linux-kernel - Re: OOM notifications

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Date:	Fri, 19 Oct 2007 00:01:26 +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 10/18/2007 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.

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

Rene.

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