Treat the piece size as a weight, and perform weighted reservoir sampling as given in Algorithm A-Chao (https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Chao) Change-Id: I299d0026d9e02d03b3d2130b0f32192928e6e326
Change-Id: Icc522fd86538b8182a1b7d42c1588c32a257acaf