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

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Message-Id: <1226353458.27999.1284030023@webmail.messagingengine.com>
Date:	Mon, 10 Nov 2008 22:44:18 +0100
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"H. Peter Anvin" <hpa@...or.com>, "Ingo Molnar" <mingo@...e.hu>
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>, 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


On Mon, 10 Nov 2008 07:39:22 -0800, "H. Peter Anvin" <hpa@...or.com>
said:
> Ingo Molnar wrote:
> > * Alexander van Heukelum <heukelum@...tmail.fm> wrote:
> > 
> > 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.
> > 
> 
> For what it's worth, I tested this out, and I'm pretty sure you need to
> run a uniprocessor configuration (or system) for it to make sense --
> otherwise you end up missing too many of the interrupts.  I first tested
> this on an 8-processor system and, well, came up with nothing.
> 
> I'm going to try this later on a uniprocessor, unless Alexander beats me
> to it.

I did the rdtsctest again for the irqstubs patch you sent. The data
is at http://heukelum.fastmail.fm/irqstubs/ and the latency histogram
is http://heukelum.fastmail.fm/irqstubs/latency_hpa.png

Greetings,
    Alexander

> 	-hpa
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

-- 
http://www.fastmail.fm - Same, same, but different...

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