+static usec_t sleep_between(sd_event *e, usec_t a, usec_t b) {
+ usec_t c;
+ assert(e);
+ assert(a <= b);
+
+ if (a <= 0)
+ return 0;
+
+ if (b <= a + 1)
+ return a;
+
+ /*
+ Find a good time to wake up again between times a and b. We
+ have two goals here:
+
+ a) We want to wake up as seldom as possible, hence prefer
+ later times over earlier times.
+
+ b) But if we have to wake up, then let's make sure to
+ dispatch as much as possible on the entire system.
+
+ We implement this by waking up everywhere at the same time
+ within any given second if we can, synchronised via the
+ perturbation value determined from the boot ID. If we can't,
+ then we try to find the same spot in every a 250ms
+ step. Otherwise, we pick the last possible time to wake up.
+ */
+
+ c = (b / USEC_PER_SEC) * USEC_PER_SEC + e->perturb;
+ if (c >= b) {
+ if (_unlikely_(c < USEC_PER_SEC))
+ return b;
+
+ c -= USEC_PER_SEC;
+ }
+
+ if (c >= a)
+ return c;
+
+ c = (b / (USEC_PER_MSEC*250)) * (USEC_PER_MSEC*250) + (e->perturb % (USEC_PER_MSEC*250));
+ if (c >= b) {
+ if (_unlikely_(c < USEC_PER_MSEC*250))
+ return b;
+
+ c -= USEC_PER_MSEC*250;
+ }
+
+ if (c >= a)
+ return c;
+
+ return b;
+}
+