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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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30
test/simulation/106-refclock
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@@ -89,9 +89,9 @@ Root delay : 0\.000000001 seconds
check_file_messages "20.* PPS1.*- N " 60 63 refclocks.log || test_fail
rm -f tmp/refclocks.log
min_sync_time=80
max_sync_time=180
chronyc_start=220
min_sync_time=180
max_sync_time=260
chronyc_start=270
client_conf="
refclock SHM 0 refid NMEA offset 0.35 delay 0.1
refclock PPS /dev/pps0
@@ -112,6 +112,30 @@ Root delay : 0\.000000001 seconds
check_file_messages "20.* PPS1.*[0-9] N " 800 960 refclocks.log || test_fail
check_file_messages "20.* PPS1.*- N " 50 63 refclocks.log || test_fail
rm -f tmp/refclocks.log
min_sync_time=80
max_sync_time=180
chronyc_start=220
# Swapped order of SHM and PPS impacts first accepted sample
client_conf="
refclock PPS /dev/pps0
refclock SHM 0 refid NMEA offset 0.35 delay 0.1
logdir tmp
log refclocks
maxupdateskew 10000"
run_test || test_fail
check_chronyd_exit || test_fail
check_source_selection || test_fail
check_sync || test_fail
check_chronyc_output "^Reference ID.*50505330 \(PPS0\)
Stratum.*: 1
.*
Root delay : 0\.000000001 seconds
.*$" || test_fail
check_file_messages "20.* PPS0.*[0-9] N " 800 960 refclocks.log || test_fail
check_file_messages "20.* PPS0.*- N " 50 63 refclocks.log || test_fail
fi
export CLKNETSIM_PHC_JITTER_OFF=$[2 * 25 * 492]