From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Date: Tue, 25 Mar 2008 23:49:27 +0000 Subject: Re: larger default page sizes... Message-Id: <20080325.164927.249210766.davem@davemloft.net> List-Id: References: <20080325.162244.61337214.davem@davemloft.net> <87tziu5q37.wl%peter@chubb.wattle.id.au> In-Reply-To: <87tziu5q37.wl%peter@chubb.wattle.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: peterc@gelato.unsw.edu.au Cc: clameter@sgi.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, torvalds@linux-foundation.org, ianw@gelato.unsw.edu.au From: Peter Chubb 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.