linux-kernel - Re: Scalability requirements for sysv ipc

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Date:	Tue, 25 Mar 2008 17:13:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com, Nadia Derbey <Nadia.Derbey@...l.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Scalability requirements for sysv ipc

On Tue, 2008-03-25 at 16:50 +0100, Mike Galbraith wrote:
> On Sat, 2008-03-22 at 20:08 +0100, Manfred Spraul wrote:
> 
> > just the normal performance of 2.6.25-rc3 is abyssimal, 55 to 60% slower 
> > than 2.6.18.8:
> 
> After manually reverting 3e148c79938aa39035669c1cfa3ff60722134535,
> 2.6.25.git scaled linearly, but as you noted, markedly down from earlier
> kernels with this benchmark.  2.6.24.4 with same revert, but all
> 2.6.25.git ipc changes piled on top still performed close to 2.6.22, so
> I went looking.  Bisection led me to..
> 
> 8f4d37ec073c17e2d4aa8851df5837d798606d6f is first bad commit
> commit 8f4d37ec073c17e2d4aa8851df5837d798606d6f
> Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date:   Fri Jan 25 21:08:29 2008 +0100
> 
>     sched: high-res preemption tick
> 
>     Use HR-timers (when available) to deliver an accurate preemption tick.
> 
>     The regular scheduler tick that runs at 1/HZ can be too coarse when nice
>     level are used. The fairness system will still keep the cpu utilisation 'fair'
>     by then delaying the task that got an excessive amount of CPU time but try to
>     minimize this by delivering preemption points spot-on.
> 
>     The average frequency of this extra interrupt is sched_latency / nr_latency.
>     Which need not be higher than 1/HZ, its just that the distribution within the
>     sched_latency period is important.
> 
>     Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> 
> :040000 040000 ab225228500f7a19d5ad20ca12ca3fc8ff5f5ad1 f1742e1d225a72aecea9d6961ed989b5943d31d8 M     arch
> :040000 040000 25d85e4ef7a71b0cc76801a2526ebeb4dce180fe ae61510186b4fad708ef0211ac169decba16d4e5 M     include
> :040000 040000 9247cec7dd506c648ac027c17e5a07145aa41b26 950832cc1dc4d30923f593ecec883a06b45d62e9 M     kernel
> 
> ...and I verified it via :-/ echo 7 > sched_features in latest.  That
> only bought me roughly half though, so there's a part three in there
> somewhere.

Ouch, I guess hrtimers are just way expensive on some hardware... 

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