refclock: stop requiring 4 samples in median filter

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.

doc/chrony.conf.adoc

@@ -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
+This option sets the maximum number of samples that can be stored in the median
-noise in the measurements. With each poll about 40 percent of the stored
+filter between polls of the source. The filter combines about 60 percent of the
-samples are discarded and one final sample is calculated as an average of the
+samples closest to the median offset into one sample. If more samples are
-remaining samples. If the length is 4 or more, at least 4 samples have to be
+received by the driver between polls, the oldest samples will be dropped. One
-collected between polls. For lengths below 4, the filter has to be full. The
+sample per poll is sufficient for the source to be selectable. The default
-default is 64. With drivers that perform their own polling (PPS, PHC, SHM), the
+is 64. Note that the PHC driver has additional filtering based on the reading
-maximum value is adjusted to the number of driver polls per source poll, i.e.
+delay.
-2^(_poll_ - _dpoll_).
 *prefer*:::
 Prefer this source over other selectable sources without the *prefer* option.
 *noselect*:::

refclock.c

@@ -233,12 +233,10 @@ RCL_AddRefclock(RefclockParameters *params)
     if (inst->driver_poll > inst->poll)
       inst->driver_poll = inst->poll;
 
+    /* Adjust the filter length to save memory if the expected number
+       of samples is smaller */
     max_samples = 1 << (inst->poll - inst->driver_poll);
     if (max_samples < params->filter_length) {
-      if (max_samples < 4) {
-        LOG(LOGS_WARN, "Setting filter length for %s to %d",
-            UTI_RefidToString(inst->ref_id), max_samples);
-      }
       params->filter_length = max_samples;
     }
   }
@@ -246,11 +244,9 @@ RCL_AddRefclock(RefclockParameters *params)
   if (inst->driver->init && !inst->driver->init(inst))
     LOG_FATAL("refclock %s initialisation failed", params->driver_name);
 
-  /* Require the filter to have at least 4 samples to produce a filtered
+  /* Don't require more than one sample per poll and combine 60% of the
-     sample, or be full for shorter lengths, and combine 60% of samples
+     samples closest to the median offset */
-     closest to the median */
+  inst->filter = SPF_CreateInstance(1, params->filter_length, params->max_dispersion, 0.6);
-  inst->filter = SPF_CreateInstance(MIN(params->filter_length, 4), params->filter_length,
-                                    params->max_dispersion, 0.6);
 
   inst->source = SRC_CreateNewInstance(inst->ref_id, SRC_REFCLOCK, 0, params->sel_options,
                                        NULL, params->min_samples, params->max_samples,

test/simulation/106-refclock

@@ -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
+	min_sync_time=180
-	max_sync_time=180
+	max_sync_time=260
-	chronyc_start=220
+	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]