KVM Forum 2025

Modern processors are increasingly adopting chiplet-based architectures that distribute CPU cores across multiple chiplets, each containing one or more core complexes (CCX) with a shared last-level cache. Inter-chiplet communication penalties can significantly degrade workload performance. While the current Linux scheduler has NUMA awareness and NUCA (non-uniform cache access) awareness through its scheduling domain hierarchy, it lacks adequate consideration for the significantly higher inter-CCX communication penalties inherent in chiplet architectures. This leads to suboptimal VM placement and therefore, degraded performance.

This talk proposes an enhanced VM scheduling framework designed for chiplet-based processors.
1. The framework utilizes lightweight monitoring techniques, such as hardware performance counters, to monitor cache efficiency, memory access patterns and inter-chiplet communication metrics. These insights help formulate informed policies for VM placement and vCPU group migrations.
2. The framework implements intelligent VM vCPU group placement strategies that optimize the initial allocation of vCPU groups and their associated contexts, such as vhost and other datapath threads, across chiplet boundaries. The algorithm balances maximizing chiplet locality while minimizing intra-cache contention. The algorithm also studies the dynamic behaviour of VM placements in overloaded cases.
3. These performance measurements guide the assignment of optimal virtual topologies to guests, improving performance through chiplet-locality-aware decisions starting at the guest scheduler level.