Reshaping Rollout Workloads for Asynchronous RL Post-Training on Heterogeneous Accelerators

Abstract

Reinforcement learning (RL) post-training increasingly relies on long-horizon, multi-turn rollouts. As post-training jobs outgrow a single cluster, rollout pools assembled across clusters introduce hardware heterogeneity. Rollout scheduling must serve two stakeholders: the hardware needs high aggregate decode throughput, while each trajectory needs to finish quickly. The tension arises from the memory-bandwidth-bound nature of autoregressive decoding. A large active batch amortizes weight reads for high throughput but leaves each trajectory a smaller bandwidth share and a longer completion time. The scheduling objective is therefore specialization, letting different workers serve different roles. Heterogeneous hardware further enables this specialization. High-bandwidth accelerators favor long-context work, while cost-efficient accelerators sustain large batches. Workload evolution makes this specialization difficult to sustain, and dynamic reassignment faces a circular dependency because a move's benefit depends on subsequent placement decisions. We present CadenceRL, which bypasses this dependency through structural workload reshaping rather than per-move benefit estimation. Pacing replaces long-context trajectories with shorter ones, providing a structurally positive transformation that sustains large active batches for high throughput. When accumulated staleness demands faster completion, concentration directs the residual long-context tail onto high-affinity workers. Late-bound KV preparation stages accumulated prefixes before a destination is selected. On heterogeneous rollout pools, CadenceRL improves decode throughput by up to 48% and reduces P95 trajectory latency by up to 64%. Adding high-bandwidth accelerators reduces tail latency, while adding cost-efficient accelerators increases throughput, without manual routing configuration.