Re: [RFC][PATCH 1/2][memcg] moving memcg's node info array tovirtually contiguous array

[KAMEZAWA Hiroyuki]

On Thu, 22 Jul 2010 14:43:56 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> On Wed, 21 Jul 2010 19:58:31 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> 
> > These are just a _toy_ level patches yet. My final purpose is to use indexed array
> > for mem_cgroup itself, it has IDs.
> > 
> > Background:
> >   memory cgroup uses struct page_cgroup for tracking all used pages. It's defined as
> > ==
> > struct page_cgroup {
> >         unsigned long flags;
> >         struct mem_cgroup *mem_cgroup;
> >         struct page *page;
> >         struct list_head lru;           /* per cgroup LRU list */
> > };
> > ==
> >   and this increase the cost of per-page-objects dramatically. Now, we have
> >   troubles on this object.
> >   1.  Recently, a blkio-tracking guy wants to add "blockio-cgroup" information
> >       to page_cgroup. But our concern is extra 8bytes per page.
> >   2.  At tracking dirty page status etc...we need some trick for safe access
> >       to page_cgroup and memcgroup's information. For example, a small seqlock.
> > 
> > Now, each memory cgroup has its own ID (0-65535). So, if we can replace
> > 8byte of pointer "pc->mem_cgroup" with an ID, which is 2 bytes, we may able
> > to have another room. (Moreover, I think we can reduce the number of IDs...)
> > 
> > This patch is a trial for implement a virually-indexed on-demand array and
> > an example of usage. Any commetns are welcome.
> > 

Hi,
> So, your purpose is to:
> 
> - make the size of mem_croup small(by [2/2])
It's just an example to test virt-array. I don't convice it can
save memory or make something fast. and I found a bug in free routine.)


> - manage all the mem_cgroup in virt-array indexed by its ID(it would be faster
>   than using css_lookup)
yes.

> - replace pc->mem_cgroup by its ID and make the size of page_cgroup small
> 
yes.

Final style I'm thinking is
	struct page_cgroup {
		unsigned long flags;
		spinlock_t	lock;  # for lock_page_cgroup()
		unsigned short memcg;
		unsigned short blkio;
		struct page *page;
		struct list_head list;
	};
This will be benefical in 64bit. About 32bit, I may have to merge some fields.
Or I may have to add some "version" field for updating memcg's statistics
without locks. memcg field may be able to be moved onto high-bits of "flags"
because it's stable value unless it's not under move_charge. 
(IIUC, at move_charge, memcg is off-LRU and there are no race with AcctLRU bit
 v.s. pc->mem_cgroup field. With other flags, lock_page_cgroup() works enough.)

Anyway, race with move_charge() will be the last enemy for us to track 
dirty pages etc...at least, this kind of "make room" job is required, I feel.

There are many things to be considered, but I'm a bit in hurry. I'd like to do
some preparation before Mel at el rewrites memory-reclaim+writeback complelety.


Thanks,
-Kame

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