linux-kernel - Re: [discuss] memrlimit - potential applications that can use

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Message-Id: <20080821195915.f1ecd012.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 21 Aug 2008 19:59:15 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andrea Righi <righi.andrea@...il.com>,
	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [discuss] memrlimit - potential applications that can use

On Thu, 21 Aug 2008 15:56:41 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Thu, 21 Aug 2008 08:55:52 +0530
> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> > 
> >>>>> So, before we expand the use of those features to control groups by
> >>>>> adding a bunch of new code, let's make sure that there will be users
> >>>> for
> >>>>> it and that those users have no better way of doing it.
> >>>> I am all ears to better ways of doing it. Are you suggesting that overcommit was
> >>>> added even though we don't actually need it?
> >>> It serves a purpose, certainly.  We have have better ways of doing it
> >>> now, though.  "So, before we expand the use of those features to
> >>> control groups by adding a bunch of new code, let's make sure that there
> >>> will be users for it and that those users have no better way of doing
> >>> it."
> >>>
> >>> The one concrete user that's been offered so far is postgres.  I've
> >> No, you've been offered several, including php and apache that use memory limits.
> >>
> >>> suggested something that I hope will be more effective than enforcing
> >>> overcommit.  
> > 
> > I'm sorry I miss the point. My concern on memrlimit (for overcommiting) is that
> > it's not fair because an application which get -ENOMEM at mmap() is just someone
> > unlucky.
> 
> It can happen today with overcommit turned on. Why is it unlucky?
> 
Today's overcommit is also unlucky ;) 

For example) process A and B is under a memrlimit.
 process A no memory leak, it often calls malloc() and free().
 process B does memory leak, 100MB per night.

process A cannot do anything when it notices malloc() returns NULL.
It controls his memory usage perfectly. He is unlucky and will die.
process B can use up VSZ which is freed by process A.

(OOM-killer, is disliked by everyone, have some kind of fairness.
 It checks usage.)

>  I think it's better to trigger some notifier to application or daemon
> > rather than return -ENOMEM at mmap(). Notification like "Oh, it seems the VSZ
> > of total application exceeds the limit you set. Although you can continue your
> > operation, it's recommended that you should fix up the  situation".
> > will be good.
> > 
> 
> So you are suggesting that when we are running out of memory (as defined by our
> current resource constraints), we don't return -ENOMEM, but instead we now
> handle a new event that states that we are running out of memory?
> 
Not "running out of memory" Just "VSZ is over the limit you set/expected".

My point is an application witch can handle NULL returned by malloc() is
not very popular, I think.

Sorry for noise.

Thanks,
-Kame

> NOTE: I am not opposed to the event, it can be useful for container
> administrators to know how to size their containers, not to application
> developers who want to auto-tune their applications (see my comment on autonomic
> computing in an earlier thread) or to applications that want to make sure they
> don't OOM without the system administrator having to do oom_adj for every
> important application.
> 


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