linux-kernel - Re: [PATCH 09/10] Change table chaining layout

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Message-ID: <20071022234622.GT19691@waste.org>
Date:	Mon, 22 Oct 2007 18:46:23 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	mingo@...e.hu, Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: [PATCH 09/10] Change table chaining layout

On Mon, Oct 22, 2007 at 11:52:51PM +0100, Alan Cox wrote:
> On Mon, 22 Oct 2007 16:47:07 -0500
> Matt Mackall <mpm@...enic.com> wrote:
> 
> > On Mon, Oct 22, 2007 at 05:21:30PM -0400, Jeff Garzik wrote:
> > > Alan Cox wrote:
> > > >Why can't we just make the list one item longer than the entry count and
> > > >stick a NULL on the end of it like normal people ?
> > > 
> > > Certainly seems safer than the current "let's run off the end of the 
> > > list if anything bad happens" setup...  And I do not think allocating 
> > > n+1 scatterlist entries will have much of a negative impact.
> > 
> > It'll mean m-1 scatterlists fit on a slab.
> 
> Is that really a credible space issue ?

Yes. Especially if m is 2 or 1. A scatterlist on 64-bit x86 looks like
it takes 32 bytes, which means 128 elements fit on a page. One more
spills - ouch!

But maybe chaining means this doesn't matter any more. Maybe we can
even pick a nice moderate sg size and reduce the number of mempools we
need for these things.

-- 
Mathematics is the supreme nostalgia of our time.
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