From mboxrd@z Thu Jan 1 00:00:00 1970 Mime-Version: 1.0 (Apple Message framework v752.3) In-Reply-To: <20071116170014.595B2108A1@mail.cse.psu.edu> References: <20071116170014.595B2108A1@mail.cse.psu.edu> Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed Message-Id: <569F1E30-446D-4B63-8D30-8AD7AE65EF94@utopian.net> Content-Transfer-Encoding: 7bit From: Joshua Wood Subject: Re: [9fans] Current status of amd64 port? Date: Fri, 16 Nov 2007 15:10:39 -0800 To: 9fans@cse.psu.edu Topicbox-Message-UUID: fd7cb5fe-ead2-11e9-9d60-3106f5b1d025 > Intel or AMD? I've found AMDs memory architecture for 64bit stuff to > pretty much stomp Intel nearly every time. Happened to be amd. Completeness wasn't needed, so there was no intel testing. > Not handy, but I used to build MPI applications on Opterons in 32bit > mode, then build them in 64bit mode, and nearly always get a > performance boost out of the 64bit. Which was the exact opposite that > I'd get on PPC 32 vs 64 bit. I think you're seeing that the ppc always had all those registers, 32 or 64-bit. What goes on, to oversimplify, is that when you compile code 64-bit for the amd64, you suddenly have bunches of new registers, and [some] things go faster. When you compile code 64-bit for the ppc, you didn't gain any registers you didn't already have. With no built-in performance gain, you're left with only the results of the increased cache pressure from larger everything in your 64-bit executable. Your observation that things slow down is, as you said, readily heard from others. But that's just it; the performance isn't the point; the point is you can address >4G of memory now. It might be worth observing that the "real" POWER 64-bit isa tends to appear in configurations with massive L2 and L3 cache -- not the meg or two you get with the x86 64bit stuff, amd or intel. I'm repeating the sin of performance discussion without numbers -- but if you check apple's own "64 bit myths" factsheet you can see the same information. Moreover, I'm so OT, but the general theme of the observations seems to me to underline the point Erik was making: 64- bit is meaningless in a vacuum, lacks demonstrable numbers as a performance magic bullet, and in the end, is a way to physically address more memory. Nothing more, nothing less. -- Josh