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

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Date:	Thu, 13 Nov 2008 17:18:21 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Matt Mackall <mpm@...enic.com>
CC:	Alexander van Heukelum <heukelum@...tmail.fm>,
	Ingo Molnar <mingo@...e.hu>, 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

Matt Mackall wrote:
> On Mon, 2008-11-10 at 21:00 -0800, H. Peter Anvin wrote:
>> Okay, after spending most of the day trying to get something that isn't
>> completely like white noise (interesting problem, otherwise I'd have
>> given up long ago) I did, eventually, come up with something that looks
>> like it's significant.  I did a set of multiple runs, and am looking for
>> the "waterfall points" in the cumulative statistics.
>>
>> http://www.zytor.com/~hpa/baseline-hpa-3000-3600.pdf
>>
>> This particular set of data points was gathered on a 64-bit kernel, so I
>> didn't try the segment technique.
>>
>> It looks to me that the collection of red lines is enough to the left of
>> the black ones that one can assume there is a significant effect,
>> probably by about a cache miss worth of time.
> 
> This graph is a little confusing. Is the area under each curve here
> supposed to be a constant?
> 

No, they reflect individual runs.  They start at 1 at the top left and
drop to 0 at the far right in each case.  What matters is the horizontal
position of large vertical drops.

> Is this latency from all interrupts as seen by userspace? Or does a
> particular interrupt dominate?
> 

All interrupts, but rather inherently the difference between interrupt
handlers is going to be bigger than the differences between
implementations of the same handler.  I *believe* all the interrupts
you're seeing in that graph are probably timer interrupts.  The other
major interrupt source that was active on the system was USB.

	-hpa
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