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systemclock: Use clock_nanosleep for higher accuracy

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The various wait implementation have a latency ranging from 50 to 500+
microseconds. While this is not a major issue when dealing with a low number of
waits per second (for ex: video), it does introduce a non-negligeable jitter for
synchronization of higher packet rate systems.

The `clock_nanosleep` syscall does offer a lower-latency waiting system but is
unfortunately blocking, so we don't want to use it in all scenarios nor for too
long.

This patch makes GstSystemClock use clock_nanosleep (if available) as such:
* Any wait below 500us uses it
* Any wait below 2ms will first use the regular waiting system and then
  clock_nanosleep

  #	modified:   gst/gstsystemclock.c

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/688>
This commit is contained in:
Edward Hervey 2020-10-30 16:58:52 +01:00 committed by Edward Hervey
parent b6df564251
commit 17feeb1bd6
2 changed files with 30 additions and 5 deletions
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34
gst/gstsystemclock.c
View file

@ -961,11 +961,34 @@ gst_system_clock_id_wait_jitter_unlocked (GstClock * clock,
while (TRUE) { while (TRUE) {
gboolean waitret; gboolean waitret;
/* now wait on the entry, it either times out or the cond is signalled. #ifdef HAVE_CLOCK_NANOSLEEP
* The status of the entry is BUSY only around the wait. */ if (diff <= 500 * GST_USECOND) {
waitret = /* In order to provide more accurate wait, we will use BLOCKING
GST_SYSTEM_CLOCK_ENTRY_WAIT_UNTIL ((GstClockEntryImpl *) entry, clock_nanosleep for any deadlines at or below 500us */
mono_ts * 1000 + diff); struct timespec end;
GST_TIME_TO_TIMESPEC (entryt, end);
GST_SYSTEM_CLOCK_ENTRY_UNLOCK ((GstClockEntryImpl *) entry);
waitret =
clock_nanosleep (CLOCK_MONOTONIC, TIMER_ABSTIME, &end, NULL) == 0;
GST_SYSTEM_CLOCK_ENTRY_LOCK ((GstClockEntryImpl *) entry);
} else {
if (diff < 2 * GST_MSECOND) {
/* For any deadline within 2ms, we first use the regular non-blocking
wait by reducing the diff accordingly */
diff -= 500 * GST_USECOND;
}
#endif
/* now wait on the entry, it either times out or the cond is signalled.
* The status of the entry is BUSY only around the wait. */
waitret =
GST_SYSTEM_CLOCK_ENTRY_WAIT_UNTIL ((GstClockEntryImpl *) entry,
mono_ts * 1000 + diff);
#ifdef HAVE_CLOCK_NANOSLEEP
}
#endif
/* get the new status, mark as DONE. We do this so that the unschedule /* get the new status, mark as DONE. We do this so that the unschedule
* function knows when we left the poll and doesn't need to wakeup the * function knows when we left the poll and doesn't need to wakeup the
@ -1004,6 +1027,7 @@ gst_system_clock_id_wait_jitter_unlocked (GstClock * clock,
/* reschedule if gst_cond_wait_until returned early or we have to reschedule after /* reschedule if gst_cond_wait_until returned early or we have to reschedule after
* an unlock*/ * an unlock*/
mono_ts = g_get_monotonic_time ();
now = gst_clock_get_time (clock); now = gst_clock_get_time (clock);
diff = GST_CLOCK_DIFF (now, entryt); diff = GST_CLOCK_DIFF (now, entryt);

1
meson.build
View file

@ -236,6 +236,7 @@ check_functions = [
'pselect', 'pselect',
'getpagesize', 'getpagesize',
'clock_gettime', 'clock_gettime',
'clock_nanosleep',
'strnlen', 'strnlen',
# These are needed by libcheck # These are needed by libcheck
'getline', 'getline',