From owner-freebsd-smp Mon Jul 26 19:48:57 1999 Delivered-To: freebsd-smp@freebsd.org Received: from apollo.backplane.com (apollo.backplane.com [209.157.86.2]) by hub.freebsd.org (Postfix) with ESMTP id 1567F150CB for ; Mon, 26 Jul 1999 19:48:51 -0700 (PDT) (envelope-from dillon@apollo.backplane.com) Received: (from dillon@localhost) by apollo.backplane.com (8.9.3/8.9.1) id TAA49515; Mon, 26 Jul 1999 19:47:33 -0700 (PDT) (envelope-from dillon) Date: Mon, 26 Jul 1999 19:47:33 -0700 (PDT) From: Matthew Dillon Message-Id: <199907270247.TAA49515@apollo.backplane.com> To: Poul-Henning Kamp Cc: "Eric J. Schwertfeger" , Cosmic 665 , freebsd-smp@FreeBSD.ORG Subject: Re: Overclocking References: <55719.933019101@critter.freebsd.dk> Sender: owner-freebsd-smp@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org This is how fabs generally work in regards to chip speeds: * The fab makes a chip * The chip is tested. Due to doping inconsistancies and other issues the chips that come off the fab will only test to certain speeds. * The chips are placed into buckets based on how fast they test to. But then marketing and distribution come into play. In recent years fabs have gotten a lot more reliable and this has resulted in higher and better yields. So lets say in a 10000 lot Intel winds up with 8000 chips that test to 500MHz and 2000 that test to 400MHz. But lets also say that Intel needs to ship 5000 500MHz chips and 5000 400MHz chips. What Intel (and all chip manufacturers) will do is throw some of the higher-testing chips into the lower-MHz rating in order to cover their orders. When you purchase a 400Mhz chip all you are guarenteed is that it can run at 400MHz. It is possible that your chip was tested to 500MHz but you can't tell for sure. Intel's FAB has gotten good enough that they physically changed the celeron design in order to be able to laser or PROM a limit to the frequency multiplier to prevent people from overclocking the chips after people found out that they could do it reliably. Now, this is why you *DON'T* want to overclock: When a chip is tested the frequency limit is determined by a number of factors both internal and external. For example, a single line within the register file or a single bit in the cache might be just a tad too slow and limit the overall frequency the chip can sustain. Or it could be an external signal that doesn't quite meet spec above a certain frequency. The problem is that usually the failure is something relatively minor in the chip, but which can have major consequences to the execution of instructions. Worse, the failure condition is not necessarily reliably reproduceable. You may believe that your overclocked chip is working correctly, but there is a good chance that it isn't, quite. If you overclock a chip you can wind up with wierd failures that take days or even weeks to show up, or failures that show up as arithmatic miscalculations. For example, the FP unit might begin to produce slightly incorrect results, or certain L1 cache situations might fail to the produce the correct data (this was a serious problem with a lot of 486's that Intel inadequately tested. They would work fine w/ Windows, but would fail under UNIX because UNIX utilized chip features that would tickle the cache bugs). In regards to heat dissipation: Heat dissipation is not usually a problem when you are overclocking a chip but still leave it under the maximum clock rating that the manufacturer makes, at least as long as the package type remains the same as the package type the manufacturer uses in their high-end products. I think for most Intel cpu's the package type is the same for nearly all speed grades of a particular chip class. If you overclock a chip beyond the maximum rating sold by the manufacturer in the package type in question, you *can* melt it. Overheating also introduces additional noise on the die and, even worse, may cause the gates making up the die to slow down. Thus some people see chips "fail" after being on for a period of time, and then work again after the computer's been off for a while. -Matt Matthew Dillon :In message , "Eric J. :Schwertfeger" writes: :> :>Much of the practicality of overclocking comes from the family of chips. :>The same fabrication process is used for PPGA Celerons from 333mhz up to :>the latest 500mhz, so I wouldn't expect you to damage CPU or motherboard :>overclocking a PPGA 333 by 50%, as long as you didn't tweak the voltage in :>order to make it run. : :It can be said as simple as this: "You Are Wrong". Running the chip :at higher clock will lead to increased heat generation, which isn't a :good thing for your silicon. : : :Rule #1: : Do not Overclock. : :Rule #2: : If you overclock, do not complain that things don't work. : : :-- :Poul-Henning Kamp FreeBSD coreteam member :phk@FreeBSD.ORG "Real hackers run -current on their laptop." :FreeBSD -- It will take a long time before progress goes too far! : : :To Unsubscribe: send mail to majordomo@FreeBSD.org :with "unsubscribe freebsd-smp" in the body of the message : To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-smp" in the body of the message