Lukewarm serverless functions: characterization and optimization

Serverless computing has emerged as a widely-used paradigm for running services in the cloud. In serverless, developers organize their applications as a set of functions, which are invoked on-demand in response to events, such as an HTTP request. To avoid long start-up delays of launching a new function instance, cloud providers tend to keep recently-triggered instances idle (or warm) for some time after the most recent invocation in anticipation of future invocations. Thus, at any given moment on a server, there may be thousands of warm instances of various functions whose executions are interleaved in time based on incoming invocations. This paper observes that (1) there is a high degree of interleaving among warm instances on a given server; (2) the individual warm functions are invoked relatively infrequently, often at the granularity of seconds or minutes; and (3) many function invocations complete within a few milliseconds. Interleaved execution of rarely invoked functions on a server leads to thrashing of each function's microarchitectural state between invocations. Meanwhile, the short execution time of a function impedes amortization of the warm-up latency of the cache hierarchy, causing a 31--114% increase in CPI compared to execution with warm microarchitectural state. We identify on-chip misses for instructions as a major contributor to the performance loss. In response we propose Jukebox, a record-and-replay instruction prefetcher specifically designed for reducing the start-up latency of warm function instances. Jukebox requires just 32KB of metadata per function instance and boosts performance by an average of 18.7% for a wide range of functions, which translates into a corresponding throughput improvement.