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SubjectRe: [PATCH -mm 1/5] quick lookup memcg by ID
On Tue, 3 Aug 2010 08:52:16 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-08-02 19:13:04]:
>
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> >
> > Now, memory cgroup has an ID per cgroup and make use of it at
> > - hierarchy walk,
> > - swap recording.
> >
> > This patch is for making more use of it. The final purpose is
> > to replace page_cgroup->mem_cgroup's pointer to an unsigned short.
> >
> > This patch caches a pointer of memcg in an array. By this, we
> > don't have to call css_lookup() which requires radix-hash walk.
> > This saves some amount of memory footprint at lookup memcg via id.
> >
>
> It is a memory versus speed tradeoff, but if the number of created
> cgroups is low, it might not be all that slow, besides we do that for
> swap_cgroup anyway - no?
>

In following patch, pc->page_cgroup is changed from pointer to ID.
Then, this lookup happens in lru_add/del, for example.
And, by this, we can place all lookup related things to __read_mostly.
With css_lookup(), we can't do it and have to be afraid of cache
behavior.

I hear there are a users who create 2000+ cgroups and considering
about "low number" user here is not important.
This patch is a help for getting _stable_ performance even when there are
many cgroups.

Thanks,
-Kame



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