linux-kernel - Re: oom kill oddness.

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Message-Id: <20060928171706.bee0c50b.akpm@osdl.org>
Date:	Thu, 28 Sep 2006 17:17:06 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: oom kill oddness.

On Fri, 29 Sep 2006 01:03:16 +0200 (CEST)
Roman Zippel <zippel@...ux-m68k.org> wrote:

> Hi,
> 
> On Wed, 27 Sep 2006, Dave Jones wrote:
> 
> > So I have two boxes that are very similar.
> > Both have 2GB of RAM & 1GB of swap space.
> > One has a 2.8GHz CPU, the other a 2.93GHz CPU, both dualcore.
> > 
> > The slower box survives a 'make -j bzImage' of a 2.6.18 kernel tree
> > without incident. (Although it takes ~4 minutes longer than a -j2)
> > 
> > The faster box goes absolutely nuts, oomkilling everything in sight,
> > until eventually after about 10 minutes, the box locks up dead,
> > and won't even respond to pings.
> > 
> > Oh, the only other difference - the slower box has 1 disk, whereas the
> > faster box has two in RAID0.   I'm not surprised that stuff is getting
> > oom-killed given the pathological scenario, but the fact that the
> > box never recovered at all is a little odd.  Does md lack some means
> > of dealing with low memory scenarios ?
> 
> I think I see the same thing on the other end on slow machines, here it 
> only takes a single compile job, which doesn't quite fit into memory and 
> another task (like top) which occasionally wakes up and tries to allocate 
> memory and then kills the compile job - that's very annoying.
> 
> AFAICT the basic problem is that "did_some_progress" in __alloc_pages() is 
> rather local information, other processes can still make progress and keep 
> this process from making progress, which gets grumpy and starts killing. 
> What's happing here is that most memory is either mapped or in the swap 
> cache, so we have a race between processes trying to free memory from the 
> cache and processes mapping memory back into their address space.

Kernel versions please, guys.  There have been a lot of oom-killer changes
post-2.6.18.

> If someone wants to play with the problem, the example program below 
> triggers the problem relatively easily (booting with only little ram 
> helps), it starts a number of readers, which should touch a bit more 
> memory than is available and a few writers, which occasionally allocate 
> memory.
> 

How much ram, how much swap?

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