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[dm-devel] Re: [PATCH 4/7] bio-cgroup: Split the cgroup memory subsystem into two parts



On Thu, 07 Aug 2008 16:25:12 +0900 (JST)
Hirokazu Takahashi <taka valinux co jp> wrote:
> > >I've just noticed that most of overhead comes from the spin-locks
> > >when reclaiming the pages inside mem_cgroups and the spin-locks to
> > >protect the links between pages and page_cgroups.
> > Overhead between page <-> page_cgroup lock is cannot be catched by
> > lock_stat now.Do you have numbers ?
> > But ok, there are too many locks ;(
> 
> The problem is that every time the lock is held, the associated
> cache line is flushed.
I think "page" and "page_cgroup" is not so heavly shared object in fast path.
foot-print is also important here.
(anyway, I'd like to remove lock_page_cgroup() when I find a chance)

> 
> > >The latter overhead comes from the policy your team has chosen
> > >that page_cgroup structures are allocated on demand. I still feel
> > >this approach doesn't make any sense because linux kernel tries to
> > >make use of most of the pages as far as it can, so most of them
> > >have to be assigned its related page_cgroup. It would make us happy
> > >if page_cgroups are allocated at the booting time.
> > >
> > Now, multi-sizer-page-cache is discussed for a long time. If it's our
> > direction, on-demand page_cgroup make sense.
> 
> I don't think I can agree to this.
> When multi-sized-page-cache is introduced, some data structures will be
> allocated to manage multi-sized-pages. 
maybe no. it will be encoded into struct page.
> I think page_cgroups should be allocated at the same time.
> This approach will make things simple.
yes, of course.

> 
> It seems like the on-demand allocation approach leads not only
> overhead but complexity and a lot of race conditions.
> If you allocate page_cgroups when allocating page structures,
> You can get rid of most of the locks and you don't have to care about
> allocation error of page_cgroups anymore.
> 
> And it will also give us flexibility that memcg related data can be
> referred/updated inside critical sections.
> 
But it's not good for the systems with small "NORMAL" pages.
This discussion should be done again when more users of page_group appears and
it's overhead is obvious.

Thanks,
-Kame




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