Date: Thu, 30 Jan 2003 05:00:45 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Peter Wemm <peter@wemm.org> Cc: "Andrew R. Reiter" <arr@watson.org>, Scott Long <scott_long@btc.adaptec.com>, arch@FreeBSD.ORG Subject: Re: PAE (was Re: bus_dmamem_alloc_size()) Message-ID: <3E3921FD.BA247304@mindspring.com> References: <20030130083020.05CC12A8A1@canning.wemm.org>
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Peter Wemm wrote: > > Peter Wemm? > > We've been tinkering with it, but frankly, PAE isn't as useful to us as a > larger virtual address space would be. eg: ia64 or x86-64. There have been > various experiments and parts of the problem worked on, but nothing even > remotely complete. Most of what I've tinkered with is available in the p4 > tree. > > Reliable sources tell me that this should change soon, but it is not from > Y!. I'll leave it up to the folks involved to add more details if they > think it appropriate. In the mean time, please stop sending people in our > direction. No problem. The reason I don't send them in my own direction is that I personally don't find it useful, either, so I didn't do anything much on it, past a proof-of-concept. The PSE36 actually seems marginally more useful, but in both cases, the resulting code is not nearly as useful as a large address space would be, and the overhead is prohibitively expensive for it to be useful. IMO, unless you use TSS for task switching, and are willing to eat the overhead of not being able to access task memory simultaneously, and jamming all shared memory into a small window of shared address space, the idea is totally screwed (shared libraries are particularly problematic, given where they are mapped). For some reason, people think PAE/PSE36 is a substitute for 64bit architectures, when it comes to accessing more memory; in reality, the hardware design is bad enough that it's not really possible to access the memory simultaneously, and that's the most interesting (and useful) application for "more memory". In general, I sent people your way because you guys were talking about playing with it in the context of the one place it might be considered useful (running too many processes on one machine, and eating the associated overhead), since it's not really possible to make the additional physical memory a DMA target on most hardware, even if you have a 64bit PCI controller available for all your network and disk controllers. If anything, I have *more* respect for you guys for abandoning it... 8-) 8-). -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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