Messages in this thread | | | Date | Thu, 24 Sep 1998 21:41:08 -0500 (CDT) | From | Oliver Xymoron <> | Subject | Re: Interesting scheduling times - NOT |
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On Thu, 24 Sep 1998, Larry McVoy wrote:
> : I only mentioned that because Larry mentioned normal distributions of > : results in a previous message and it struck me as strange to expect a > : normal distribution of scheduler latencies. He was arguing against using > : the minimum time from a benchmark, as I recall. There are a number of > : places this is useful number, for instance, the round trip times from > : ping. The minimum rtt is a rather hard limit and tells you what the best > : latency you can expect is. > > Well, this is a fine theory - and it was what I thought when I started > measuring things - but it's wrong in practice. If you take a suite of > tests, lmbench for example, and do a bunch of runs and scatter plot them > and stare at them you'll see patterns emerging. Now if the pattern was > that most run times clustered around the min, then my feeling is that > the min is the right number. Wherever they cluster up is the number I > wanted because that was the number mostly likely to be seen.
Again, I agree that generally the average is the number that's interesting. But earlier you seemed to imply that the minimum is not generally a meaningful number, because they were way out on the tails of normal distributions. Even if that is true (and many distributions that are the combination of many events will take a bell shaped curve because of the binomial theorem, but at the same time many aren't), that doesn't mean that the minimum is meaningless. It may generally be a poor number to use for benchmarks, but it may be able to tell you about room for improvement, underlying physical constraints, etc. See the ping example.
-- "Love the dolphins," she advised him. "Write by W.A.S.T.E.."
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