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SubjectRe: Avoiding OOM on overcommit...?
David Whysong <dwhysong@physics.ucsb.edu>:
> On Mon, 20 Mar 2000, Jesse Pollard wrote:
>
> >On Mon, 20 Mar 2000, David Whysong wrote:
> >>
> >>That's very misleading. In fact if you give the overcommitted system the
> >>same amount of VM, it will work just fine.
> >>
> >>In other words, turning off overcommit isn't what saves you. You added
> >>more memory!
> >
> >I guaranteed that the memory allocated could be used. I didn't just add
> >more memory. Just adding more memory will still allow the system to fail,
> >it may take longer, it may not happen as often. But it can still happen.
>
> The only reason your non-overcommit situation doesn't fail is because you
> gave that system more memory than the overcommitted system.

No. just adding more memory would still allow the OOM to occur. and possibly
for the same reason. Runaway memory allocators are not stopped by adding
memory. They are stopped by resource limits. I just want the ability to
define that limit on a per-user basis so that I can prevent it from affecting
other users.

In a system that doesn't support resource quotas, just adding memory
doesn't prevent the OOM. It just delays it. There is no control.

> In order to make a reasonable comparison, you must keep the total VM
> constant. The failure modes for a non-overcommitted system are a superset
> of the failure modes of an overcommitted system.

NOPE - it's a subset. The OOM condition affects many users, not one. It can
affect the usability of the system. It can crash/reboot the system.

non-overcommit:
1. aborts at least one user process
2. may abort multiple processes, but all belonging to the user over quota

overcommit:
1. aborts at least one user process
2. may abort multiple processes belonging to different users
3. may abort system processes
4. may force reboot.

> Stated another way: for a fixed quantity of virtual memory, in low memory
> situations, a system without overcommit will ALWAYS have a failure before
> or at the same point as an overcommitted system.

yes.
-------------------------------------------------------------------------
Jesse I Pollard, II
Email: pollard@navo.hpc.mil

Any opinions expressed are solely my own.

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