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

[Vivek Goyal]

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)

bdi_thres ~= per_memory_cgroup_dirty * bdi_fraction

But bdi_nr_reclaimable and bdi_nr_writeback stats are still global.

So for the same number of dirty pages system wide on this bdi, we will be
triggering writeouts much more aggressively if somebody has created few
memory cgroups and tasks are running in those cgroups.

I guess it might cause performance regressions in case of small file
writeouts because previously one could have written the file to cache and
be done with it but with this patch set, there are higher changes that
you will be throttled to write the pages back to disk.

I guess we need two pieces to resolve this.
	- BDI stats per cgroup.
	- Writeback of inodes from same cgroup.

I think BDI stats per cgroup will increase the complextiy.

I am still setting up the system to test whether we see any speedup in
writeout of large files with-in a memory cgroup with small memory limits.
I am assuming that we are expecting a speedup because we will start
writeouts early and background writeouts probably are faster than direct
reclaim?

Thanks
Vivek



> 
> BTW, in other thread, I'm now proposing this style. 
> ==
> +void mem_cgroup_update_stat(struct page *page, int idx, bool charge)
> +{
> +	struct page_cgroup *pc;
> +
> +	pc = lookup_page_cgroup(page);
> +	if (unlikely(!pc))
> +		return;
> +
> +	if (trylock_page_cgroup(pc)) {
> +		__mem_cgroup_update_stat(pc, idx, charge);
> +		unlock_page_cgroup(pc);
> +	}
> +	return;
> ==
> 
> Then, it's not problem that check pc->mem_cgroup is root cgroup or not
> without spinlock.
> ==
> void mem_cgroup_update_stat(struct page *page, int idx, bool charge)
> {
> 	pc = lookup_page_cgroup(page);
> 	if (unlikely(!pc) || mem_cgroup_is_root(pc->mem_cgroup))
> 		return;	
> 	...
> }
> ==
> This can be handle in the same logic of "lock failure" path.
> And we just do ignore accounting.
> 
> There are will be no spinlocks....to do more than this,
> I think we have to use "struct page" rather than "struct page_cgroup".
> 
> Thanks,
> -Kame
> 
> 
> 
> 
> 
> 
> 
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