[Devel] Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v7)

[Andrea Righi]

On Mon, Mar 15, 2010 at 11:36:12AM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon, 15 Mar 2010 00:26:37 +0100
> Andrea Righi <arighi at develer.com> wrote:
> 
> > Control the maximum amount of dirty pages a cgroup can have at any given time.
> > 
> > Per cgroup dirty limit is like fixing the max amount of dirty (hard to reclaim)
> > page cache used by any cgroup. So, in case of multiple cgroup writers, they
> > will not be able to consume more than their designated share of dirty pages and
> > will be forced to perform write-out if they cross that limit.
> > 
> > The overall design is the following:
> > 
> >  - account dirty pages per cgroup
> >  - limit the number of dirty pages via memory.dirty_ratio / memory.dirty_bytes
> >    and memory.dirty_background_ratio / memory.dirty_background_bytes in
> >    cgroupfs
> >  - start to write-out (background or actively) when the cgroup limits are
> >    exceeded
> > 
> > This feature is supposed to be strictly connected to any underlying IO
> > controller implementation, so we can stop increasing dirty pages in VM layer
> > and enforce a write-out before any cgroup will consume the global amount of
> > dirty pages defined by the /proc/sys/vm/dirty_ratio|dirty_bytes and
> > /proc/sys/vm/dirty_background_ratio|dirty_background_bytes limits.
> > 
> > Changelog (v6 -> v7)
> > ~~~~~~~~~~~~~~~~~~~~~~
> >  * introduce trylock_page_cgroup() to guarantee that lock_page_cgroup()
> >    is never called under tree_lock (no strict accounting, but better overall
> >    performance)
> >  * do not account file cache statistics for the root cgroup (zero
> >    overhead for the root cgroup)
> >  * fix: evaluate cgroup free pages as at the minimum free pages of all
> >    its parents
> > 
> > Results
> > ~~~~~~~
> > The testcase is a kernel build (2.6.33 x86_64_defconfig) on a Intel Core 2 @
> > 1.2GHz:
> > 
> > <before>
> >  - root  cgroup:	11m51.983s
> >  - child cgroup:	11m56.596s
> > 
> > <after>
> >  - root cgroup:		11m51.742s
> >  - child cgroup:	12m5.016s
> > 
> > In the previous version of this patchset, using the "complex" locking scheme
> > with the _locked and _unlocked version of mem_cgroup_update_page_stat(), the
> > child cgroup required 11m57.896s and 12m9.920s with lock_page_cgroup()+irq_disabled.
> > 
> > With this version there's no overhead for the root cgroup (the small difference
> > is in error range). I expected to see less overhead for the child cgroup, I'll
> > do more testing and try to figure better what's happening.
> > 
> Okay, thanks. This seems good result. Optimization for children can be done under
> -mm tree, I think. (If no nack, this seems ready for test in -mm.)

OK, I'll wait a bit to see if someone has other fixes or issues and post
a new version soon including these small changes.

Thanks,
-Andrea
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