linux-kernel - Re: Large stack usage in fs code (especially for PPC64)

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Message-Id: <20081117133137.616cf287.akpm@linux-foundation.org>
Date:	Mon, 17 Nov 2008 13:31:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	paulus@...ba.org, benh@...nel.crashing.org,
	linuxppc-dev@...abs.org, mingo@...e.hu, tglx@...utronix.de,
	linux-mm@...ck.org
Subject: Re: Large stack usage in fs code (especially for PPC64)

On Mon, 17 Nov 2008 13:23:23 -0800 (PST)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Mon, 17 Nov 2008, Andrew Morton wrote:
> > 
> > Far be it from me to apportion blame, but THIS IS ALL LINUS'S FAULT!!!!! :)
> > 
> > I fixed this six years ago.  See http://lkml.org/lkml/2002/6/17/68
> 
> Btw, in that thread I also said:
> 
>   "If we have 64kB pages, such architectures will have to have a bigger 
>    kernel stack. Which they will have, simply by virtue of having the very 
>    same bigger page. So that problem kind of solves itself."
> 
> and that may still be the "right" solution - if somebody is so insane that 
> they want 64kB pages, then they might as well have a 64kB kernel stack as 
> well. 

I'd have thought so, but I'm sure we're about to hear how important an
optimisation the smaller stacks are ;)

> Trust me, the kernel stack isn't where you blow your memory with a 64kB 
> page. You blow all your memory on the memory fragmentation of your page 
> cache. I did the stats for the kernel source tree a long time ago, and I 
> think you wasted something like 4GB of RAM with a 64kB page size.
> 

Yup.  That being said, the younger me did assert that "this is a neater
implementation anyway".  If we can implement those loops without
needing those on-stack temporary arrays then things probably are better
overall.

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