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

[KAMEZAWA Hiroyuki]

On Tue, 10 Jun 2008 21:57:03 +0900 (JST)
yamamoto at valinux.co.jp (YAMAMOTO Takashi) wrote:
> > > 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.
> 
> regardless of how to handle task moves,
> it's important to provide information to help users
> to avoid unreasonable cgroup/task placement.
> otherwise, they will be surprised by OOM-killer etc anyway.
> 
yes.

> having said that, if you decide to put too large tasks into
> a cgroup with too small limit, i don't think that there are
> many choices besides OOM-kill and allowing "exceed".
> 
IMHO, allowing exceed is harmfull without changing the definition of "limit".
"limit" is hard-limit, now, not soft-limit. Changing the defintion just for
this is not acceptable for me. 
Maybe "move" under limit itself is crazy ops....Hmm...

Should we allow task move when the destination cgroup is unlimited ?
Isn't it useful ?



> actually, i think that #3 and #5 are somewhat similar.
> a big difference is that, while #5 shrinks the cgroup immediately,
> #3 does it later.  in case we need to do OOM-kill, i prefer to do it
> sooner than later.
> 
#3 will not cause OOM-killer, I hope...A user can notice memory shortage.



> > > > 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 ;)
> 
> i don't claim that #5 is a perfect solution for everyone. :)
> 

Maybe there will no perfect solution ;)

Thanks,
-Kame

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