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

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Message-ID: <49318822.1070100@redhat.com>
Date:	Sat, 29 Nov 2008 20:21:22 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Alexander van Heukelum <heukelum@...lshack.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	lguest@...abs.org, 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>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Lguest] [PATCH RFC/RFB] x86_64,	i386: interrupt dispatch changes

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)..

I'm sorry, but the whole effort is misguided, in my opinion.  If you 
want to optimize, try reducing the number of interrupts that occur 
rather than saving a few cycles in the interrupt path.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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