  In particle-in-cell simulations, excessive or even unfeasible computational
demands can be caused by the growth of the number of particles in the course of
prolific ionization or cascaded pair production due to the effects of quantum
electrodynamics. Here we discuss how one can organize a dynamic rearrangement
of the ensemble to reduce the number of macroparticles, while maintaining
acceptable sampling of an arbitrary particle distribution. The approaches of
merging and thinning as well as their variants are discussed and the aspects of
use are considered.
