Date: Tue, 4 Apr 2006 10:54:58 +0200 (CEST) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-amd64@FreeBSD.ORG Subject: Re: Status of NX bit support. Message-ID: <200604040854.k348swpW087545@lurza.secnetix.de> In-Reply-To: <44318E3F.6080808@rogers.com>
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Mike Jakubik <mikej@rogers.com> wrote: > John Baldwin wrote: > > On Sunday 02 April 2006 14:48, Mike Jakubik wrote: > > > I was wondering what the status of the NX bit support is. Is the pmap.c > > > code still broken or is support enabled and functioning by default? > > > > I don't think the status has changed. > > Well that sucks.. I guess then there really is no reason for someone to > run in amd64 mode unless you need more than 4GB of ram. Rather: Unless you need more than 32 bit of address space. That's a significant difference. You can take advantage of the 64 bit address space even if you don't have more than 4 GB of RAM. On the other hand, if you only need more than 4 GB of RAM, but don't require a larger address space, you could run in 32 bits with PAE enabled (although that's a crutch which I'd rather avoid). Apart from that, in 64 bit mode there are more registers available, which gcc takes advantage of to generate more efficient code. However, on the other hand, the additional registers also have to be saved and restored upon context switches, which costs CPU cycles. Also, long data types take up more memory, which can reduce cache efficiency. Whether a specific usage benfits from 64 bit mode (in terms of performance) needs to be checked by benchmarking it, and then deciding whether FreeBSD/i386 or FreeBSD/amd64 should be used. Of course, there are cases where you don't have a choice: If software is required which hasn't been ported to 64bit, you're stuck with /i386. If you need more than 32 bit of address space, you must use /amd64. If you need both, you're out of luck. :-) As you can see, there are quite a lot of advantages and disadvantages to both /i386 and /amd64 (and I've certainly missed a few). The NX bit and the 4 GB barrier aren't the only ones by far. Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd Any opinions expressed in this message may be personal to the author and may not necessarily reflect the opinions of secnetix in any way. "Emacs ist für mich kein Editor. Für mich ist das genau das gleiche, als wenn ich nach einem Fahrrad (für die Sonntagbrötchen) frage und einen pangalaktischen Raumkreuzer mit 10 km Gesamtlänge bekomme. Ich weiß nicht, was ich damit soll." -- Frank Klemm, de.comp.os.unix.discussion
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