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SubjectRe: gettimeofday() resolution in Linux?
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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