sources: add configurable limit for jitter

The maxjitter directive sets the maximum allowed jitter of the sources
to not be rejected by the source selection algorithm. This prevents
synchronisation with sources that have a small root distance, but their
time is too variable. By default, the maximum jitter is 1 second.

diff --git a/conf.c b/conf.c
index 384fc56bca2aec0fafff9eef0c78f99d14a11678..be3a189d5cf15b5983e02561d55491412b4cab9a 100644 (file)
--- a/conf.c
+++ b/conf.c
@@ -90,6 +90,7 @@ static double max_drift = 500000.0; /* in ppm */
 static double max_slew_rate = 1e6 / 12.0; /* in ppm */
 
 static double max_distance = 3.0;
+static double max_jitter = 1.0;
 static double reselect_distance = 1e-4;
 static double stratum_weight = 1e-3;
 static double combine_limit = 3.0;
@@ -510,6 +511,8 @@ CNF_ParseLine(const char *filename, int number, char *line)
     parse_double(p, &max_distance);
   } else if (!strcasecmp(command, "maxdrift")) {
     parse_double(p, &max_drift);
+  } else if (!strcasecmp(command, "maxjitter")) {
+    parse_double(p, &max_jitter);
   } else if (!strcasecmp(command, "maxsamples")) {
     parse_int(p, &max_samples);
   } else if (!strcasecmp(command, "maxslewrate")) {
@@ -1551,6 +1554,14 @@ CNF_GetMaxDistance(void)
 
 /* ================================================== */
 
+double
+CNF_GetMaxJitter(void)
+{
+  return max_jitter;
+}
+
+/* ================================================== */
+
 double
 CNF_GetReselectDistance(void)
 {

diff --git a/conf.h b/conf.h
index 13e58b595e297c6e1803175df8fb1e8c7d836a28..d24166503c18ca20e5659a10f480bf22e16bfbf8 100644 (file)
--- a/conf.h
+++ b/conf.h
@@ -90,6 +90,7 @@ extern double CNF_GetCorrectionTimeRatio(void);
 extern double CNF_GetMaxSlewRate(void);
 
 extern double CNF_GetMaxDistance(void);
+extern double CNF_GetMaxJitter(void);
 extern double CNF_GetReselectDistance(void);
 extern double CNF_GetStratumWeight(void);
 extern double CNF_GetCombineLimit(void);

diff --git a/doc/chrony.conf.adoc b/doc/chrony.conf.adoc
index 30c77c9a5981e6722133ed48a64ccea6d8619b16..123666ad0612f04477a4d2ad63e776a114f98d2b 100644 (file)
--- a/doc/chrony.conf.adoc
+++ b/doc/chrony.conf.adoc
@@ -600,6 +600,13 @@ Setting *maxdistance* to a larger value can be useful to allow synchronisation
 with a server that only has a very infrequent connection to its sources and can
 accumulate a large dispersion between updates of its clock.
 
+[[maxjitter]]*maxjitter* _jitter_::
+The *maxjitter* directive sets the maximum allowed jitter of the sources to not
+be rejected by the source selection algorithm. This prevents synchronisation
+with sources that have a small root distance, but their time is too variable.
++
+By default, the maximum jitter is 1 second.
+
 [[minsources]]*minsources* _sources_::
 The *minsources* directive sets the minimum number of sources that need to be
 considered as selectable in the source selection algorithm before the local

diff --git a/sources.c b/sources.c
index 55e6a8221a8133aec20d574b40c52d7a15a1fe0d..5c9bbe3d80e08497c28c3db21da3de2b8b30cb2b 100644 (file)
--- a/sources.c
+++ b/sources.c
@@ -71,12 +71,12 @@ typedef enum {
   SRC_UNSELECTABLE,     /* Has noselect option set */
   SRC_BAD_STATS,        /* Doesn't have valid stats data */
   SRC_BAD_DISTANCE,     /* Has root distance longer than allowed maximum */
+  SRC_JITTERY,          /* Had std dev larger than allowed maximum */
   SRC_WAITS_STATS,      /* Others have bad stats, selection postponed */
   SRC_STALE,            /* Has older samples than others */
   SRC_ORPHAN,           /* Has stratum equal or larger than orphan stratum */
   SRC_UNTRUSTED,        /* Overlaps trusted sources */
   SRC_FALSETICKER,      /* Doesn't agree with others */
-  SRC_JITTERY,          /* Scatter worse than other's dispersion (not used) */
   SRC_WAITS_SOURCES,    /* Not enough sources, selection postponed */
   SRC_NONPREFERRED,     /* Others have prefer option */
   SRC_WAITS_UPDATE,     /* No updates, selection postponed */
@@ -159,6 +159,7 @@ static int selected_source_index; /* Which source index is currently
 #define DISTANT_PENALTY 32
 
 static double max_distance;
+static double max_jitter;
 static double reselect_distance;
 static double stratum_weight;
 static double combine_limit;
@@ -184,6 +185,7 @@ void SRC_Initialise(void) {
   max_n_sources = 0;
   selected_source_index = INVALID_SOURCE;
   max_distance = CNF_GetMaxDistance();
+  max_jitter = CNF_GetMaxJitter();
   reselect_distance = CNF_GetReselectDistance();
   stratum_weight = CNF_GetStratumWeight();
   combine_limit = CNF_GetCombineLimit();
@@ -665,6 +667,12 @@ SRC_SelectSource(SRC_Instance updated_inst)
       continue;
     }
 
+    /* And the same applies for the estimated standard deviation */
+    if (si->std_dev > max_jitter) {
+      sources[i]->status = SRC_JITTERY;
+      continue;
+    }
+
     sources[i]->status = SRC_OK; /* For now */
 
     if (sources[i]->reachability && max_reach_sample_ago < first_sample_ago)