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 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? 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.
 
> 
> > > Case 2)
> > >   swap is an object which can be shared.
> > > 
> > 
> > Good point, I expect the user to account all cached pages as shared -
> > no
> Maybe yes if we explain it's so ;)
> 
> ?
> > 
> > > Case 3)
> > >   1. a page known as "A" is mapped by process-X under group-A.
> > >   2. its mapped by process-Y under group-B(now, shared and charged under group-A)
> > >   3. Do copy-on-write by process-X.
> > >      Now, "A" is mapped only by B but accoutned under group-A.
> > >      This case is ignored intentionally, now.
> > 
> > Yes, that is the original design
> > 
> > >      Do you want to call try_charge() both against group-A and group-B
> > >      under process-X's page fault ?
> > > 
> > 
> > No we don't, but copy-on-write is caught at page_rmap_dup() - no?
> > 
> Hmm, if we don't consider group-B, maybe we can.
> But I wonder counting is overkill..
> 
> 
> > > There will be many many corner case.
> > > 
> > > 
> > > > Motivation for shared page accounting
> > > > -------------------------------------
> > > > 1. Memory cgroup administrators will benefit from the knowledge of how
> > > >    much of the data is shared, it helps size the groups correctly.
> > > > 2. We currently report only the pages brought in by the cgroup, knowledge
> > > >    of shared data will give a complete picture of the actual usage.
> > > > 
> > > 
> > > Motivation sounds good. But counting this in generic rmap will have tons of
> > > troubles and slow-down.
> > > 
> > > I bet we should prepare a file as
> > >   /proc/<pid>/cgroup_maps
> > > 
> > > And show RSS/RSS-owned-by-us per process. Maybe this feature will be able to be
> > > implemented in 3 days.
> > 
> > Yes, we can probably do that, but if we have too many processes in one
> > cgroup, we'll need to walk across all of them in user space. One other
> > alternative I did not mention is to walk the LRU like we walk page
> > tables and look at page_mapcount of every page, but that will be
> > very slow.
> 
> Can't we make use of information in mm_counters ? (As I shown in above)
> (see set/get/add/inc/dec_mm_counters())
>

I've seen them, might be a good way to get started, except some corner
cases mentioned above. 

-- 
	Balbir

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