On Fri, 17 Mar 2000, Jesse Pollard wrote:
> Paul Jakma <paul.jakma@compaq.com>
> > On Thu, 16 Mar 2000, James Sutherland wrote:
> >
> > > No. *ANY* memory allocation system can run out of memory. Avoiding
> > > "overcommitting" would make the OOM situation arise SOONER (and more
> > > frequently), as well as killing performance.
> > >
> >
> > well, it's a more a question of whether you make promises that you might
> > not be able to keep. If you do (ie overcommit) then it's your
> > (kernel) problem. If you don't, it's not.
> >
> > Without overcommit the /system/ can run out memory, of course - it's
> > finite - but it's no longer a kernel problem.
> >
> > > Right... Now we'll try this on the university's central Unix system, shall
> > > we? Let's see... 6000 users, 2Gb RAM+swap. They get about 300K each.
> > > That's ALMOST enough to log in with!
> >
> > well then get more ram/swap. But at least it has become a hw issue.
>
> It's still not right - the assumption that you have 6000 concurrent users
> on a 2GB based system is unreasonable. It is much more likely that there
> is only 40-50 conccurent users. If the limit is 40 then it becomes 2GB/40
> or ~50MB per user not 300K. After that what you have is a management decision
> on system expansion, AND the data to back that decision. I have worked in
> such an environment, and that's the way it is.
I never said 6000 CONCURRENT users. BUT the system is perfectly happy
without any limits on the number of users. We could have 200 users using
an AVERAGE of 5Mb each quite happily - if they are all running the same
couple of programs (a typical workload) overcommit makes the difference
between needing 2Gb of swap and 20Gb, without *ANY* difference in
functionality.
The enormous overhead disabling overcommit would incur, without ANY
benefits, are simply not practical.
> > > The alternative just isn't practical in any real-world situation, except
> > > more-or-less single user boxes.
> >
> > the alternative is very practical. boxes where oracle runs as one user /
> > apache as another for example. You might want to lock down something
> > that you know can cause problems.. etc..
> >
> > > Even then, that single user will still run
> > > out of memory - it's just his quota, rather than a physical limit.
> > >
> >
> > obviously.
> >
> > > So his processes STILL end up dying randomly, but they do it sooner rather
> > > than later. Hrm. Wow.
> > >
> >
> > but then it's the users problem - not the OS..
> >
> > > On any realistically specced box, it is almost zero already.
> > >
> >
> > no it's not. a plain linux box today has a terrible tendency to die if
> > an app misbehaves wrt to memory. It's that bad. (unless you think 1GB of
> > RAM + 4GB's+ of swap is a reasonable spec for a desktop)
> >
> > per process limits go a long long way to solving this (easy with
> > pam_limits) but not far enough. But per user limits would be a huge
> > improvement...
> >
> > > No, it's a risk with *EVERY* OS.
> >
> > no it's not. A non overcommiting OS doesn't run out of VM for
> > processes. it cleanly grants or denies memory requests. What the app
> > does after that is not a kernel problem.
>
> I agree that is is not a problem with the os.
> It can also prevent additional logins if the resources for the new login
> are not available to the specific user attempting to log in. This is done
> in a LOT of UNIX systems, as long as they have functioning, per user,
> resource controls.
Oh dear. The problem here is when the system runs out of VM and has to
start denying malloc() requests. *NOT* that it has already allocated more
VM than it has - it has allocated the maximum amount it is able to, and
now needs to claw some back for legitimate processes.
Overcommit is, if anything, a partial SOLUTION here - not part of the
problem.
James.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Thu Mar 23 2000 - 21:00:20 EST