linux-kernel - Re: x86: 4kstacks default

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Message-ID: <480B7B1E.7010707@firstfloor.org>
Date:	Sun, 20 Apr 2008 19:19:26 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jörn Engel <joern@...fs.org>
CC:	Willy Tarreau <w@....eu>, Mark Lord <lkml@....ca>,
	Adrian Bunk <bunk@...nel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: x86: 4kstacks default

Jörn Engel wrote:
> On Sun, 20 April 2008 16:19:29 +0200, Andi Kleen wrote:
>> Only if you believe that 4K stack pages are a worthy goal.
>> As far as I can figure out they are not. They might have been
>> a worthy goal on crappy 2.4 VMs, but these times are long gone.
>>
>> The "saving memory on embedded" argument also does not 
>> quite convince me, it is unclear if that is really
>> a significant amount of memory on these systems and if that 
>> couldn't be addressed better (e.g. in running generally
>> less kernel threads).  I don't have numbers on this,
>> but then the people who made this argument didn't have any
>> either :) 
> 
> It is not uncommon for embedded systems to be designed around 16MiB.

But these are SoC systems. Do they really run x86?
(note we're talking about an x86 default option here)

Also I suspect in a true 16MB system you have to strip down
everything kernel side so much that you're pretty much outside
the "validated by testers" realm that Adrian cares about.

> When dealing in those dimensions, savings of 100k are substantial.  In
> some causes they may be the difference between 16MiB or 32MiB, which
> translates to manufacturing costs.  In others it simply means that the
> system can cache 

If you need the stack you don't have any less cache foot print.
If you don't need it you don't have any either.

-Andi
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