linux-kernel - Re: larger default page sizes...

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Message-Id: <20080325.164927.249210766.davem@davemloft.net>
Date:	Tue, 25 Mar 2008 16:49:27 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	peterc@...ato.unsw.edu.au
Cc:	clameter@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-ia64@...r.kernel.org, torvalds@...ux-foundation.org,
	ianw@...ato.unsw.edu.au
Subject: Re: larger default page sizes...

From: Peter Chubb <peterc@...ato.unsw.edu.au>
Date: Wed, 26 Mar 2008 10:41:32 +1100

> It's actually harder than it looks.  Ian Wienand just finished his
> Master's project in this area, so we have *lots* of data.  The main
> issue is that, at least on Itanium, you have to turn off the hardware
> page table walker for hugepages if you want to mix superpages and
> standard pages in the same region. (The long format VHPT isn't the
> panacea we'd like it to be because the hash function it uses depends
> on the page size).  This means that although you have fewer TLB misses
> with larger pages, the cost of those TLB misses is three to four times
> higher than with the standard pages.

If the hugepage is more than 3 to 4 times larger than the base
page size, which it almost certainly is, it's still an enormous
win.

> Other architectures (where the page size isn't tied into the hash
> function, so the hardware walked can be used for superpages) will have
> different tradeoffs.

Right, admittedly this is just a (one of many) strange IA64 quirk.
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