On Wed 30-03-11 10:59:21, Ying Han wrote:

On Wed, Mar 30, 2011 at 1:18 AM, Michal Hocko [off-list ref] wrote:

quoted

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

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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.

quoted

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.

Michal,

Reading through your example, sounds to me you can accomplish the
"guarantee" of the high priority service using existing
memcg mechanisms.

Assume you have the service named cgroup-A which needs memory
"guarantee". Meantime we want to launch cgroup-B with no memory
"guarantee". What you want is to have cgroup-B uses the slack memory
(not being allocated by cgroup-A), but also volunteer to give up under
system memory pressure.

This would require a "guarantee" that no pages are reclaimed from a
group if that group is under its soft limit, right? I am thinking if we
can achieve that without too many corner cases when cgroups (process's
accounted memory) don't leave out much for other memory used by the
kernel.
That was my concern so I made that isolation rather opt-in without
modifying the current reclaim logic too much (there are, of course,
parts that can be improved).

So continue w/ my previous post, you can consider the following
configuration in 32G machine. We can only have resident size of
cgroup-A as much as the machine capacity.

cgroup-A :  limit_in_bytes =32G soft_limit_in_bytes = 32G
cgroup-B : limit_in_bytes =20G  soft_limit_in_bytes = 0G

To be a little bit extreme, there shouldn't be memory pressure on
cgroup-A unless it grows above the machine capacity. If the global
memory contention is triggered by cgroup-B, we should steal pages from
it always.

However, the current implementation of soft_limit needs to be improved
for the example above. Especially when we start having lots of cgroups
running w/ different limit setting, we need to have soft_limit being
efficient and we can eliminate the global lru scanning. 

Lots of groups is really an issue because we can end up in a situation
when everybody is under the limit while there is not much memory left
for the kernel. Maybe sum(soft_limit) < kernel_treshold condition would
solve this.

The later one breaks the isolation.

Sorry, I don't understand. Why would elimination of the global lru
scanning break isolation? Or am I misreading you?

Thanks
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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