Date: Tue, 05 Nov 2002 00:39:53 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Richard Sharpe <rsharpe@ns.aus.com> Cc: Matt <mattl3@earthlink.net>, hackers@FreeBSD.ORG Subject: Re: max phy mem known working with FreeBSD 4.x Message-ID: <3DC783D9.1511B859@mindspring.com> References: <Pine.LNX.4.33.0211051916580.4835-100000@ns.aus.com>
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Richard Sharpe wrote: > Well, the P4 does have an address extension that allows addressing of up > to 64GB, EPA or something like that. This is useless to discuss in the context of 4.3, or even 4.x, unless Paul Saab is doing his work in 4.7, and it makes it into 4.8. Given that we are pretty much forced to do all new work 0n -current in order to get committed (or everyone would be working on 4.x instead), it'll end up a forward port with no back-port (expect it after the 5.x release, if Yahoo gives it back to FreeBSD). > However, it requires some work. Would be nice in large Samba servers, > though, to be able to cache enormous amounts of file data :-) It would be useless for this purpose. Specifically, you would need to guarantee a locality of reference for clients that you could not guarantee, for this to be useful at all. Mostly, you would be swapping the PSE window around all the time, shooting down TLB's and cache lines, and you would burn up any benefits of having the data in main RAM instead of in RAM on the disk controller or the disk itself, very quickly. In addition, you would not be able to use "sendfile" or similar interfaces, and you would not be able to use scatter/gather DMA, unless you knew for certain that the card doing the DMA had 64 bit access to physical RAM. Otherwise, you would need to copy all data to be DMA'ed to/from any physical RAM above 4G to bounce buffers below 4G. The extra copying might as well be from the controller RAM or the RAM on the disk itself, than from/to RAM above 4G. The 2G limit on Alpha is basically the same issue, where the data needs to be bounced through lower memory in order to be accessible to the hardware DMA engines on the other side of the PCI bus (hence the need to convert the drivers to "bus space" to be able to use more than 2G in an Alpha box). Basically, the extra RAM makes assumptions about code that make us all realize that hardware designers don't really ask software designers why they ask for things like the ability to access more than 4G of RAM. 8-). If you want to discuss this further, please read the archives of the last 3 times the issue was discusees, on the -current and -arch lists. No matter how you look at it, though, unless you pay some schmuck to do the work, you're never going to see this in 4.3 anyway, so you might as well move the discussion over to the -current list. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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