Author: Gian-Carlo Pascutto
Date: 18:59:43 10/04/03
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On October 04, 2003 at 21:53:17, Tom Kerrigan wrote: >Well, let's say I have the latest version of Windows. Including new bugs? :-) >>>Given that, HT would have to be providing a 50% speedup for my program if it >>>were only running on one processor, which seems unrealistically high, and that >>>also means my program should run 100% faster with 4 threads (vs. 2) but in >>>reality it only runs 15% faster. >> >>I don't get your math. > >Put another way, if 2 threads are spending a bunch of time on just 1 processor, >that means the 2nd processor has a bunch of idle processing power. So if I run 4 >threads, it should use the extra processing power and run a lot faster, but it >doesn't. The conclusion is that when running 2 threads, each thread gets its own >CPU. Assuming things are scheduled randomly, you have a 1/3 chance of getting the thread on the wrong cpu, or 66% efficiency over the second CPU. Fixing this brings you to 100% efficiency on the second CPU, or a 33%/2=16% increase in efficiency overall. This perfectly in line with what you measured, if I understand your post and my math at 4am isn't totally broken. -- GCP
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