On 4/7/2012 5:49 AM, Victor Stinner wrote:
2012/4/7 Janzert<janz...@janzert.com>:
On 4/5/2012 6:32 AM, Victor Stinner wrote:
I prefer to use CLOCK_MONOTONIC, not because it is also available for
older Linux kernels, but because it is more reliable. Even if the
underlying clock source is unstable (unstable frequency), a delta of
two reads of the CLOCK_MONOTONIC clock is a result in *seconds*,
whereas CLOCK_MONOTONIC_RAW may use an unit a little bit bigger or
smaller than a second. time.monotonic() unit is the second, as written
in its documentation.
I believe the above is only true for sufficiently large time deltas. One of
the major purposes of NTP slewing is to give up some short term accuracy in
order to achieve long term accuracy (e.g. whenever the clock is found to be
ahead of real time it is purposefully ticked slower than real time).
I don't think that NTP works like that. NTP only uses very smooth adjustements:
""slewing": change the clock frequency to be slightly faster or slower
(which is done with adjtime()). Since the slew rate is limited to 0.5
ms/s, each second of adjustment requires an amortization interval of
2000 s. Thus, an adjustment of many seconds can take hours or days to
amortize."
http://www.python.org/dev/peps/pep-0418/#ntp-adjustment
Right, the description in that paragraph is exactly what I was referring
to above. :) It is unfortunate that a clock with a resolution of 1ns may
be purposefully thrown off by 500,000ns per second in the short term.
In practice you are probably correct that it is better to take the
slewed clock even though it may have purposeful short term inaccuracy
thrown in than it is to use the completely unadjusted one.
So for benchmarking it would not be surprising to be better off with the
non-adjusted clock. Ideally there would be a clock that was slewed "just
enough" to try and achieve short term accuracy, but I don't know of anything
providing that.
time.monotonic() is not written for benchmarks. It does not have the
highest frequecency, it's primary property is that is monotonic. A
side effect is that it is usually the steadiest clock.
For example, on Windows time.monotonic() has only an accuracy of 15 ms
(15 milliseconds not 15 microseconds).
Hmm, I just realized an unfortunate result of that. Since it means
time.monotonic() will be too coarse to be useful for frame rate level
timing on windows.
Janzert
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