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sd-event: instrument sd_event_run() for profiling delays 

Set SD_EVENT_PROFILE_DELAYS to activate accounting and periodic logging
of the distribution of delays between sd_event_run() calls.

Time spent in dispatching as well as time spent outside of
sd_event_run() is measured and accounted for.  Every 5 seconds a
logarithmic histogram loop iteration delays since 5 seconds previous is
logged.

This is useful in identifying the frequency and magnitude of latencies
affecting the event loop, which should be kept to a minimum.
This commit is contained in:
Vito Caputo 2015-12-07 11:28:18 -08:00 committed by Vito Caputo
parent cf6c8c46fc
commit 34b8751774
1 changed files with 39 additions and 0 deletions
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39
src/libsystemd/sd-event/sd-event.c
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@ -225,6 +225,7 @@ struct sd_event {
bool exit_requested:1;
bool need_process_child:1;
bool watchdog:1;
bool profile_delays:1;
int exit_code;
@ -236,6 +237,9 @@ struct sd_event {
unsigned n_sources;
LIST_HEAD(sd_event_source, sources);
usec_t last_run, last_log;
unsigned delays[sizeof(usec_t) * 8];
};
static void source_disconnect(sd_event_source *s);
@ -444,6 +448,11 @@ _public_ int sd_event_new(sd_event** ret) {
goto fail;
}
if (secure_getenv("SD_EVENT_PROFILE_DELAYS")) {
log_info("Event loop profiling enabled. Logarithmic histogram of event loop iterations in the range 2^0 ... 2^63 us will be logged every 5s.");
e->profile_delays = true;
}
*ret = e;
return 0;
@ -2622,6 +2631,17 @@ _public_ int sd_event_dispatch(sd_event *e) {
return 1;
}
static void event_log_delays(sd_event *e) {
char b[ELEMENTSOF(e->delays) * DECIMAL_STR_MAX(unsigned) + 1];
int i, o;
for (i = o = 0; i < ELEMENTSOF(e->delays); i++) {
o += snprintf(&b[o], sizeof(b) - o, "%u ", e->delays[i]);
e->delays[i] = 0;
}
log_info("Event loop iterations: %.*s", o, b);
}
_public_ int sd_event_run(sd_event *e, uint64_t timeout) {
int r;
@ -2630,11 +2650,30 @@ _public_ int sd_event_run(sd_event *e, uint64_t timeout) {
assert_return(e->state != SD_EVENT_FINISHED, -ESTALE);
assert_return(e->state == SD_EVENT_INITIAL, -EBUSY);
if (e->profile_delays && e->last_run) {
usec_t this_run;
unsigned l;
this_run = now(CLOCK_MONOTONIC);
l = u64log2(this_run - e->last_run);
assert(l < sizeof(e->delays));
e->delays[l]++;
if (this_run - e->last_log >= 5*USEC_PER_SEC) {
event_log_delays(e);
e->last_log = this_run;
}
}
r = sd_event_prepare(e);
if (r == 0)
/* There was nothing? Then wait... */
r = sd_event_wait(e, timeout);
if (e->profile_delays)
e->last_run = now(CLOCK_MONOTONIC);
if (r > 0) {
/* There's something now, then let's dispatch it */
r = sd_event_dispatch(e);