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SubjectRe: Scalability requirements for sysv ipc
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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@chello.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@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.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.
-Mike
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