[Devel] Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6)

[Andrea Righi]

On Fri, Mar 12, 2010 at 08:42:30AM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 11 Mar 2010 10:03:07 -0500
> Vivek Goyal <vgoyal at redhat.com> wrote:
> 
> > On Thu, Mar 11, 2010 at 06:25:00PM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 11 Mar 2010 10:14:25 +0100
> > > Peter Zijlstra <peterz at infradead.org> wrote:
> > > 
> > > > On Thu, 2010-03-11 at 10:17 +0900, KAMEZAWA Hiroyuki wrote:
> > > > > On Thu, 11 Mar 2010 09:39:13 +0900
> > > > > KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> > > > > > > The performance overhead is not so huge in both solutions, but the impact on
> > > > > > > performance is even more reduced using a complicated solution...
> > > > > > > 
> > > > > > > Maybe we can go ahead with the simplest implementation for now and start to
> > > > > > > think to an alternative implementation of the page_cgroup locking and
> > > > > > > charge/uncharge of pages.
> > > > 
> > > > FWIW bit spinlocks suck massive.
> > > > 
> > > > > > 
> > > > > > maybe. But in this 2 years, one of our biggest concerns was the performance.
> > > > > > So, we do something complex in memcg. But complex-locking is , yes, complex.
> > > > > > Hmm..I don't want to bet we can fix locking scheme without something complex.
> > > > > > 
> > > > > But overall patch set seems good (to me.) And dirty_ratio and dirty_background_ratio
> > > > > will give us much benefit (of performance) than we lose by small overheads.
> > > > 
> > > > Well, the !cgroup or root case should really have no performance impact.
> > > > 
> > > > > IIUC, this series affects trgger for background-write-out.
> > > > 
> > > > Not sure though, while this does the accounting the actual writeout is
> > > > still !cgroup aware and can definately impact performance negatively by
> > > > shrinking too much.
> > > > 
> > > 
> > > Ah, okay, your point is !cgroup (ROOT cgroup case.)
> > > I don't think accounting these file cache status against root cgroup is necessary.
> > > 
> > 
> > I think what peter meant was that with memory cgroups created we will do
> > writeouts much more aggressively.
> > 
> > In balance_dirty_pages()
> > 
> > 	if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> > 		break;
> > 
> > Now with Andrea's patches, we are calculating bdi_thres per memory cgroup
> > (almost)
> hmm.
> 
> > 
> > bdi_thres ~= per_memory_cgroup_dirty * bdi_fraction
> > 
> > But bdi_nr_reclaimable and bdi_nr_writeback stats are still global.
> > 
> Why bdi_thresh of ROOT cgroup doesn't depend on global number ?

Very true. mem_cgroup_has_dirty_limit() must always return false in case
of root cgroup, so that global numbers are used.

Thanks,
-Andrea
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