  We study the distribution of switching times in spin-transfer switching
induced by sub-ns current pulses in pillar-shaped spin-valves. The pulse
durations leading to switching follow a comb-like distribution, multiply-peaked
at a few most probable, regularly spaced switching durations. These durations
reflect the precessional nature of the switching, which occurs through a
fluctuating integer number of precession cycles. This can be modeled
considering the thermal variance of the initial magnetization orientations and
the occurrence of vanishing total torque in the possible magnetization
trajectories. Biasing the spin-valve with a hard axis field prevents some of
these occurrences, and can provide an almost perfect reproducibility of the
switching duration.
