Date: Wed, 8 Nov 2000 12:28:30 -0600 (CST) From: Jonathan Lemon <jlemon@flugsvamp.com> To: asmodai@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Re: PSE/PAE support Message-ID: <200011081828.eA8ISUj13784@prism.flugsvamp.com> In-Reply-To: <local.mail.freebsd-hackers/20001108150722.L80971@lucifer.bart.nl>
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In article <local.mail.freebsd-hackers/20001108150722.L80971@lucifer.bart.nl> you write: >Reading some documents and sources I came to the following conclusion: > >We support 4 Kbyte pages and 4 Mbyte pages, but we do not support 2 >Mbyte pages. On IA-32 at least. > >Is there a reason to? > >I could understand that some people would prefer 2 Mbyte pages in some >situations. But looking at pmap.c I see we only test CPUID_PSE and >don't use CPUID_PAE anywhere. My initial feeling is that it wouldn't win us anything. We use a two-level page table, with the first level covering a 4MB page, and the second level containing 1024 pte's, each covering 4K. Since each pte takes up 4 bytes, by using a 4MB page, we eliminate an entire page of pte's at the second level. With a 2M page, we would still need a second level page entry, plus some code to differentiate between 4K entries and a "half-page" 2M entry. But I'm by no means a vm hacker, so I may be wrong here. -- Jonathan To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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