode, the kernel tracks how much
VM
it actually needs (worst-case, reflected in Committed_AS in
/proc/meminfo)
and compares that to a commit limit that reflects ram and swap.
if you don't use overcommit_memory=2, you are basically borrowing VM
space in hopes of not needing it. that can still be reasonable,
considering
how often processes have a lot of shared VM, and how many processes
allocate but never touch lots of pages. but you have to ask yourself:
would I like a system that was actually _using_ 16 GB of swap? if you
have 16x disks, perhaps, but 16G will suck if you only have 1 disk.
at least for overcommit_memory != 2, I don't see the point of
configuring
a lot of swap, since the only time you'd use it is if you were
thrashing.
sort of a "quality of life" argument.
But what are the reccomendations of modern praxis ?
it depends a lot on the size variance of your jobs, as well as their
real/virtual ratio. the kernel only enforces RLIMIT_AS
(vsz in ps),assuming a 2.6 kernel - I forget whether 2.4 did
RLIMIT_RSS or not.
if you use overcommit_memory=2, your desired max VM size determines
the amount of swap. otherwise, go with something modest - memory size
or so. but given that the smallest reasonable single disk these days
is probably about 320GB, it's hard to justify being _too_ tight.
:-) The disks we use in nodes is SATA WD/10K RPM w/70 GB :-))
We didn't set overcommit_memory=2, but really use strongly restricted
scheduling police for SGE batch jobs using only few applications. We
have only batch jobs (no interactive), moreover - practically only
*long batch jobs*. As a result we have summary VM (requested per node)
equal (or lower) than RAM. There is practically zero swap activity.
The only exclusion are (seldom executed) small test jobs,
non-parallelized, mainly for check of input data. They use small RAM
amount. So it looks for me that I may set even lower than 1.5*RAM swap
size (I think RAM+4G = 20G will be enough).
In message from Walid <[EMAIL PROTECTED]> (Tue, 10 Jun 2008
19:27:43 +0300):
Hi,
For an 8GB dual socket quad core node, choosing in the kick start
file --recommended instead of specifying size RHEL5 allocates 1GB of
memory. our developers say that they should not swap as this will
cause an overhead, and they try to avoid it as much as possible
OpenSuSE 10.3 recommends swap size=2 GB only, but I don't know,
performs SuSE inst software some estimation of server RAM or no.
Yours
Mikhail
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