Lazy Asynchronous I/O for Event-Driven Servers

We introduce Lazy Asynchronous I/O (LAIO), a new asynchronous I/O interface that is well suited to event-driven programming. LAIO is general in the sense that it applies to all blocking I/O operations. Furthermore, it is lazy in the sense that it creates a continuation only when an operation actually blocks, and it notifies the application only when a blocked operation completes in its entirety. These features make programming high-performance, event-driven servers using LAIO considerably easier than with previous interfaces.

We describe a user-level implementation of LAIO, relying only on kernel support for scheduler activations, a facility present in many Unix-like systems.

We compare the performance of web servers implemented using LAIO to the performance obtained with previous interfaces. For workloads with an appreciable amount of disk I/O, LAIO performs substantially better than the alternatives, because it avoids blocking entirely. In one such case, the peak throughput with LAIO is 24% higher than the next best alternative. For in-memory workloads it performs comparably.