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

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Message-ID: <4933AD51.5000202@redhat.com>
Date:	Mon, 01 Dec 2008 11:24:33 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	lguest@...abs.org, 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

Rusty Russell wrote:
> 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.
>
>   

Four bytes was the smallest sane option.  Three bytes involved 
instruction opcodes overlap.

>> 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.
>   

Once it's done there's no reason not to commit it.  But the effort 
expended to do it is gone, without any measurable return.


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

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