linux-kernel - Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

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Message-ID: <20081110085846.GG22392@elte.hu>
Date:	Mon, 10 Nov 2008 09:58:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alexander van Heukelum <heukelum@...tmail.fm>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, lguest@...abs.org,
	jeremy@...source.com, Steven Rostedt <srostedt@...hat.com>,
	Mike Travis <travis@....com>
Subject: Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

* Alexander van Heukelum <heukelum@...tmail.fm> wrote:

> Hi all,
> 
> I have spent some time trying to find out how expensive the 
> segment-switching patch was. I have only one computer available at 
> the time: a "Sempron 2400+", 32-bit-only machine.
> 
> Measured were timings of "hackbench 10" in a loop. The average was 
> taken of more than 100 runs. Timings were done for two seperate 
> boots of the system.

hackbench is _way_ too noisy to measure such cycle-level differences 
as irq entry changes cause. It also does not really stress interrupts 
- it only stresses networking, the VFS and the scheduler.

a better test might have been to generate a ton of interrupts, but 
even then it's _very_ hard to measure it properly. The best method is 
what i've suggested to you early on: run a loop in user-space and 
observe irq costs via RDTSC, as they happen. Then build a histogram 
and compare the before/after histogram. Compare best-case results as 
well (the first slot of the histogram), as those are statistically 
much more significant than a noisy average.

Measuring such things in a meaningful way is really tricky business. 
Using hackbench to measure IRQ entry micro-costs is like trying to 
take a photo of a delicate flower at night, by using an atomic bomb as 
the flash-light: you certainly get some sort of effect to report, but 
there's not many nuances left in the picture to really look at ;-)

	Ingo
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