linux-kernel - Re: [PATCH v3] oom-kill: add lowmem usage aware oom kill handling

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Date:	Sat, 30 Jan 2010 13:33:49 +0100
From:	Vedran Furač <vedran.furac@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, rientjes@...gle.com,
	minchan.kim@...il.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3] oom-kill: add lowmem usage aware oom kill handling

Alan Cox wrote:

>> off by default. Problem is that it breaks java and some other stuff that
>> allocates much more memory than it needs. Very quickly Committed_AS hits
>> CommitLimit and one cannot allocate any more while there is plenty of
>> memory still unused.
> 
> So how about you go and have a complain at the people who are causing
> your problem, rather than the kernel.

That would pass completely unnoticed and ignored as long as overcommit
is enabled by default.

>>> theoretical limit, but you generally need more swap (it's one of the
>>> reasons why things like BSD historically have a '3 * memory' rule).
>> Say I have 8GB of memory and there's always some free, why would I need
>> swap?
> 
> So that all the applications that allocate tons of address space and
> don't use it can swap when you hit that corner case, and as a result you
> don't need to go OOM. You should only get an OOM when you run out of
> memory + swap.

Yes, but unfortunately using swap makes machine crawl with huge disk IO
every time you access some application you haven't been using for a few
hours. So recently more and more people are disabling it completely with
positive experience.

>>> So sounds to me like a problem between the keyboard and screen (coupled
>> Unfortunately it is not. Give me ssh access to your computer (leave
>> overcommit on) and I'll kill your X with anything running on it.
> 
> If you have overcommit on then you can cause stuff to get killed. Thats
> what the option enables.

s/stuff/wrong stuff/

> It's really very simple: overcommit off you must have enough RAM and swap
> to hold all allocations requested. Overcommit on - you don't need this
> but if you do use more than is available on the system something has to
> go.
> 
> It's kind of like banking  overcommit off is proper banking, overcommit
> on is modern western banking.

Hehe, yes and you know the consequences.

If you look at malloc(3) you would see this:

"This means that when malloc() returns non-NULL there is no guarantee
that the memory really is available.  This is a really bad bug."

So, if you don't want to change the OOM algorithm why not fixing this
bug then? And after that change the proc(5) manpage entry for
/proc/sys/vm/overcommit_memory into something like:

0: heuristic overcommit (enable this if you have memory problems with
                          some buggy software)
1: always overcommit, never check
2: always check, never overcommit (this is the default)

Regards,
Vedran


-- 
http://vedranf.net | a8e7a7783ca0d460fee090cc584adc12

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