Meta Research

Just-In-Time (JIT) compilation is often employed in Virtual Machines (VMs) to translate their virtual-machine languages into real-machine code. This approach not only brings portability, but it also enables aggressive compiler optimizations based on runtime behavior observed via profiling. The downside of JIT compilation, compared to Ahead-Of-Time native compilation, is that the profiling and compilation overheads are incurred during execution. To mitigate these overheads, previous work have proposed sharing either profile data or final JIT compiled code across VM executions. Unfortunately, these techniques have drawbacks, including steady-state performance degradation and difficulty of use. To address these issues, this paper presents the Jump-Start mechanism implemented inside the HipHop Virtual Machine (HHVM). Jump-Start is a practical approach to share VM profile data at a large scale, being used to power one of the largest websites in the world. In this paper, we argue for HHVM’s Jump-Start approach, describe it in detail, and present steady-state optimizations built on top of it. Running the Facebook website, we demonstrate that Jump-Start effectively solves the warmup problem in HHVM, reducing the server capacity loss during warmup by 54.9%, while also improving steady-state performance by 5.4%.