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

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Date:	Sat, 19 May 2007 03:25:30 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Lameter <clameter@....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 11:14:26AM -0700, Christoph Lameter wrote:
> On Fri, 18 May 2007, Nick Piggin wrote:
> 
> > However we don't have to let those 8 bytes go to waste: we can use them
> > to store the virtual address of the page, which kind of makes sense for
> > 64-bit, because they can likely to use complicated memory models.
> 
> That is not a valid consideration anymore. There is virtual memmap update 
> pending with the sparsemem folks that will simplify things.
> 
> > Many batch operations on struct page are completely random, and as such, I
> > think it is better if each struct page fits completely into a single
> > cacheline even if it means being slightly larger.
> 
> Right. That would simplify the calculations.

It isn't the calculations I'm worried about, although they'll get simpler
too. It is the cache cost.

> > Don't let this space go to waste though, we can use page->virtual in order
> > to optimise page_address operations.
> 
> page->virtual is a benefit if the page is cache hot. Otherwise it may 
> cause a useless lookup.

It would be very rare for the page not to be in L1 cache at this point,
because we've likely taken a reference on it and/or locked it or moved it
between lists etc.

> I wonder if there are other uses for the free space?

Hugh points out that we should make _count and _mapcount atomic_long_t's,
which would probably be a better use of the space once your vmemmap goes
in.
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