linux-kernel - Re: [RFC][PATCH] VM: per-user overcommit policy

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Date:	Tue,  8 May 2007 00:49:57 +0200 (MEST)
From:	Andrea Righi <righiandr@...rs.sourceforge.net>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] VM: per-user overcommit policy

Alan Cox wrote:
>> - allow uid=1001 and uid=1002 (common users) to allocate memory only if the
>>   total committed space is below the 50% of the physical RAM + the size of
>>   swap:
>> root@...t # echo 1001:2:50 > /proc/overcommit_uid
>> root@...t # echo 1002:2:50 > /proc/overcommit_uid
> 
> There are some fundamental problems with this model - the moment you mix
> strict overcommit with anything else it ceases to be a strict overcommit
> and you might as well use existing overcommit rules for most stuff
> 
> The other thing you are sort of faking is per user resource management -
> which is a subset of per group of users resource management which is
> useful - eg "students can't hog the machine"
> 
> I don't see that this is the right approach compared with the container
> work and openvz work that is currently active and far more flexible.
> 

Obviously I was not proposing a nice theoretical model, my work is more similar
to a quick and dirty hack that could resolve some problems (at least in my case)
like the crash of critical services due to OOM-killing (or due to the failure of
a malloc() when OOM-killer is disabled).

When $VERY_CRITICAL_DAEMON dies *all* the users blame the sysadmin [me]. If a
user application dies because a malloc() returns NULL, the sysadmin [I] can
blame the user saying: "hey! _you_ tried to hog the machine and _your_
application is not able to handle the NULL result of the malloc()s!"... :-)

A solution could be to define the critical processes unkillable via
/proc/<pid>/oom_adj, but the per-process approach doesn't resolve all the
possible cases and it's quite difficult to manage in big environments, like HPC
clusters.

Anyway, it seems that I need to deepen my knowledge about the recent development
of process containers and openvz...

Thanks,
-Andrea
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