linux-kernel - Re: [rfc] increase struct page size?!

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Message-ID: <20070519013013.GC15569@wotan.suse.de>
Date:	Sat, 19 May 2007 03:30:13 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-arch@...r.kernel.org
Subject: Re: [rfc] increase struct page size?!

On Fri, May 18, 2007 at 10:42:30AM +0100, David Howells wrote:
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > I'd like to be the first to propose an increase to the size of struct page
> > just for the sake of increasing it!
> 
> Heh.  I'm surprised you haven't got more adverse reactions.
> 
> > If we add 8 bytes to struct page on 64-bit machines, it becomes 64 bytes,
> > which is quite a nice number for cache purposes.
> 
> Whilst that's true, if you have to deal with a run of contiguous page structs
> (eg: the page allocator, perhaps) it's actually less efficient because it
> takes more cache to do it.  But, hey, it's a compromise whatever.
> 
> In the scheme of things, if we're mostly dealing with individual page structs
> (as I think we are), then yes, I think it's probably a good thing to do -
> especially with larger page sizes.

Yeah, we would end up eating about 12.5% more cachelines for contiguous
runs of pages... but that only kicks in after we've touched 8 of them I
think, and by that point the accesses should be very prefetchable.

I think the average of 75% more cachelines touched for random accesses
is going to outweigh the contiguous batch savings, but that's just a
guess at this point.
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