Abstract: Recent studies have shown that if the overhead associated with process migration is not high when compared to the estimated lifetime of processes, migrating isolated processes to underutilized workstations can improve system performance. However, if tightly-coupled processes from groups of communicating processes or other existing dependencies are separated, system performance can decrease due to increased communication cost. Current load balancers do not make effective use of communication dependencies in their algorithms. We have implemented a communication-sensitive load balancer for DUNES-a user-level distributed operating system-that uses run-time communication pattern between processes when balancing load. We examine communication-sensitive load balancing under different workload conditions and compare its performance against a load balancer that is not communication-aware. Our results show that a communication-sensitive load balancer performs substantially better than a load balancer that is not communication-aware under workloads that have a "good-mix" of CPU-bound and I/O-bound processes.