Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-05-22 at 11:22 +0200, Peter Zijlstra wrote:
> 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.

Ah, should I read this as 'n' lmbench instances on a single cpu? So that
we get multiple inheritance sets mixed and have to switch between them?