Towards Swift Serverless LLM Cold Starts with ParaServe

Abstract: With the surge in number of LLMs, the industry turns to serverless computing for LLM inference serving. However, serverless LLM serving suffers from significant cold start latency and service level objective (SLO) violations due to the substantial model size, which leads to prolonged model fetching time from remote storage. We present ParaServe, a serverless LLM serving system that minimizes cold start latency through the novel use of pipeline parallelism. Our insight is that by distributing model parameters across multiple GPU servers, we can utilize their aggregated network bandwidth to concurrently fetch different parts of the model. ParaServe adopts a two-level hierarchical design. At the cluster level, ParaServe determines the optimal degree of parallelism based on user SLOs and carefully places GPU workers across servers to reduce network interference. At the worker level, ParaServe overlaps model fetching, loading, and runtime initialization to further accelerate cold starts. Additionally, ParaServe introduces pipeline consolidation, which merges parallel groups back to individual workers to maintain optimal performance for warm requests. Our comprehensive evaluations under diverse settings demonstrate that ParaServe reduces the cold start latency by up to 4.7x and improves SLO attainment by up to 1.74x compared to baselines.