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

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Message-Id: <200812011502.37170.rusty@rustcorp.com.au>
Date:	Mon, 1 Dec 2008 15:02:35 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	lguest@...abs.org
Cc:	Avi Kivity <avi@...hat.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	jeremy@...source.com, LKML <linux-kernel@...r.kernel.org>,
	Mike Travis <travis@....com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

On Sunday 30 November 2008 04:52:41 Avi Kivity wrote:
> Alexander van Heukelum wrote:
> > I now did the benchmarks for the same -rc6 with hpa's 4-byte stubs
> > too. Same machine. It's significantly better than the other two
> > options in terms of speed. It takes about 7% less cpu to handle
> > the interrupts. (0.64% cpu instead of 0.69%.) I have to run now,
> > I'll let interpreting the histogram to someone else ;).
>
> This is noise. 0.05% cpu on a 1GHz machine servicing 1000 interrupt/sec
> boils down to 500 cycles/interrupt.  These changes shouldn't amount to
> so much (and I doubt you have 1000 interrupts/sec with a single disk)..

Sure, but smallest cache wins.  Which is why I thought hpa chose the 3 byte 
option.

> I'm sorry, but the whole effort is misguided, in my opinion.

Respectfully disagree.  I wouldn't do it, but it warms my heart that others 
are.  It's are not subtractive from other optimization efforts.

Cheers,
Rusty.
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