Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v6)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-16 20:58:30]:

> Balbir Singh wrote:
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-16
> > 20:10:41]:
> >> >> At least, this check will be necessary in v7, I think.
> >> >> shrink_slab() should be called.
> >> >
> >> > Why do you think so? So here is the design
> >> >
> >> > 1. If a cgroup was using over its soft limit, we believe that this
> >> >    cgroup created overall memory contention and caused the page
> >> >    reclaimer to get activated.
> >> This assumption is wrong, see below.
> >>
> >> >    If we can solve the situation by
> >> >    reclaiming from this cgroup, why do we need to invoke shrink_slab?
> >> >
> >> No,
> >> IIUC, in big server, inode, dentry cache etc....can occupy Gigabytes
> >> of memory even if 99% of them are not used.
> >>
> >> By shrink_slab(), we can reclaim unused but cached slabs and make
> >> the kernel more healthy.
> >>
> >
> > But that is not the job of the soft limit reclaimer.. Yes if no groups
> > are over their soft limit, the regular action will take place.
> >
> Oh, yes, it's not job of memcg but it's job of memory management.
> 
> 
> >>
> >> > If the concern is that we are not following the traditional reclaim,
> >> > soft limit reclaim can be followed by unconditional reclaim, but I
> >> > believe this is not necessary. Remember, we wake up kswapd that will
> >> > call shrink_slab if needed.
> >> kswapd doesn't call shrink_slab() when zone->free is enough.
> >> (when direct recail did good jobs.)
> >>
> >
> > If zone->free is high why do we need shrink_slab()? The other way
> > of asking it is, why does the soft limit reclaimer need to call
> > shrink_slab(), when its job is to reclaim memory from cgroups above
> > their soft limits.
> >
> Why do you consider that softlimit is called more than necessary
> if shrink_slab() is never called ?

A run away application can do that. Like I mentioned with the tests I
did for your patches. Soft limits were at 1G/2G and the applications
(two) tried to touch all the memory in the system. The point is that
shrink_slab() will be called if the normal system experiences
watermark issues, soft limits will tackle/control cgroups running out
of their soft limits and causing memory contention to take place.


-- 
	Balbir

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