linux-kernel - Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

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Date:	Wed, 18 Jul 2007 19:17:14 +0200
From:	Rene Herman <rene.herman@...il.com>
To:	Matt Mackall <mpm@...enic.com>
CC:	Ray Lee <ray-lk@...rabbit.org>, Bodo Eggert <7eggert@....de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jesper Juhl <jesper.juhl@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH][RFC] 4K stacks default, not a debug thing any more...?

On 07/18/2007 06:54 PM, Matt Mackall wrote:

> You can expect the distribution of file sizes to follow a gamma
> distribution, with a large hump towards the small end of the spectrum
> around 1-10K, dropping off very rapidly as file sizes grow.

Okay.

>> Not too sure then that 8K wouldn't be something I'd want, given fewer 
>> pagefaults and all that...
> 
> Fewer minor pagefaults, perhaps. Readahead already deals with most of
> the major pagefaults that larger pages would.

Mmm, yes.

> Anyway, raising the systemwide memory overhead by up to 15% seems an 
> awfully silly way to address the problem of not being able to allocate a
> stack when you're down to your last 1 or 2% of memory!

Well, I've seen larger pagesizes submerge in more situations, specifically 
in allocation overhead -- ie, making the struct page's fit in lowmem for 
hugemem x86 boxes was the first I heard of it. But yes, otherwise (also) 
mostly database loads which obviously have moved to 64-bit since.

Pagecache tail-packing seems like a promising idea to deal with the downside 
of larger pages but I'll admit I'm not particularly sure how many _up_ sides 
to them are left on x86 (not -64) now that's becoming a legacy architecture 
(and since you just shot down the pagefaults thing).

Rene.
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