backend_mintime
if (expect_false (waittime < backend_mintime))
waittime = backend_mintime;
if (sleeptime > waittime - backend_mintime)
sleeptime = waittime - backend_mintime;
backend_mintime = 1e-3;
backend_mintime = 1e-6;
VARx(ev_tstamp, backend_mintime) /* assumed typical timer resolution */
#define backend_mintime ((loop)->backend_mintime)