Re: overcommit stuff

[Hugh Dickins <hugh@veritas.com>]

On Sat, 21 Sep 2002, Andrew Morton wrote:
> Hugh Dickins wrote:
> > On Sat, 21 Sep 2002, Andrew Morton wrote:
> > >
> > > running 10,000 tiobench threads I'm showing 23 gigs of
> > > `Commited_AS'.  Is this right?  Those pages are shared,
> > > and if they're not PROT_WRITEable then there's no way in
> > > which they can become unshared?   Seems to be excessively
> > > pessimistic?
>  
> > Committed_AS certainly errs on the pessimistic side, that's
> > what it's about.  How much swap do you have i.e. is 23GB
> > committed impossible, or just surprising to you?  Does the
> > number go back to what it started off from when you kill
> > off the tests?  How are "those pages" allocated e.g. what
> > mmap args?
> 
> I have 7G physical, 4G swap.

When I wondered if impossible, of course I was overlooking
that you wouldn't be running with strict commit limitation,
so "impossible" is quite difficult to reach.

> "those pages" were just used by some scruffy perl script 
> running `./tiotest &' ten thousand times.  I assume it's
> shared executable text.

When I run tiotest here, /proc/<pid>/maps shows a little over
2MB of rwxp or rw-p areas, all to be counted in Committed_AS.
So 23GB for 10,000 of them sounds reasonable.  You think you
have less PROT_WRITE or less MAP_PRIVATE than I'm seeing?

> It seems very unlikely (impossible?) that those pages will
> ever become unshared.

I expect it's very unlikely (short of application bugs) that
those pages would become unshared; but they have been mapped
in such a way that the process is entitled to unshare them,
therefore they have been counted.  A good example of why
Linux does not impose strict commit accounting, and why
you may choose not to use Alan's strict accounting policy.

Hugh

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