Date: Fri, 12 Mar 2004 15:26:56 -0500 From: Jem Matzan <valour@thejemreport.com> To: freebsd-amd64@freebsd.org Subject: Re: Peer review of AMD64/FreeBSD article Message-ID: <40521D10.7060204@thejemreport.com> In-Reply-To: <40521A9E.8070808@jrv.org> References: <4051A841.9020205@thejemreport.com> <40521A9E.8070808@jrv.org>
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James R. Van Artsalen wrote: > Jem Matzan wrote: > >> I've just finished writing this article comparing performance between >> an Athlon64 in 32-bit and 64-bit mode using FreeBSD: > > > Intel would be thrilled were Prescott to "idle" at 60 F anywhere other > than outdoors in an Antarctic winter: alas, 60 C sounds more likely > (but still seems astoundingly high for a halted processor). > > Your Prescott probably isn't doctored, but it is the case that early > steppings of a CPU are always faster than later steppings: bug fixes > to the silicon or control store patches by ROM POST rarely speed it up. > > The Prescott performance variation may indeed be due to thermal > issues. I think Prescott slows down in response to thermal overload > (AMD just enters a non-resumable halt - AMD's is a safety mechanism to > protect the motherboard and CPU). It is not out of the question that > Prescotts is regularly bumping up against thermal limits and running > slow briefly. I find this hard to believe, but no harder to believe > than a 60 C halted processor... Test by *lightly* preheating CPU > cooler air intake with a well-aimed hairdryer to and see if that hurts > performance. > > It may be worth mentioning that theoretically the usual win from > 64-bit mode comes not from the fact that registers and reg ops are 64 > bit but rather because more registers are available when in 64-bit > mode. This is a huge win for a compiler which is nearly asphyxiated > in register allocation by the i386. > > It might be worth mentioning that a powerful differentiator (between > i386 and amd64 is maximum memory. With AMD64 you can keep on adding > RAM after 4 GB as long as it wins. A database-driven web site might > win substantially by having an 8 GB resident working set > *in-process*. The max for i386 is around 3 GB; the practical max for > amd64 is about 15 GB and growing (Tyan Thunder K8W with 8x 2GB > DIMMs). This is beyond the scope of your tests but might be worth > mentioning. > > _______________________________________________ > freebsd-amd64@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-amd64 > To unsubscribe, send any mail to "freebsd-amd64-unsubscribe@freebsd.org" > > Doh -- yes, that's a mistake on my part, should be 60 degrees C, not F. More like ~144 degrees F. I wish I could reliably measure the temperature under load in FreeBSD. I could do it in Windows but the software temp readers are sometimes very inaccurate. Intel included a special dashboard utility with the press kit (probably standard issue for the Prescott retail processors on the driver CD) but I didn't try it out. I'll add this information to the article -- thanks. -Jem
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