Date: Thu, 2 Aug 2001 05:51:35 -0300 (BRST) From: Rik van Riel <riel@conectiva.com.br> To: Terry Lambert <tlambert2@mindspring.com> Cc: craig <craiglei@pasia.com.cn>, <freebsd-hackers@FreeBSD.ORG> Subject: Re: How to visit physical memory above 4G? Message-ID: <Pine.LNX.4.33L.0108020543070.5582-100000@duckman.distro.conectiva> In-Reply-To: <3B68F190.AB04ACB3@mindspring.com>
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On Wed, 1 Aug 2001, Terry Lambert wrote: > > craig wrote: > > > > > > I know PIII can support 64G physical memory. In FreeBSD how can I visit such > > range memory(4G-64G) ? > > The short answer is "you can't". > > The longer answer is that you end up having to window it using > segmentation; Only if you want to use it all within one process. You can have multiple 2 GB (that's the maximum process size in FreeBSD, right?) programs at the same time, happily using all physical memory. Only the FreeBSD memory management subsystem doesn't support it (yet?). > This basically means that the memory is useless as a DMA target > or source for disk controllers or gigabit ethernet cards, and is > pretty useless for swap, ... > So for limited uses in data intensive applications, it might be > usable, And for those data intensive applications, it is very useful indeed... > But to directly answer your question: by rewriting much of the > low core virtual memory and page mapping handling code to know > about segmentation. Not just that. There is a more insidious problem with the FreeBSD VM code and support of huge machines. The part of handling the PAE extended page table format and mapping high memory pages in and out of KVA (kernel virtual address) memory to copy stuff is easy. Problem is that you'll have to fit all of FreeBSD's VM data structures in the 2GB of KVA. This just isn't going to fit with the size the data structures have today ... So in order to support huge memory machines "right", you'd have to put a number of FreeBSD's VM data structures on a rather strict diet. regards, Rik -- Executive summary of a recent Microsoft press release: "we are concerned about the GNU General Public License (GPL)" http://www.surriel.com/ http://www.conectiva.com/ http://distro.conectiva.com/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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