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netclientclock: Filter RTTs based on the median of the last RTTs before considering them at all

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This improves accuracy on wifi or similar networks, where the RTT can go very
high up for a single observation every now and then. Without filtering them
away completely, they would still still modify the average RTT, and thus all
clock estimations.
This commit is contained in:
Sebastian Dröge 2015-06-06 14:31:16 +02:00
parent 42b16c9e3c
commit f7a7a569cd
1 changed files with 47 additions and 0 deletions
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47
libs/gst/net/gstnetclientclock.c
View file

@ -60,6 +60,8 @@
#include <gio/gio.h>
#include <string.h>
GST_DEBUG_CATEGORY_STATIC (ncc_debug);
#define GST_CAT_DEFAULT (ncc_debug)
@ -74,6 +76,8 @@ GST_DEBUG_CATEGORY_STATIC (ncc_debug);
/* Maximum number of clock updates we can skip before updating */
#define MAX_SKIPPED_UPDATES 5
#define MEDIAN_PRE_FILTERING_WINDOW 9
enum
{
PROP_0,
@ -101,6 +105,8 @@ struct _GstNetClientClockPrivate
GstClockTime rtt_avg;
GstClockTime minimum_update_interval;
guint skipped_updates;
GstClockTime last_rtts[MEDIAN_PRE_FILTERING_WINDOW];
gint last_rtts_missing;
gchar *address;
gint port;
@ -195,6 +201,7 @@ gst_net_client_clock_init (GstNetClientClock * self)
priv->roundtrip_limit = DEFAULT_ROUNDTRIP_LIMIT;
priv->minimum_update_interval = DEFAULT_MINIMUM_UPDATE_INTERVAL;
priv->skipped_updates = 0;
priv->last_rtts_missing = MEDIAN_PRE_FILTERING_WINDOW;
}
static void
@ -307,6 +314,16 @@ gst_net_client_clock_get_property (GObject * object, guint prop_id,
}
}
static gint
compare_clock_time (const GstClockTime * a, const GstClockTime * b)
{
if (*a < *b)
return -1;
else if (*a > *b)
return 1;
return 0;
}
static void
gst_net_client_clock_observe_times (GstNetClientClock * self,
GstClockTime local_1, GstClockTime remote, GstClockTime local_2)
@ -327,6 +344,9 @@ gst_net_client_clock_observe_times (GstNetClientClock * self,
GstClockTime orig_internal_time, orig_external_time, orig_rate_num,
orig_rate_den;
GstClockTime max_discont;
GstClockTime last_rtts[MEDIAN_PRE_FILTERING_WINDOW];
GstClockTime median;
gint i;
GST_OBJECT_LOCK (self);
rtt_limit = self->priv->roundtrip_limit;
@ -351,6 +371,33 @@ gst_net_client_clock_observe_times (GstNetClientClock * self,
goto bogus_observation;
}
for (i = 1; i < MEDIAN_PRE_FILTERING_WINDOW; i++)
self->priv->last_rtts[i - 1] = self->priv->last_rtts[i];
self->priv->last_rtts[i - 1] = rtt;
if (self->priv->last_rtts_missing) {
self->priv->last_rtts_missing--;
} else {
memcpy (&last_rtts, &self->priv->last_rtts, sizeof (last_rtts));
g_qsort_with_data (&last_rtts,
MEDIAN_PRE_FILTERING_WINDOW, sizeof (GstClockTime),
(GCompareDataFunc) compare_clock_time, NULL);
median = last_rtts[MEDIAN_PRE_FILTERING_WINDOW / 2];
/* FIXME: We might want to use something else here, like only allowing
* things in the interquartile range, or also filtering away delays that
* are too small compared to the median. This here worked well enough
* in tests so far.
*/
if (rtt > 2 * median) {
GST_LOG_OBJECT (self,
"Dropping observation, long RTT %" GST_TIME_FORMAT " > 2 * median %"
GST_TIME_FORMAT, GST_TIME_ARGS (rtt), GST_TIME_ARGS (median));
goto bogus_observation;
}
}
/* Track an average round trip time, for a bit of smoothing */
/* Always update before discarding a sample, so genuine changes in
* the network get picked up, eventually */