[Haskell-cafe] Re: System.CPUTime and picoseconds

Aren't Doubles evil? Integer is a nice type, Haskell filosofy compliant. Doubles are not CDoubles, IEEE, infinite precision or anything long term meaninfull. (Warning: non-expert opinion.)

I've found the pico second accuracy useful in working with 'rate equivalent' real time systems. Systems where the individual timings (their jitter) is not critical but the long term rate should be accurate - the extra precision helps with keeping the error accumulation under control.

When you are selling something (like data bandwidth) and you are pacing the data stream on a per packet basis you definitely want any error to accumulate slowly - you are in the 10^10 events per day range here.

Neil

On 12 Jan 2009, at 00:00, Lennart Augustsson wrote:

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On Sun, Jan 11, 2009 at 8:28 PM, ChrisK wrote:

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An Double or Int64 are both 8 bytes and counts with picoseconds precision for 2.5 hours to 106 days. Going to 12 byte integer lets you count to 3.9 billion years (signed). Going to 16 byte integer is over 10^38 years.

Lennart Augustsson wrote:

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A double has 53 bits in the mantissa which means that for a running time of about 24 hours you'd still have picoseconds. I doubt anyone cares about picoseconds when the running time is a day.

The above is an unfounded claim about the rest of humanity.

It's not really about humanity, but about physics. The best known clocks have a long term error of about 1e-14. If anyone claims to have made a time measurement where the accuracy exceeds the precision of a double I will just assume that this person is a liar.

For counting discrete events, like clock cycles, I want something like Integer or Int64. For measuring physical quantities, like CPU time, I'll settle for Double, because we can't measure any better than this (this can of course become obsolete, but I'll accept that error).

-- Lennart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe