Re: [RFC] Low overhead patches for the memory cgroup controller (v2)

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

* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2009-06-01 13:25:05]:

> I'm sorry for my very late reply.
> 
> I've been working on the stale swap cache problem for a long time as you know :)
> 
> On Sun, 17 May 2009 12:15:43 +0800, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-05-16 02:45:03]:
> > 
> > > I think set/clear flag here adds race condtion....because pc->flags is
> > > modfied by
> > >   pc->flags = pcg_dafault_flags[ctype] in commit_charge()
> > > you have to modify above lines to be
> > > 
> > >   SetPageCgroupCache(pc) or some..
> > >   ...
> > >   SetPageCgroupUsed(pc)
> > > 
> > > Then, you can use set_bit() without lock_page_cgroup().
> > > (Currently, pc->flags is modified only under lock_page_cgroup(), so,
> > >  non atomic code is used.)
> > >
> > 
> > Here is the next version of the patch
> > 
> > 
> > Feature: Remove the overhead associated with the root cgroup
> > 
> > From: Balbir Singh <balbir@linux.vnet.ibm.com>
> > 
> > This patch changes the memory cgroup and removes the overhead associated
> > with accounting all pages in the root cgroup. As a side-effect, we can
> > no longer set a memory hard limit in the root cgroup.
> > 
> I agree to this idea itself.
>

Thanks!
 
> > A new flag is used to track page_cgroup associated with the root cgroup
> > pages. A new flag to track whether the page has been accounted or not
> > has been added as well. Flags are now set atomically for page_cgroup,
> > pcg_default_flags is now obsolete, but I've not removed it yet. It
> > provides some readability to help the code.
> > 
> > Tests:
> > 1. Tested lightly, previous versions showed good performance improvement 10%.
> > 
> You should test current version :)
> And I think you should test this patch under global memory pressure too
> to check whether it doesn't cause bug or under/over flow of something, etc.
> memcg's LRU handling about SwapCache is different from usual one.
> 

OK, I've tested it using my stress tool, but I'll modify to add some
of the things you've pointed out.

> > NOTE:
> > I haven't got the time right now to run oprofile and get detailed test results,
> > since I am in the middle of travel.
> > 
> > Please review the code for functional correctness and if you can test
> > it even better. I would like to push this in, especially if the %
> > performance difference I am seeing is reproducible elsewhere as well.
> > 
> > Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> > ---
> > 
> >  include/linux/page_cgroup.h |   12 ++++++++++++
> >  mm/memcontrol.c             |   42 ++++++++++++++++++++++++++++++++++++++----
> >  mm/page_cgroup.c            |    1 -
> >  3 files changed, 50 insertions(+), 5 deletions(-)
> > 
> > 
> > diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> > index 7339c7b..ebdae9a 100644
> > --- a/include/linux/page_cgroup.h
> > +++ b/include/linux/page_cgroup.h
> > @@ -26,6 +26,8 @@ enum {
> >  	PCG_LOCK,  /* page cgroup is locked */
> >  	PCG_CACHE, /* charged as cache */
> >  	PCG_USED, /* this object is in use. */
> > +	PCG_ROOT, /* page belongs to root cgroup */
> > +	PCG_ACCT, /* page has been accounted for */
> >  };
> >  
> Those new flags are protected by zone->lru_lock, right ?
> If so, please add some comments.
> And I'm not sure why you need 2 flags. Isn't PCG_ROOT enough for you ?
>

Nope.. the accounting is independent of charge/uncharge.
 
-- 
	Balbir

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