linux-kernel - Re: OOM notifications

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Message-ID: <4717DB45.8000604@keyaccess.nl>
Date:	Fri, 19 Oct 2007 00:16:37 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Rik van Riel <riel@...hat.com>
CC:	Marcelo Tosatti <marcelo@...ck.org>, linux-kernel@...r.kernel.org,
	drepper@...hat.com
Subject: Re: OOM notifications

On 10/19/2007 12:01 AM, Rene Herman wrote:
> On 10/18/2007 11:18 PM, Rik van Riel wrote:
> 
>> On Thu, 18 Oct 2007 23:06:52 +0200
>> Rene Herman <rene.herman@...access.nl> wrote:
>>
>>> They don't -- that's why I asked if you need both scenario's active
>>> at the same time. SIGDANGER would just be SIGPLEASEFREEALLYOUCAN with
>>> the operator deciding through setting the level at which point
>>> applications get it.
>>>
>>> Or put differently; what's the additional value of notifying an
>>> application that the system is about to go balistic when you've
>>> already asked it to free all it could earlier? SIGSEEDAMNITITOLDYOUSO?
>>
>> The first threshold - "we are about to swap" - means the application
>> frees memory that it can.  Eg. free()d memory that glibc has not yet
>> given back to the kernel, or JVM running the garbage collector, or ...
>>
>> The second threshold - "we are out of memory" - means that the first
>> approach has failed and the system needs to do something else. On an
>> embedded system, I would expect some application to exit or maybe
>> restart itself.
> 
> That first threshold sounds fine yes. To me, the second mostly sounds 
> like a job for SIGTERM though.
> 
> The OOM killer could after it selected the task for killing first try a 
> TERM on it to give a chance to exit gracefully and only when that 
> doesn't help make it eligible for killing on a second round through the 
> badness calculation.
> 
> You could moreover _never_ make a task eligible for killing before it 
> received a SIGTERM, thereby guaranteeing that everyone got the SIGTERM 
> before killing anything, and it seems SIGTERM would be a more focussed 
> version of SIGDANGER2 then.

Well, no, that "guarantee" is fairly badly formulated but I mean "before 
everyone got a SIGTERM" ofcourse. That is, first do the same selection as 
now but don't send KILL but TERM and mark the task as having received a TERM 
already and make it not eligible anymore. Only when there are no TERM 
eligible tasks anymore, start sending KILL.

> Would at least forego any need for multiplexing the DANGER signal.

Rene.
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