linux-kernel - Re: Memory overcommit

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Date:	Wed, 14 Oct 2009 13:51:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	vedran.furac@...il.com
Cc:	Vedran Furač <vedranf@...ranf.mine.nu>,
	linux-kernel@...r.kernel.org
Subject: Re: Memory overcommit

On Tue, 13 Oct 2009 19:13:34 +0200
Vedran Furač <vedranf@...ranf.mine.nu> wrote:

> KAMEZAWA Hiroyuki wrote:
> > I guess the reason of default value is that the kernel assume
> > processes will not always use all mmaped range. There will be unused
> > range in process's virtual memory and it can be big.
> > 
> > For example, typical case in a server, when you run multi-thread
> > program (like java VM),
> > 
> > - stack per thread - malloc() arena per thread
> > 
> > can makes difference among size-of-mapped-range v.s. used-pages
> > bigger. I saw Gigabytes of unused range on ia64 host,...statck size
> > was big.
> 
> Yes, I noticed that JVM allocates gigabytes but then uses less than 10%
> of that and, as a consequence, eclipse sometimes fails to start although
> there's plenty of free memory. So overcommiting is some kind of a
> workaround for broken software that allocate not what they need but what
> they might need in some rare occurrences. I would rather like fixing
> this userland software than risking OOM situations and random killing of
> innocent processes.
> 

In my understanding, mmap() is just for requesting virtual address space.
Not for requesting memory in these days.



> > And if strict check(vm.ovecommit_memory=2) is used, mmap() return
> > -ENOMEM whenever it hits limit.
> 
> % strace -f -e mmap java -version
> [...]
> mmap(NULL, 996147200, PROT_READ|PROT_WRITE|PROT_EXEC,
> MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = -1 ENOMEM (Cannot
> allocate memory)
> 
> And that should be fine.
> 
It's not fine for me ;)

> > Against random-kill, you may have 2 choices.
> > 
> > 1. use  /proc/<pid>/oom_adj 2. use  memory cgroup.
> > 
> > Something more easy-to-use method may be appriciated. We have above 2
> > now.
> 
> These are just bad workarounds for bad OOM algorithm. I tested this
> little program on multiple systems (including windows) without any
> tweaking and linux behavior is, unfortunately *the worst*.  :/
> 
> 
Yes, they are workaround. You can use /etc/sysctl.conf.
But if making it default _now_, many threaded programs will not work.
 
But I agree, OOM killer should be sophisticated.
Please give us a sample program/test case which causes problem.
linux-mm@...ck.org may be a better place. lkml has too much traffic.

Regards,
-Kame

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