On Sun, 2005-09-18 at 13:18 +1000, Hamish Moffatt wrote: > On Sat, Sep 17, 2005 at 06:49:55PM -0700, lordSauron wrote: > > pentium 4s use a 21 stage pipeline or something like that... so they > > take approximately 21 clock cycles to get anything done. AMD uses > > about 7 stages (or something in that neighbourhood) so if you divide > > 2.8 by 21 and 2.0 (my Athlon64) by 7, you get a really interesting > > breakdown. You'll certainly find a HUGE increase in performance, > > That's a terrible simplification. Yes, it takes longer to get the first Not only is it a simplification, it's wrong. > result (21 cycles versus 7) but the idea of the pipeline is that you can > get a result every clock cycle after that. But when you context-switch or branch, the pipeline gets dirty, and the new process needs to fill up the pipeline. Short pipelines like in Athlon & G4 are easier on branching, but other techniques like speculative fetching and OOE mitigate that somewhat. And then, deep pipelines let you ramp up the clock much easier than do short pipelines. Don't know why, though. > The latency is higher but the > throughput is also higher (more clock cycles per second). I've always wondered if a simple single-tasking OS like extended DOS would be a perfect match for the P4. With loop-unrolling and a 2MB cache, it would chew thru CPU-inten- sive code like a buzz saw thru balsa. -- ----------------------------------------------------------------- Ron Johnson, Jr. Temporarily not of Jefferson, LA USA PGP Key ID 8834C06B I prefer encrypted mail. Vegetarian - an old Indian word meaning 'lousy hunter'.
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