Re: [RFI] Shared accounting for memory resource controller

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-04-07 17:24:19]:

> On Tue, 7 Apr 2009 13:33:55 +0530
> Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-04-07 16:33:31]:
> > 
> > > On Tue, 7 Apr 2009 12:48:25 +0530
> > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > 
> > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-04-07 16:00:14]:
> > > > 
> > > > > On Tue, 7 Apr 2009 12:07:22 +0530
> > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > > > > 
> > > > > > Hi, All,
> > > > > > 
> > > > > > This is a request for input for the design of shared page accounting for
> > > > > > the memory resource controller, here is what I have so far
> > > > > > 
> > > > > 
> > > > > In my first impression, I think simple counting is impossible.
> > > > > IOW, "usage count" and "shared or not" is very different problem.
> > > > > 
> > > > > Assume a page and its page_cgroup.
> > > > > 
> > > > > Case 1)
> > > > >   1. a page is mapped by process-X under group-A
> > > > >   2. its mapped by process-Y in group-B (now, shared and charged under group-A)
> > > > >   3. move process-X to group-B
> > > > >   4. now the page is not shared.
> > > > > 
> > > > 
> > > > By shared I don't mean only between cgroups, it could be a page shared
> > > > in the same cgroup
> > > > 
> > > Hmm, is it good information ?
> > > 
> > > Such kind of information can be calucated by
> > > ==
> > >    rss = 0;
> > >    for_each_process_under_cgroup() {
> > >        mm = tsk->mm
> > >        rss += mm->anon_rss;
> > >    }
> > >    some_of_all_rss = rss;
> > >    
> > >    shared_ratio = mem_cgrou->rss *100 / some_of_all_rss.
> > > ==
> > >    if 100%, all anon memory are not shared.
> > >
> > 
> > Why only anon? 
> 
> no serious intention.
> Just because you wrote "expect the user to account all cached pages as shared" ;)

OK, I think we should mention that we can treat unmapped cache as
shared :)

> 
> > This seems like a good idea, except when we have a page
> > charged to a cgroup and the task that charged it has migrated, in that
> > case sum_of_all_rss will be 0.
> > 
> Yes. But we don't move pages at task-move under expectation that moved
> process will call fork() soon.
> "task move" has its own problem, so ignoring it for now is a choice.
> That kind of troubls can be treated when we fixes "task move".
> (or fix "task move" first.)
>

Yes, but the point I was making was that we could have pages left over
without tasks remaining, in the case of shared pages. I think we can
handle them suitably, probably an implementation issue. 

-- 
	Balbir

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