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SubjectRe: OOM Killer don't works at all if the system have >gigabytes memory (was Re: [PATCH] mm: check zone->all_unreclaimable in all_unreclaimable())
On Thu, 12 May 2011, Minchan Kim wrote:

> > processes a 1% bonus for every 30% of memory they use as proposed
> > earlier.)
>
> I didn't follow earlier your suggestion.
> But it's not formal patch so I expect if you send formal patch to
> merge, you would write down the rationale.
>

Yes, I'm sure we'll still have additional discussion when KOSAKI-san
replies to my review of his patchset, so this quick patch was written only
for CAI's testing at this point.

In reference to the above, I think that giving root processes a 3% bonus
at all times may be a bit aggressive. As mentioned before, I don't think
that all root processes using 4% of memory and the remainder of system
threads are using 1% should all be considered equal. At the same time, I
do not believe that two threads using 50% of memory should be considered
equal if one is root and one is not. So my idea was to discount 1% for
every 30% of memory that a root process uses rather than a strict 3%.

That change can be debated and I think we'll probably settle on something
more aggressive like 1% for every 10% of memory used since oom scores are
only useful in comparison to other oom scores: in the above scenario where
there are two threads, one by root and one not by root, using 50% of
memory each, I think it would be legitimate to give the root task a 5%
bonus so that it would only be selected if no other threads used more than
44% of memory (even though the root thread is truly using 50%).

This is a heuristic within the oom killer badness scoring that can always
be debated back and forth, but I think a 1% bonus for root processes for
every 10% of memory used is plausible.

Comments?

> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -160,7 +160,7 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
> >         */
> >        if (p->flags & PF_OOM_ORIGIN) {
> >                task_unlock(p);
> > -               return 1000;
> > +               return 10000;
> >        }
> >
> >        /*
> > @@ -177,32 +177,32 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
> >        points = get_mm_rss(p->mm) + p->mm->nr_ptes;
> >        points += get_mm_counter(p->mm, MM_SWAPENTS);
> >
> > -       points *= 1000;
> > +       points *= 10000;
> >        points /= totalpages;
> >        task_unlock(p);
> >
> >        /*
> > -        * Root processes get 3% bonus, just like the __vm_enough_memory()
> > -        * implementation used by LSMs.
> > +        * Root processes get 1% bonus per 30% memory used for a total of 3%
> > +        * possible just like LSMs.
> >         */
> >        if (has_capability_noaudit(p, CAP_SYS_ADMIN))
> > -               points -= 30;
> > +               points -= 100 * (points / 3000);
> >
> >        /*
> >         * /proc/pid/oom_score_adj ranges from -1000 to +1000 such that it may
> >         * either completely disable oom killing or always prefer a certain
> >         * task.
> >         */
> > -       points += p->signal->oom_score_adj;
> > +       points += p->signal->oom_score_adj * 10;
> >
> >        /*
> >         * Never return 0 for an eligible task that may be killed since it's
> > -        * possible that no single user task uses more than 0.1% of memory and
> > +        * possible that no single user task uses more than 0.01% of memory and
> >         * no single admin tasks uses more than 3.0%.
> >         */
> >        if (points <= 0)
> >                return 1;
> > -       return (points < 1000) ? points : 1000;
> > +       return (points < 10000) ? points : 10000;
> >  }
> >
> >  /*
> > @@ -314,7 +314,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> >                         */
> >                        if (p == current) {
> >                                chosen = p;
> > -                               *ppoints = 1000;
> > +                               *ppoints = 10000;
>
> Scattering constant value isn't good.
> You are proving it now.
> I think you did it since this is not a formal patch.
> I expect you will define new value (ex, OOM_INTERNAL_MAX_SCORE or whatever)
>

Right, we could probably do something like

#define OOM_SCORE_MAX_FACTOR 10
#define OOM_SCORE_MAX (OOM_SCORE_ADJ_MAX * OOM_SCORE_MAX_FACTOR)

in mm/oom_kill.c, which would then be used to replace all of the constants
above since OOM_SCORE_ADJ_MAX is already defined to be 1000 in
include/linux/oom.h.
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