PROCA: Programmable Probabilistic Processing Unit Architecture with Accept/Reject Prediction & Multicore Pipelining for Causal Inference

Causal inference is an important field in data science and cognitive artificial intelligence. It requires the construction of complex probabilistic models to describe the causal relationships between random variables. Probabilistic models rely on probabilistic programming as a flexible framework. However, the computing speed of probabilistic programming is often hindered by the extensive use of Markov chain Monte Carlo (MCMC) algorithms, even though they are powerful in Bayesian inference. To accelerate MCMC, this work presents PROCA, a programmable MCMC-based probabilistic processing unit architecture. PROCA exploits processing-in-memory function units to generate new samples of Markov chains. PROCA is programmable to execute the computation for arbitrary forms of posterior distribution formulas that software probabilistic programming frameworks support. We develop a novel accept/reject prediction methodology to accelerate the sequential MCMC computation, thereby introducing efficient multi-core pipelining methods. We implement and validate the PROCA architecture with commercial process development kits. The implementation is evaluated based on 9 representative benchmarks, covering PyMC official tutorial probabilistic problems, single-variable probabilistic problems, and real-world causal inference problems. Our comprehensive experiments demonstrate that PROCA achieves a speedup of 172~4871 $\times$ compared to Intel Xeon Gold CPU, $42 \sim 1058 \times$ compared to NVIDIA A100 GPU, and $1.765 \times$ over state-of-the-art MCMC accelerators, respectively. PROCA achieves comparable statistical robustness to the software probabilistic programming frameworks. Compared with state-of-the-art MCMC domain-specific accelerators, our design boosts the energy efficiency by $9.47 \times$.