Rethinking MC Scheduling Domain for Arm64 Servers - Dietmar Eggemann (OSPM26)

OSPM26 Summit - https://retis.sssup.it/ospm-summit/ Cambridge (UK) on April 14-16, 2026 Title: Rethinking MC Scheduling Domain for Arm64 Servers Speaker: Dietmar Eggemann Modern Arm64 servers increasingly use mesh-based interconnects that conflict with assumptions in the Linux Fair Scheduler's wakeup fast-path. Unlike typical x86 systems, many Arm64 platforms lack an LLC shared by a moderate subset of CPUs. As a result, the MC (LLC) scheduling domain is either missing, very small, or spans the entire machine. This mismatch degrades the effectiveness of the Fair scheduler's wakeup fast-path, including select_idle_sibling() → select_idle_cpu() and the SIS_UTIL mechanism, which relies on MC-domain utilization to bound the CPU search space. These optimizations were tuned for x86 servers with MC domains of roughly 25–45 CPUs and often fail to engage or scale poorly on modern Arm64 systems. We evaluate the hypothesis that wakeup performance depends less on LLC topology itself and more on the size of the CPU search domain. Through experiments on multiple Arm64 servers, we analyze wakeup latency, load balance, and throughput, and discuss introducing artificially sliced MC scheduling domains to restore fast-path effectiveness. Sponsored by: Arm - https://www.arm.com/ Scuola Superiore Sant'Anna (Pisa) - https://www.santannapisa.it/