Grolar: Efficient LLM Training on Heterogeneous Clusters

Large language models (LLMs) require vast amounts of GPU compute to train, but limited availability and high costs of GPUs make homogeneous clusters impractical for many organizations. Instead, assembling heterogeneous clusters by pooling together GPUs of different generations allows them to achieve higher aggregate compute and make use of all available GPUs. However, training on heterogeneous clusters presents significant challenges. The workload must be carefully partitioned such that GPUs in the cluster with limited compute, memory, or network bandwidth do not bottleneck the training process. Existing heterogeneous training systems cannot do so efficiently since they integrate data, pipeline, and tensor parallelism in a way that trades off communication for memory overhead. Combining vanilla data parallelism with pipeline parallelism is communication-efficient but results in high memory overhead from replicating model parameters. Alternatively, using sharded data parallelism or tensor parallelism reduces memory overhead but increases communication overhead when combined with pipeline parallelism. To address this problem, we designed Grolar, a system that uses Pipeline-Efficient ZeRO DP, a novel integration of pipeline parallelism and data parallelism that is both communication- and memory-efficient. Grolar uses a planner to automatically find an optimized training configuration from the vast search space of possibilities on heterogeneous clusters, and our evaluation shows that Grolar achieves up to 3× higher training throughput than state-of-the-art systems across representative heterogeneous training scenarios.