Date: Thu, 02 Aug 2001 22:15:39 -0700 From: Terry Lambert <tlambert2@mindspring.com> To: Charles Randall <crandall@matchlogic.com> Cc: Rik van Riel <riel@conectiva.com.br>, craig <craiglei@pasia.com.cn>, freebsd-hackers@FreeBSD.ORG Subject: Re: How to visit physical memory above 4G? Message-ID: <3B6A337B.5A81232B@mindspring.com> References: <5FE9B713CCCDD311A03400508B8B30130828F234@bdr-xcln.corp.matchlogic.com>
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Charles Randall wrote: > > From: Terry Lambert [mailto:tlambert2@mindspring.com] > >I have yet to see one person using it for anything. So far, > >it is nothing more than marketing fodder: I haven't seen one > >motherboard capable of more than 4G worth of SIMMs. > > The Dell PowerEdge 6450 supports 8 GB of RAM. > > http://www.dell.com/us/en/biz/products/model_pedge_pedge_6400.htm > > If I understand your comments in a few follow-up messages > correctly you're saying that this effort may be better spent > by working on an IA-64 port and making it support large memory > configurations? The IA64 intrinsically supports a physical address space of 2^64, so an address space of 2^36 vs. 2^32 is spectacularly unimpressive. > Can you elaborate? Yeah, the overhead in doing this will up the CPU utilization to the point where it becomes fairly useless to do the swapping to and from above 4G, vs. just swapping normally. The costs involved in doing DMA to/from the memory region above 4G will be incredible, unless the address space is both exported, and known, to the PCI bus; even then, it could only work for 64 bit cards, since 32 bith cards will only be able to address the first 4G of physical memory. I can think of one or two uses for the memory, assuming the ability to DMA into and out of it with a 64 bit card, and the ability to shove a 1G or 2G window around in it so the kernel can get at the memory when it needs to, but the overhead seems to me to be high enough that you are better off buying a Sibytes card, running NetBSD on the MIPS processors on the thing, plugging in 16G of RAM, and calling your PC a "control processor". -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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