At 21:51 -0500 07/24/2002, R.A. Cantrell wrote: >on 7/24/02 9:44 PM, Jeff Walther at [EMAIL PROTECTED] wrote: > >> If your G3 doesn't have software to disable the 512K cache, then you >> may wish to disable it with the jumper. Leaving the motherboard >> cache enabled when you have a backside cache in front of it >> definitely causes a performance hit. It's not as much as 10%, but >> it's still a hit. >Is the 512 soldered on the MoBo? >-- >All the best, >R.A. Cantrell >
Yes. To make my previous post clearer. I meant that I can't see that installing the J38 jumper would get you any advantage, because even with the MB 512K cache active all the CPU cards I've tested seem to perform up to their maximum possible bus speed anyway. So I can't see what one would gain, bus speed wise, by disabling the motherboard cache with the J38 jumper. However, bus speed is not all there is to performance. If you install a G3 or G4 with a backside cache and you leave the motherboard cache enabled, you will suffer a performance penalty. Checking the cache for data which the CPU wants takes time, which costs performance. This lost time is made up because whenever the needed data is in the cache, accessing the cache is much faster than going to memory. So a cache hit (info is in the cache) saves time, but a cache miss costs time. Obviously, the larger your ratio of hits to misses the better your performance. A good caching scheme can generate hits more than 85% of the time. If you put a smaller or same size cache behind a larger or same size cache you generate a lot more misses. The machine checks the backside cache and a hit is fine, you get the data from the backside cache. But on every miss on the backside cache, the machine would then go and check the motherboard cache which is both slower and likely to also be a miss, because it is smaller than the backside cache, and so unlikely to contain anything that the backside cache lacked. So almost every miss penalty on the backside is more than doubled if the motherboard cache is still working. Most (all?) G3/G4 upgrades include software which will disable the motherboard cache for you, though. So there really shouldn't be any need to install the J38 jumper, unless OSX doesn't have a motherboard cache disabling utility or something. If I have a motherboard on my bench, I usually install the pins at J38 so that a removeable jumper can be used if wanted, but that's just because I like to solder. Jeff Walther -- SuperMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | Service & Replacement Parts [EMAIL PROTECTED] | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> SuperMacs list info: <http://lowendmac.com/supermacs/list.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/supermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
