linux-kernel - Re: [patch 10/10] Scheduler profiling

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Date:	Sat, 7 Jul 2007 19:59:39 +0200
From:	Adrian Bunk <bunk@...sta.de>
To:	Willy Tarreau <w@....eu>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 10/10] Scheduler profiling - Use immediate values

On Sat, Jul 07, 2007 at 07:20:12PM +0200, Willy Tarreau wrote:
> On Sat, Jul 07, 2007 at 07:01:57PM +0200, Adrian Bunk wrote:
> > On Sat, Jul 07, 2007 at 11:45:20AM -0400, Frank Ch. Eigler wrote:
>...
> > You always have to decide between some debug code and some small bit of 
> > performance. There's a reason why options to disable things like BUG() 
> > or printk() are in the kernel config menus hidden behind CONFIG_EMBEDDED
> > although they obviously have some performance impact.
> 
> It is not for the CPU performance they can be disabled, but for the code
> size which is a real problem on embedded system. While you often have
> mem/cpu_mhz ratios around 1GB/1GHz on servers and desktops, you more often
> have ratios like 16MB/500MHz which is 1:32 of the former. That's why you
> optimize for size at the expense of speed on such systems.

The latter is not true for my two examples.

CONFIG_PRINTK=n, CONFIG_BUG=n will obviously make the kernel both 
smaller and faster. [1]

> Regards,
> Willy

cu
Adrian

[1] faster due to less code to execute and positive cache effects due to 
    the smaller code [2]
[2] whether the "faster" is big enough that it is in any way measurable 
    is a different question

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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