Messages in this thread | | | Date | Thu, 10 Apr 2008 18:07:14 +0200 | From | "Bart Van Assche" <> | Subject | Re: gettimeofday() resolution in Linux? |
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On Thu, Apr 10, 2008 at 4:40 PM, Jack Harvard <jack.harvard@googlemail.com> wrote: > I would like to ask a few questions about how Linux keeps time. > > As far as I understand, > 1. Linux's time resolution is 10ms, as defined by HZ=100. > 2. gettimeofday() can get time in microseconds, but I'm not sure about > the accuracy of the time finer than 10ms. Sometimes gettimeofday( ) > can even give me microseconds results rolled backwards in time, which > I suspect could be caused by its accuracy. My question here is "how > accurate is the time from gettimeofday()" > 3. If I want to increase the time resolution to 1ms, I can possibly > change HZ=1000, but if I want 1usec resolution, how can I do that? It > would be too busy for the processor to handle so frequent timer > interrupts if I just increase HZ=1000000.
You should use clock_gettime(CLOCK_MONOTONIC) instead of gettimeofday() for interval timing. As soon as NTP software is running, the result of gettimeofday() can jump forward or backward in time. The result of clock_gettime() is guaranteed to be monotonic, and additionally the resolution of clock_gettime() is higher than gettimeofday() -- 1 ns instead of 1 us.
Changing HZ only makes sense on very old hardware that does not have a TSC or equivalent register (e.g. the Intel 486 CPU).
Bart.
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