linux-kernel - Re: running of out memory => kernel crash

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Message-ID: <CAK1hOcM5u-zB7fUnR5QVJGBrEnLMhK9Q+EmWBknThga70UQaLw@mail.gmail.com>
Date:	Thu, 18 Aug 2011 14:44:56 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Pavel Ivanov <paivanof@...il.com>
Cc:	Mahmood Naderan <nt_mahmood@...oo.com>,
	David Rientjes <rientjes@...gle.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	"\"linux-kernel@...r.kernel.org\"" <linux-kernel@...r.kernel.org>,
	"\"linux-mm@...ck.org\"" <linux-mm@...ck.org>
Subject: Re: running of out memory => kernel crash

On Thu, Aug 18, 2011 at 4:18 AM, Pavel Ivanov <paivanof@...il.com> wrote:
>>> Why "killing" does not appear here? Why it try to "find some
>>> recently used page"?
>>
>> Because killing is the last resort. As long as kernel can free
>> a page by dropping an unmodified file-backed page, it will do that.
>> When there is nothing more to drop, and still more free pages
>> are needed, _then_ kernel will start oom killing.
>
> I have a little concern about this explanation of yours. Suppose we
> have some amount of more or less actively executing processes in the
> system. Suppose they started to use lots of resident memory. Amount of
> memory they use is less than total available physical memory but when
> we add total size of code for those processes it would be several
> pages more than total size of physical memory. As I understood from
> your explanation in such situation one process will execute its time
> slice, kernel will switch to other one, find that its code was pushed
> out of RAM, read it from disk, execute its time slice, switch to next
> process, read its code from disk, execute and so on. So system will be
> virtually unusable because of constantly reading from disk just to
> execute next small piece of code. But oom will never be firing in such
> situation. Is my understanding correct?

Yes.

> Shouldn't it be considered as an unwanted behavior?

Yes. But all alternatives (such as killing some process) seem to be worse.

-- 
vda
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