Apple's Clutch Scheduler: Intra-Cluster Thread Scheduling Design for XNU Kernel

Summary

This document describes Apple's Clutch Scheduler, an intra-cluster thread scheduling design for the XNU kernel. It explains how the scheduler addresses the challenge of balancing latency-sensitive workloads (like UI interactions and multimedia) with starvation avoidance for lower priority batch workloads (like photo sync and source compilation). The traditional Mach scheduler's priority-based approach is contrasted with the Clutch Scheduler's design that groups threads into 'clutches' for more efficient CPU allocation across different workload types on Apple's heterogeneous processor architectures.