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

[Michal Hocko]

On Tue 29-03-11 09:09:24, KAMEZAWA Hiroyuki wrote:
> On Mon, 28 Mar 2011 13:44:30 +0200
> Michal Hocko <mhocko@xxxxxxx> wrote:
> 
> > On Mon 28-03-11 20:03:32, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 28 Mar 2011 11:39:57 +0200
> > > Michal Hocko <mhocko@xxxxxxx> wrote:
> > [...]
> > > 
> > > Isn't it the same result with the case where no cgroup is used ?
> > 
> > Yes and that is the point of the patchset. Memory cgroups will not give
> > you anything else but the top limit wrt. to the global memory activity.
> > 
> > > What is the problem ?
> > 
> > That we cannot prevent from paging out memory of process(es), even though
> > we have intentionaly isolated them in a group (read as we do not have
> > any other possibility for the isolation), because of unrelated memory
> > activity.
> > 
> Because the design of memory cgroup is not for "defending" but for 
> "never attack some other guys".

Yes, I am aware of the current state of implementation. But as the
patchset show there is not quite trivial to implement also the other
(defending) part.

> 
> 
> > > Why it's not a problem of configuration ?
> > > IIUC, you can put all logins to some cgroup by using cgroupd/libgcgroup.
> > 
> > Yes, but this still doesn't bring the isolation.
> > 
> 
> Please explain this more.
> Why don't you move all tasks under /root/default <- this has some limit ?

OK, I have tried to explain that in one of the (2nd) patch description.
If I move all task from the root group to other group(s) and keep the
primary application in the root group I would achieve some isolation as
well. That is very much true. But then there is only one such a group.
What if we need more such groups? I see this solution more as a misuse
of the current implementation of the (special) root cgroup.

> > > Maybe you just want "guarantee".
> > > At 1st thought, this approarch has 3 problems. And memcg is desgined
> > > never to prevent global vm scans,
> > > 
> > > 1. This cannot be used as "guarantee". Just a way for "don't steal from me!!!"
> > >    This just implements a "first come, first served" system.
> > >    I guess this can be used for server desgines.....only with very very careful play.
> > >    If an application exits and lose its memory, there is no guarantee anymore.
> > 
> > Yes, but once it got the memory and it needs to have it or benefits from
> > having it resindent what-ever happens around then there is no other
> > solution than mlocking the memory which is not ideal solution all the
> > time as I have described already.
> > 
> 
> Yes, then, almost all mm guys answer has been "please use mlock".

Yes. As I already tried to explain, mlock is not the remedy all the
time. It gets very tricky when you balance on the edge of the limit of
the available memory resp. cgroup limit. Sometimes you rather want to
have something swapped out than being killed (or fail due to ENOMEM).
The important thing about swapped out above is that with the isolation
it is only per-cgroup.

> > > 2. Even with isolation, a task in memcg can be killed by OOM-killer at
> > >    global memory shortage.
> > 
> > Yes it can but I think this is a different problem. Once you are that
> > short of memory you can hardly ask from any guarantees.
> > There is no 100% guarantee about anything in the system.
> > 
> 
> I think you should put tasks in root cgroup to somewhere. It works perfect
> against OOM. And if memory are hidden by isolation, OOM will happen easier.

Why do you think that it would happen easier? Isn't it similar (from OOM
POV) as if somebody mlocked that memory?

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