linux-kernel - Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

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Date:	Fri, 19 Nov 2010 01:02:04 +0100
From:	Samuel Thibault <samuel.thibault@...-lyon.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mike Galbraith <efault@....de>, Hans-Peter Jansen <hpj@...la.net>,
	linux-kernel@...r.kernel.org,
	Lennart Poettering <mzxreary@...inter.de>, david@...g.hm,
	Dhaval Giani <dhaval.giani@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Vivek Goyal <vgoyal@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

Linus Torvalds, le Thu 18 Nov 2010 15:51:35 -0800, a écrit :
> On Thu, Nov 18, 2010 at 3:43 PM, Samuel Thibault
> <samuel.thibault@...-lyon.org> wrote:
> >
> > What overhead? The implementation of cgroups is actually already
> > hierarchical.
> 
> Well, at least the actual group creation overhead.
> 
> If it's a "only at setsid()", that's a fairly rare thing (although I
> think somebody might want to run something like the AIM7 benchmark - I
> have this memory of it doing lots of tty tests).
> 
> Or if it's only at "user launches new program from window manager",
> that's rare too.
> 
> But if you do it per process group, now you're doing one for each
> command invocation in a shell, for example.

Well, if it's from an interactive shell, it's not really a problem :)

But when it's from a script it can become one, yes. But are cgroups so
expensive?

> If you're doing things per thread, you've already lost.

Not per thread, per process, i.e. put threads of the same process in the
same cgroup. Again, I would have thought that creating a cgroup is very
lightweight in front of a fork(). If not, maybe we are just looking for
another, more lightweight container information that the scheduler would
use [1], and keep more heavyweight containers for the non-automatic
creation way.

> Also, remember the goal: it was never about some theoretical end
> result. It's all about a simple heuristic that makes things work
> better. Trying to do that "perfectly" totally and utterly misses the
> whole point.

Sure.  Using sid should already be quite good, but including the uid
information as well should be easily even better.

Samuel

[1] Actually, I happen to have written a PhD thesis on this, that's why
I'm popping up:)
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