MPI-Rockstar: a Hybrid MPI and OpenMP Parallel Implementation of the Rockstar Halo finder
Tomoyuki Tokuue, Tomoaki Ishiyama, Ken Osato, Satoshi Tanaka, Peter Behroozi · The Journal of Open Source Software · 2026
According to the concordance cosmological theory, structure formation and evolution proceeds hierarchically in the Universe.Smaller-scale dark matter structures gravitationally collapse first everywhere in the Universe, then merge into larger-scale structures.Such dense gravitationally bound structures of dark matter are called halos.Halos can host smaller halos, so-called subhalos (or substructures).Cosmological 𝑁-body simulations are vital in understanding the formation and evolution of halos and subhalos.Halo/subhalo finders are a post-processing step to identify those dense structures in the particle dataset of cosmological 𝑁-body simulations.MPI-Rockstar is a massively parallel halo finder based on the Rockstar phase-space temporal halo finder code (Behroozi et al., 2013), which is one of the most extensively used halo finding codes.Compared to the original code, parallelized by a primitive socket communication library, we parallelized it in a hybrid way using the Message Passing Interface (MPI) and OpenMP, which is suitable for analysis on the hybrid shared-and distributed-memory environments of modern supercomputers.This implementation can easily handle the analysis of more than a trillion particles on more than 100,000 parallel processes, enabling the production of a huge dataset for the next generation of cosmological surveys.