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

[Michal Hocko <mhocko@suse.cz>]

On Mon 17-12-12 12:54:15, Johannes Weiner wrote:
> 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.

I think this is for a longer discussion.

> 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.

Yes, this is reasonable. This is in no way a cleanup so it would just
delay otherwise very nice cleanup.

Thanks!
-- 
Michal Hocko
SUSE Labs

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