* Michal Hocko [off-list ref] [2011-03-30 10:18:53]:

On Tue 29-03-11 21:23:10, Balbir Singh wrote:

quoted

On 03/28/11 16:33, KAMEZAWA Hiroyuki wrote:

quoted

On Mon, 28 Mar 2011 11:39:57 +0200
Michal Hocko [off-list ref] wrote:

[...]

quoted

quoted

Isn't it the same result with the case where no cgroup is used ?
What is the problem ?
Why it's not a problem of configuration ?
IIUC, you can put all logins to some cgroup by using cgroupd/libgcgroup.

I agree with Kame, I am still at loss in terms of understand the use
case, I should probably see the rest of the patches

OK, it looks that I am really bad at explaining the usecase. Let's try
it again then (hopefully in a better way).

Consider a service which serves requests based on the in-memory
precomputed or preprocessed data. 
Let's assume that getting data into memory is rather costly operation
which considerably increases latency of the request processing. Memory
access can be considered random from the system POV because we never
know which requests will come from outside.
This workflow will benefit from having the memory resident as long as
and as much as possible because we have higher chances to be used more
often and so the initial costs would pay off.
Why is mlock not the right thing to do here? Well, if the memory would
be locked and the working set would grow (again this depends on the
incoming requests) then the application would have to unlock some
portions of the memory or to risk OOM because it basically cannot
overcommit.
On the other hand, if the memory is not mlocked and there is a global
memory pressure we can have some part of the costly memory swapped or
paged out which will increase requests latencies. If the application is
placed into an isolated cgroup, though, the global (or other cgroups)
activity doesn't influence its cgroup thus the working set of the
application.

I think one important aspect is what percentage of the memory needs to
be isolated/locked? If you expect really large parts, then we are in
trouble, unless we are aware of the exact requirements for memory and
know what else will run on the system.

If we compare that to mlock we will benefit from per-group reclaim when
we get over the limit (or soft limit). So we do not start evicting the
memory unless somebody makes really pressure on the _application_.
Cgroup limits would, of course, need to be selected carefully.

There might be other examples when simply kernel cannot know which
memory is important for the process and the long unused memory is not
the ideal choice.

There are other watermark based approaches that would work better,
given that memory management is already complicated by topology, zones
and we have non-reclaimable memory being used in the kernel on behalf
of applications. I am not ruling out a solution, just sharing ideas.
NOTE: In the longer run, we want to account for kernel usage and look
at potential reclaim of slab pages. 

-- 
	Three Cheers,
	Balbir

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