linux-kernel - Re: Scheduler regression: Too frequent timer interrupts(?)

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Date:	Fri, 17 Apr 2009 18:49:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org
Subject: Re: Scheduler regression: Too frequent timer interrupts(?)


* Peter Zijlstra <peterz@...radead.org> wrote:

> > > Is the overhead 1%? 2%? 0.5%? And how did it change from 
> > > 2.6.22 onwards? Did it go up by 0.1%, from 1% to 1.1%? Or did 
> > > the average go down by 0.05%, while increasing the spread of 
> > > events (thus fooling your cutoff)?
> > 
> > As you see in the diagrams provided there is a 4 fold increase 
> > in the number of interrupts >1usecs when going from 2.6.22 to 
> > 2.6.23. How would you measure the overhead? Time spent in the 
> > OS? Disturbance of the caches by the OS that cause the 
> > application to have to refetch data from Ram?
> 
> You could for example run an NMI profiler at 10000 Hz and collect 
> samples. Or use PMU hardware to collect numbers

it's even simpler than that: use a user-space loop and an alert() 
based signal-handler triggered measurement to count the number of 
loops user-space can execute, per second.

Such measurements can very easily be made much more precise than 
0.1%, and in a tight loop there's basically no caching issues - it 
measures pure 'cycles left for user-space' performance.

	Ingo
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