[CentOS] Setting overcommit_memory=2 kills system

[Michael Coffman]

On Fri, May 18, 2012 at 10:39 AM, Les Mikesell <lesmikesell at gmail.com>wrote:

> On Fri, May 18, 2012 at 11:24 AM, Michael Coffman
> <michael.coffman at avagotech.com> wrote:
> >
> >> If you are already overcommitted, what do you expect to happen when
> >> you say not to allow that?   The kernel doesn't have a really good way
> >> to handle that situation (or any other OOM condition for that
> >> matter...).
> >>
> >>
> > OK.  So I was confused because free shows I have plenty of memory and I
> am
> > only now noticing that Committed_AS shows a very large value.   This is
> > largely an idle system:
> >
> >             total       used       free     shared    buffers     cached
> > Mem:       2052176     951648    1100528          0     147580     626096
> > -/+ buffers/cache:     177972    1874204
> > Swap:      2052088          0    2052088
> >
> > So my real question should have been why would the Committed_AS value be
> so
> > large?
> > Committed_AS:   137197248820 kB
> >
> > On the exact same hardware with fresh build of centos5u4 and overcommit
> > turned on:
> > Committed_AS:   125716 kB
>
> I think that means an application has malloc()'d but not actually used
> a huge amount of memory.   It used to be a common practice to test the
> amount of memory available by requesting more until the request
> failed, so it could be meaningless.  If overcommit is turned off the
> app gets an error when it expects and backs off.  The Committed_AS is
> an estimate of how much RAM/swap you would be likely to need if the
> requested storage is actually used.
>
> I get how overcommit works and it's affect on malloc.  I intentionally
turn it off so malloc will return a correct value when a process can't get
enough memory.   What I currently don't understand is the large value of
Committed_AS on my centos6/rhel6 systems.

I just did a fresh build of centos6u2 and there is very little running and
the value is:
Committed_AS:   136382202700 kB

There is almost nothing running....   Filtering out kernel processes here
is the list of running processes

I tried to post the ps listing, both as inline text and as an attached
ps.txt file and the message was rejected by Symantec :(

--
>    Les Mikesell
>      lesmikesell at gmail.com
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>



-- 
-MichaelC