BeyondPlanck. III. Commander3

We describe the computational infrastructure for end-to-end Bayesian Cosmic Microwave Background (CMB) analysis implemented by the BeyondPlanck collaboration. The code is called Commander3. It provides a statistically consistent framework for global analysis of CMB and microwave observations and may be useful for a wide range of legacy, current, and future experiments. The paper has three main goals. Firstly, we provide a high-level overview of the existing code base, aiming to guide readers who wish to extend and adapt the code according to their own needs or re-implement it from scratch in a di erent programming language. Secondly, we discuss some critical computational challenges that arise within any global CMB analysis framework, for instance inmemory compression of time-ordered data, fast Fourier transform optimization, and parallelization and load-balancing. Thirdly, we quantify the CPU and RAM requirements for the current BeyondPlanck analysis, finding that a total of 1.5 TB of RAM is required for e cient analysis and that the total cost of a full Gibbs sample for LFI is 170 CPU-hrs, including both low-level processing and high-level component separation, which is well within the capabilities of current low-cost computing facilities. The existing code base is made publicly available under a GNU General Public Library (GPL) license.