Re: [RFC 0/3] Implementation of cgroup isolation

[Ying Han <yinghan@google.com>]

On Mon, Mar 28, 2011 at 5:47 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Mon, 28 Mar 2011 17:37:02 -0700
> Ying Han <yinghan@google.com> wrote:
>
>> On Mon, Mar 28, 2011 at 5:12 PM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > On Mon, 28 Mar 2011 11:01:18 -0700
>> > Ying Han <yinghan@google.com> wrote:
>> >
>> >> On Mon, Mar 28, 2011 at 2:39 AM, Michal Hocko <mhocko@suse.cz> wrote:
>> >> > Hi all,
>> >> >
>> >> > Memory cgroups can be currently used to throttle memory usage of a group of
>> >> > processes. It, however, cannot be used for an isolation of processes from
>> >> > the rest of the system because all the pages that belong to the group are
>> >> > also placed on the global LRU lists and so they are eligible for the global
>> >> > memory reclaim.
>> >> >
>> >> > This patchset aims at providing an opt-in memory cgroup isolation. This
>> >> > means that a cgroup can be configured to be isolated from the rest of the
>> >> > system by means of cgroup virtual filesystem (/dev/memctl/group/memory.isolated).
>> >>
>> >> Thank you Hugh pointing me to the thread. We are working on similar
>> >> problem in memcg currently
>> >>
>> >> Here is the problem we see:
>> >> 1. In memcg, a page is both on per-memcg-per-zone lru and global-lru.
>> >> 2. Global memory reclaim will throw page away regardless of cgroup.
>> >> 3. The zone->lru_lock is shared between per-memcg-per-zone lru and global-lru.
>> >>
>> >> And we know:
>> >> 1. We shouldn't do global reclaim since it breaks memory isolation.
>> >> 2. There is no need for a page to be on both LRU list, especially
>> >> after having per-memcg background reclaim.
>> >>
>> >> So our approach is to take off page from global lru after it is
>> >> charged to a memcg. Only pages allocated at root cgroup remains in
>> >> global LRU, and each memcg reclaims pages on its isolated LRU.
>> >>
>> >
>> > Why you don't use cpuset and virtual nodes ? It's what you want.
>>
>> We've been running cpuset + fakenuma nodes configuration in google to
>> provide memory isolation. The configuration of having the virtual box
>> is complex which user needs to know great details of the which node to
>> assign to which cgroup. That is one of the motivations for us moving
>> towards to memory controller which simply do memory accounting no
>> matter where pages are allocated.
>>
>
> I think current fake-numa is not useful because it works only at boot time.

yes and the big hassle is to manage the nodes after the boot-up.

>
>> By saying that, memcg simplified the memory accounting per-cgroup but
>> the memory isolation is broken. This is one of examples where pages
>> are shared between global LRU and per-memcg LRU. It is easy to get
>> cgroup-A's page evicted by adding memory pressure to cgroup-B.
>>
> If you overcommit....Right ?

yes, we want to support the configuration of over-committing the
machine w/ limit_in_bytes.

>
>
>> The approach we are thinking to make the page->lru exclusive solve the
>> problem. and also we should be able to break the zone->lru_lock
>> sharing.
>>
> Is zone->lru_lock is a problem even with the help of pagevecs ?

> If LRU management guys acks you to isolate LRUs and to make kswapd etc..
> more complex, okay, we'll go that way.

I would assume the change only apply to memcg users , otherwise
everything is leaving in the global LRU list.

This will _change_ the whole memcg design and concepts Maybe memcg
should have some kind of balloon driver to
> work happy with isolated lru.

We have soft_limit hierarchical reclaim for system memory pressure,
and also we will add per-memcg background reclaim. Both of them do
targeting reclaim on per-memcg LRUs, and where is the balloon driver
needed?

Thanks

--Ying

> But my current standing position is "never bad effects global reclaim".
> So, I'm not very happy with the solution.
>
> If we go that way, I guess we'll think we should have pseudo nodes/zones, which
> was proposed in early days of resource controls.(not cgroup).
>
> Thanks,
> -Kame
>
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