Author: Tom Kerrigan
Date: 12:48:46 05/21/03
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On May 21, 2003 at 13:46:26, Robert Hyatt wrote: >On May 20, 2003 at 13:52:01, Tom Kerrigan wrote: > >>On May 20, 2003 at 00:26:49, Robert Hyatt wrote: >> >>>Actually it _does_ surprise me. The basic idea is that HT provides improved >>>resource utilization within the CPU. IE would you prefer to have a dual 600mhz >>>or a single 1000mhz machine? I'd generally prefer the dual 600, although for >> >>You're oversimplifying HT. When HT is running two threads, each thread only gets >>half of the core's resources. So instead of your 1GHz vs. dual 600MHz situation, >>what you have is more like a 1GHz Pentium 4 vs. a dual 1GHz Pentium. The dual >>will usually be faster, but in many cases it will be slower, sometimes by a wide >>margin. > >Not quite. Otherwise how do you explain my NPS _increase_ when using a second >thread on a single physical cpu? > >The issue is that now things can be overlapped and more of the CPU core >gets utilized for a greater percent of the total run-time... > >If it were just 50-50 then there would be _zero_ improvement for perfect >algorithms, and a negative improvement for any algorithm with any overhead >whatsoever... > >And the 50-50 doesn't even hold true for all cases, as my test results have >shown, even though I have yet to find any reason for what is going on... Think a little bit before posting, Bob. I said that the chip's execution resources were evenly split, I didn't say that the chip's performance is evently split. That's just stupid. You have to figure in how those execution resources are utilized and understand that adding more of these resources gives you diminishing returns. -Tom
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