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refclock: stop requiring 4 samples in median filter

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Reduce the minimum number of samples required by the filter from
min(4, length) to 1.

This makes the filtering less confusing. The sample lifetime is limited
to one poll and the default filtering of the SOCK refclock (where the
maximum number of samples per poll is unknown) is identical to the other
refclocks.

A concern with potential variability in number of samples per poll below
4 is switching between different calculations of dispersion in
combine_selected_samples() in samplefilt.c.

The 106-refclock test shows how the order of refclocks in the config can
impact the first filtered sample and selection. If the PPS refclock
follows SHM, a single low-quality PPS sample is accepted in the same
poll where SHM is selected and the initial clock correction started,
which causes larger skew later and delays the first selection of the PPS
refclock.
This commit is contained in:
Miroslav Lichvar
2024-11-05 16:03:40 +01:00
parent b9b338a8df
commit 12237bf283
3 changed files with 39 additions and 20 deletions
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15
doc/chrony.conf.adoc
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@@ -661,14 +661,13 @@ value is the estimated precision of the system clock.
Maximum allowed dispersion for filtered samples (in seconds). Samples with
larger estimated dispersion are ignored. By default, this limit is disabled.
*filter* _samples_:::
This option sets the length of the median filter which is used to reduce the
noise in the measurements. With each poll about 40 percent of the stored
samples are discarded and one final sample is calculated as an average of the
remaining samples. If the length is 4 or more, at least 4 samples have to be
collected between polls. For lengths below 4, the filter has to be full. The
default is 64. With drivers that perform their own polling (PPS, PHC, SHM), the
maximum value is adjusted to the number of driver polls per source poll, i.e.
2^(_poll_ - _dpoll_).
This option sets the maximum number of samples that can be stored in the median
filter between polls of the source. The filter combines about 60 percent of the
samples closest to the median offset into one sample. If more samples are
received by the driver between polls, the oldest samples will be dropped. One
sample per poll is sufficient for the source to be selectable. The default
is 64. Note that the PHC driver has additional filtering based on the reading
delay.
*prefer*:::
Prefer this source over other selectable sources without the *prefer* option.
*noselect*:::