[Devel] Re: [RFD][PATCH] memcg: Move Usage at Task Move

[KAMEZAWA Hiroyuki]

On Tue, 10 Jun 2008 14:50:32 +0900 (JST)
yamamoto at valinux.co.jp (YAMAMOTO Takashi) wrote:

> >  3. Use Lazy Manner
> >       When the task moves, we can mark the pages used by it as
> >       "Wrong Charge, Should be dropped", and add them some penalty in the LRU.
> >     Pros.
> >       - no complicated ones.
> >       - the pages will be gradually moved at memory pressure.
> >     Cons.
> >       - A task's usage can exceed the limit for a while.
> >       - can't handle mlocked() memory in proper way.
> > 
> >  4. Allow Half-moved state and abandon rollback.
> >     Pros.
> >       - no complicated ones in the code.
> >     Cons.
> >       - the users will be in chaos.
> 
> how about:
> 
> 5. try to move charges as your patch does.
>    if the target cgroup's usage is going to exceed the limit,
>    try to shrink it.  if it failed, just leave it exceeded.
>    (ie. no rollback)
>    for the memory subsystem, which can use its OOM killer,
>    the failure should be rare.
> 

Hmm, allowing exceed and cause OOM kill ?

One difficult point is that the users cannot know they can move task
without any risk. How to handle the risk can be a point. 
I don't like that approarch in general because I don't like "exceed"
status. But implementation will be easy.

> > After writing this patch, for me, "3" is attractive. now.
> > (or using Lazy manner and allow moving of usage instead of freeing it.)
> > 
> > One reasone is that I think a typical usage of memory controller is
> > fork()->move->exec(). (by libcg ?) and exec() will flush the all usage.
> 
> i guess that moving long-running applications can be desirable
> esp. for not so well-designed systems.
> 

hmm, for not so well-designed systems....true.
But "5" has the same kind of risks for not so well-desgined systems ;)


Thanks,
-Kame

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