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[Python-Dev] PEP 410 (Decimal timestamp): the implementation is ready for a review
"Martin v. Löwis"martin at v.loewis.de
Wed Feb 15 20:56:26 CET 2012
Am 15.02.2012 19:10, schrieb Antoine Pitrou:>> Le mercredi 15 février 2012 à 18:58 +0100, Victor Stinner a écrit :>> It gives me differences smaller than 1000 ns on Ubuntu 11.10 and a>> Intel Core i5 @ 3.33GHz:>>>> $ ./a.out>> 0 s, 781 ns>> $ ./a.out>> 0 s, 785 ns>> $ ./a.out>> 0 s, 798 ns>> $ ./a.out>> 0 s, 818 ns>> $ ./a.out>> 0 s, 270 ns>> What is it supposed to prove exactly? There is a difference between> being able to *represent* nanoseconds and being able to *measure* them;> let alone give a precise meaning to them.Linux *actually* is able to measure time in nanosecond precision, eventhough it is not able to keep its clock synchronized to UTC with ananosecond accuracy.The way Linux does that is to use the time-stamping counter of theprocessor (the rdtsc instructions), which (originally) counts one unitper CPU clock. I believe current processors use slightly differentcountings (e.g. through the APIC), but still: you get a resolutionwithin the clock frequency of the CPU quartz.With the quartz in Victor's machine, a single clock takes 0.3ns, sothree of them make a nanosecond. As the quartz may not be entirelyaccurate (and also as the CPU frequency may change) you have to measurethe clock rate against an external time source, but Linux hasimplemented algorithms for that. On my system, dmesg shows[ 2.236894] Refined TSC clocksource calibration: 2793.000 MHz.[ 2.236900] Switching to clocksource tscRegards,Martin
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