Re: [RFI] Shared accounting for memory resource controller

[KAMEZAWA Hiroyuki]

On Tue, 7 Apr 2009 12:07:22 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> 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.

Case 2)
  swap is an object which can be shared.

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.
     Do you want to call try_charge() both against group-A and group-B
     under process-X's page fault ?

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.

Thanks,
-Kame

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