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

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Date:	Tue, 11 Nov 2008 10:54:39 +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:

> > OTOH, unless i'm misreading them, it's a bit hard to compare them 
> > visually: the integral of the histograms does not seem to be 
> > constant, they dont seem to be normalized.
> 
> The total number of measured intervals (between two almost-adjacent 
> rdtsc's) is exactly the same for all histograms (10^10). Almost all 
> measurements are of the "nothing happened" type, i.e., around 11 
> clock cycles on this machine. The user time spent inside the 
> rdtsctest program is almost independent of the load, but it measures 
> time spent outside of the program... But what should be attributed 
> to what effect is unclear to me at the moment.

a high-pass filter should be applied in any case, to filter out the 
"nothing happened" baseline. Eliminating every delta below 500-1000 
cycles would do the trick i think, all IRQ costs are at least 1000 
cycles.

then a low-pass filter should be applied to eliminate non-irq noise 
such as scheduling effects or expensive irqs (which are both 
uninteresting to such analysis).

and then _that_ double-filtered dataset should be normalized: the 
number of events should be made the same. (just clip the larger 
dataset to the length of the smaller dataset)

> > It should be made constant for them to be comparable. (i.e. the 
> > total number of irq hits profiled should be equal - or should be 
> > normalized with the sum after the fact)
> 
> Basically the difference between the "idle" and "hack10" versions 
> should indicate the effect of extra interrupts (timer) and 
> additional exceptions and cache effects due to context switching.

i was only looking at before/after duos, for the same basic type of 
workload. Idle versus hackbench is indeed apples to oranges.

	Ingo
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