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Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between
 identical inherited contexts
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Corey Ashford <cjashfor@linux.vnet.ibm.com>,
       Thomas Gleixner <tglx@linutronix.de>
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Date: Fri, 22 May 2009 11:22:09 +0200
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On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
> 
>                 -----Unmodified-----            With this patch series
> Counters:       none    2 HW    4H+4S   none    2 HW    4H+4S
> 
> 2 processes:
> Average         3.44    6.45    11.24   3.12    3.39    3.60
> St dev          0.04    0.04    0.13    0.05    0.17    0.19
> 
> 8 processes:
> Average         6.45    8.79    14.00   5.57    6.23    7.57
> St dev          1.27    1.04    0.88    1.42    1.46    1.42
> 
> 32 processes:
> Average         5.56    8.43    13.78   5.28    5.55    7.15
> St dev          0.41    0.47    0.53    0.54    0.57    0.81

Any clues as to why the time is still dependent on the number of
counters in the context? The approach seems to be O(1) in that it
does a simple counter context swap on sched_out and nothing on sched_in.