Time Resolution

Marcin 'Qrczak' Kowalczyk qrczak at knm.org.pl
Mon Jan 31 08:40:39 EST 2005

Seth Kurtzberg <seth at cql.com> writes:

> Also, what happens when you are using NTP? NTP might just correct
> it, but it would screw up the calculations NTP uses and it could
> start oscillating.

The NTP client (at least on Linux) adjusts the time by making a jump
only if the time is very inaccurate. If the time only drifted a bit,
it temporarily adjusts the speed of the system clock instead.

> I'm also saying that I don't like the idea of using the same
> resolution w.r.t. the system clock, because it suggests that the
> time is known to a greater precision than is actually the case.

I don't think this would be a practical problem.

> To me all this shows that the system clock needs to be handled as
> a special case, not just converted into the high resolution
> representation

Using a different representation of time just because somebody might
not be aware that the resolution of the system clock is not as good as
the representation suggests? No, this is an unnecessary complication.

Anyway, what does the "resolution of the system clock" mean? On Linux
the timeout of select(), poll() and epoll() is accurate only to the
timer interrupt (10ms on older kernels and 1ms on newer ones), yet the
gettimeofday is more precise (it returns microseconds, quite accurately;
the call itself takes 2us on my system). So even if gettimeofday is
accurate, sleeping for some time might be much less accurate.

BTW, this means that the most accurate way to make a delay is to use
select/poll/epoll to sleep for some time below the given time (i.e.
shorter by the typical largest interval by which the system makes the
delay longer than requested), then to call gettimeofday in a loop
until the given time arrives. The implementation of my language does
that under the hood.

   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/

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