linux-kernel - Re: [PATCH] mm: add config option to select the initial overcommit mode

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Date:	Fri, 13 May 2016 12:18:54 +0200
From:	Mason <slash.tmp@...e.fr>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Sebastian Frias <sf84@...oste.net>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: add config option to select the initial overcommit
 mode

On 13/05/2016 11:52, Michal Hocko wrote:
> On Fri 13-05-16 10:44:30, Mason wrote:
>> On 13/05/2016 10:04, Michal Hocko wrote:
>>
>>> On Tue 10-05-16 13:56:30, Sebastian Frias wrote:
>>> [...]
>>>> NOTE: I understand that the overcommit mode can be changed dynamically thru
>>>> sysctl, but on embedded systems, where we know in advance that overcommit
>>>> will be disabled, there's no reason to postpone such setting.
>>>
>>> To be honest I am not particularly happy about yet another config
>>> option. At least not without a strong reason (the one above doesn't
>>> sound that way). The config space is really large already.
>>> So why a later initialization matters at all? Early userspace shouldn't
>>> consume too much address space to blow up later, no?
>>
>> One thing I'm not quite clear on is: why was the default set
>> to over-commit on?
> 
> Because many applications simply rely on large and sparsely used address
> space, I guess.

What kind of applications are we talking about here?

Server apps? Client apps? Supercomputer apps?

I heard some HPC software use large sparse matrices, but is it a common
idiom to request large allocations, only to use a fraction of it?

If you'll excuse the slight trolling, I'm sure many applications don't
expect being randomly zapped by the OOM killer ;-)

> That's why the default is GUESS where we ignore the cumulative
> charges and simply check the current state and blow up only when
> the current request is way too large.

I wouldn't call denying a request "blowing up". Application will
receive NULL, and is supposed to handle it gracefully.

"Blowing up" is receiving SIGKILL because another process happened
to allocate too much memory.

Regards.