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

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Date:	Sat, 29 Nov 2008 20:58:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Alexander van Heukelum <heukelum@...lshack.com>,
	"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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch
	changes


* Avi Kivity <avi@...hat.com> 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)..
>
> 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.

the goal was not to optimize those workloads - the goal was to (try to) 
validate those irq trampoline changes / cleanups. We went with hpa's 
changes in the end which compresses the trampolines - that reduces the 
$icache footprint which is hard to measure but a very real concern on 
real workloads.

	Ingo
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