Re: [patch 2/8] mm: vmscan: disregard swappiness shortly before going OOM

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Dec 17, 2012 at 05:37:35PM +0100, Michal Hocko wrote:
> On Fri 14-12-12 19:18:51, Johannes Weiner wrote:
> > On Fri, Dec 14, 2012 at 05:13:45PM +0100, Michal Hocko wrote:
> > > On Fri 14-12-12 10:43:55, Rik van Riel wrote:
> > > > On 12/14/2012 03:37 AM, Michal Hocko wrote:
> > > > 
> > > > >I can answer the later. Because memsw comes with its price and
> > > > >swappiness is much cheaper. On the other hand it makes sense that
> > > > >swappiness==0 doesn't swap at all. Or do you think we should get back to
> > > > >_almost_ doesn't swap at all?
> > > > 
> > > > swappiness==0 will swap in emergencies, specifically when we have
> > > > almost no page cache left, we will still swap things out:
> > > > 
> > > >         if (global_reclaim(sc)) {
> > > >                 free  = zone_page_state(zone, NR_FREE_PAGES);
> > > >                 if (unlikely(file + free <= high_wmark_pages(zone))) {
> > > >                         /*
> > > >                          * If we have very few page cache pages, force-scan
> > > >                          * anon pages.
> > > >                          */
> > > >                         fraction[0] = 1;
> > > >                         fraction[1] = 0;
> > > >                         denominator = 1;
> > > >                         goto out;
> > > > 
> > > > This makes sense, because people who set swappiness==0 but
> > > > do have swap space available would probably prefer some
> > > > emergency swapping over an OOM kill.
> > > 
> > > Yes, but this is the global reclaim path. I was arguing about
> > > swappiness==0 & memcg. As this patch doesn't make a big difference for
> > > the global case (as both the changelog and you mentioned) then we should
> > > focus on whether this is desirable change for the memcg path. I think it
> > > makes sense to keep "no swapping at all for memcg semantic" as we have
> > > it currently.
> > 
> > I would prefer we could agree on one thing, though.  Having global
> > reclaim behave different from memcg reclaim violates the principle of
> > least surprise. 
> 
> Hmm, I think that no swapping at all with swappiness==0 makes some sense
> with the global reclaim as well. Why should we swap if admin told us not
> to do that?
> I am not so strong in that though because the global swappiness has been
> more relaxed in the past and people got used to that. We have seen bug
> reports already where users were surprised by a high io wait times when
> it turned out that they had swappiness set to 0 because that prevented
> swapping most of the time in the past but fe35004f changed that.
> 
> Usecases for memcg are more natural because memcg allows much better
> control over OOM and also requirements for (not) swapping are per group
> rather than on swap availability. We shouldn't push users into using
> memcg swap accounting to accomplish the same IMHO because the accounting
> has some costs and its primary usage is not to disable swapping but
> rather to keep it on the leash. The two approaches are also different
> from semantic point of view. Swappiness is proportional while the limit
> is an absolute number.

I agree with the usecase that Rik described, though: it makes sense to
go for file cache exclusively as long as the VM can make progress, but
once we are getting close to OOM, we may as well swap.  swappiness is
describing an eagerness to swap, not a limit.  Not swapping ever with
!swappiness does not allow you to do this, even with very low
swappiness settings, you can end up swapping with just little VM load.

They way swappiness works for memcg gives you TWO options to prevent
swapping entirely for individual groups, but no option to swap only in
case of emergency, which I think is the broader usecase.

But I also won't fight this in this last-minute submission so I
dropped this change of behaviour for now, it'll just be a cleanup.

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