  Boson sampling, thought to be intractable classically, can be solved by a
quantum machine composed of merely generation, linear evolution and detection
of single photons. Such an analog quantum computer for this specific problem
provides a shortcut to boost the absolute computing power of quantum computers
to beat classical ones. However, the capacity bound of classical computers for
simulating boson sampling has not yet been identified. Here we simulate boson
sampling on the Tianhe-2 supercomputer which occupied the first place in the
world ranking six times from 2013 to 2016. We computed the permanent of the
largest matrix using up to 312,000 CPU cores of Tianhe-2, and inferred from the
current most efficient permanent-computing algorithms that an upper bound on
the performance of Tianhe-2 is one 50-photon sample per ~100 min. In addition,
we found a precision issue with one of two permanent-computing algorithms.
