Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
From: Peter Zijlstra <peterz@infradead.org>
To: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Mike Galbraith <efault@gmx.de>, Hans-Peter Jansen <hpj@urpla.net>,
        linux-kernel@vger.kernel.org,
        Lennart Poettering <mzxreary@0pointer.de>,
        Linus Torvalds <torvalds@linux-foundation.org>, david@lang.hm,
        Dhaval Giani <dhaval.giani@gmail.com>, Vivek Goyal <vgoyal@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@elte.hu>, Balbir Singh <balbir@linux.vnet.ibm.com>
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Date: Fri, 19 Nov 2010 13:19:38 +0100
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On Fri, 2010-11-19 at 12:49 +0100, Peter Zijlstra wrote:
> On Fri, 2010-11-19 at 00:43 +0100, Samuel Thibault wrote:
> > What overhead? The implementation of cgroups is actually already
> > hierarchical. 
> 
> It must be nice to be that ignorant ;-) Speaking for the scheduler
> cgroup controller (that being the only one I actually know), most all
> the load-balance operations are O(n) in the number of active cgroups,
> and a lot of the cpu local schedule operations are O(d) where d is the
> depth of the cgroup tree.
> 
> [ and that's with the .38 targeted code, current mainline is O(n ln(n))
> for load balancing and truly sucks on multi-socket ]
> 
> You add a lot of pointer chasing to all the scheduler fast paths and
> there is quite significant data size bloat for even compiling with the
> controller enabled, let alone actually using the stuff.
> 
> But sure, treat them as if they were free to use, I guess your machine
> is fast enough.

In general though, I think you can say that: cgroups ass overhead.
Simply because you add constraints, this means you need to 1) account
more, 2) enforce constraints. Both have definite non-zero cost in both
data and time.

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