It is generally believed that quantum computers can perform certain tasks faster than their classical counterparts. Identifying the resource that potentially enables this speedup is of particular interest in quantum information science. In this talk, by using the well-developed theory of phase-space quasiprobability distributions, I present two sufficient conditions for efficient classical simulation of quantum-optics experiments. These conditions show that the negativity of the phase-space quasiprobability distributions is an essential resource for quantum speedup. I will then discuss the applications of this formalism to boson-sampling experiments, and show that above some thresholds for loss and noise, the experiments are classically simulatable.