Re: [LSF/MM TOPIC] memory control groups

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 17 Jan 2011 20:14:00 +0100
Johannes Weiner <hannes@cmpxchg.org> wrote:

> Hello,
> 
> on the MM summit, I would like to talk about the current state of
> memory control groups, the features and extensions that are currently
> being developed for it, and what their status is.
> 
> I am especially interested in talking about the current runtime memory
> overhead memcg comes with (1% of ram) and what we can do to shrink it.
> 
> In comparison to how efficiently struct page is packed, and given that
> distro kernels come with memcg enabled per default, I think we should
> put a bit more thought into how struct page_cgroup (which exists for
> every page in the system as well) is organized.
> 
> I have a patch series that removes the page backpointer from struct
> page_cgroup by storing a node ID (or section ID, depending on whether
> sparsemem is configured) in the free bits of pc->flags.
> 
> I also plan on replacing the pc->mem_cgroup pointer with an ID
> (KAMEZAWA-san has patches for that), and move it to pc->flags too.
> Every flag not used means doubling the amount of possible control
> groups, so I have patches that get rid of some flags currently
> allocated, including PCG_CACHE, PCG_ACCT_LRU, and PCG_MIGRATION.
> 
> [ I meant to send those out much earlier already, but a bug in the
> migration rework was not responding to my yelling 'Marco', and now my
> changes collide horribly with THP, so it will take another rebase. ]
> 
> The per-memcg dirty accounting work e.g. allocates a bunch of new bits
> in pc->flags and I'd like to hash out if this leaves enough room for
> the structure packing I described, or whether we can come up with a
> different way of tracking state.
> 

I see that there are requests for shrinking page_cgroup. And yes, I think
we should do so. I think there are trade-off between performance v.s.
memory usage. So, could you show the numbers when we discuss it ?

BTW, I think we can...

- PCG_ACCT_LRU bit can be dropped.(I think list_empty(&pc->lru) can be used.
                ROOT cgroup will not be problem.)
- pc->mem_cgroup can be replaced with ID.
  But move it into flags field seems difficult because of races.
- pc->page can be replaced with some lookup routine.
  But Section bit encoding may be something mysterious and look up cost
  will be problem.
- PCG_CACHE bit is a duplicate of information of 'page'. So, we can use PageAnon()
- I'm not sure PCG_MIGRATION. It's for avoiding races.

Note: we'll need to use 16bits for blkio tracking.

Another idea is dynamic allocation of page_cgroup. It may be able to be a help
for THP enviroment but will not work well (just adds overhead) against file cache
workload.

Anwyay, my priority of development for memcg this year is:

 1. dirty ratio support.
 2. Backgound reclaim (kswapd)
 3. blkio tracking.

Diet of page_cgroup should be done in step by step. We've seen many level down
when some new feature comes to memory cgroup. 

Thanks,
-Kame

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