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Date: Fri, 19 Nov 2010 07:55:48 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>,
        Mike Galbraith <efault@gmx.de>, Hans-Peter Jansen <hpj@urpla.net>,
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        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Ingo Molnar <mingo@elte.hu>, Balbir Singh <balbir@linux.vnet.ibm.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups
Message-ID: <20101119125548.GC24411@Krystal>
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* Peter Zijlstra (peterz@infradead.org) wrote:
> 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.

I really think you meant "add" here ? (Hey! The keys were next to each other!)
;)

> 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.

Yep, this looks like one of these perpetual throughput vs latency trade-offs.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com