Exception-less system calls for event-driven servers

Event-driven architectures are currently a popular design choice for scalable, high-performance server applications. For this reason, operating systems have invested in efficiently supporting non-blocking and asynchronous I/O, as well as scalable event-based notification systems. We propose the use of exception-less system calls as the main operating system mechanism to construct high-performance event-driven server applications. Exceptionless system calls have four main advantages over traditional operating system support for event-driven programs: (1) any system call can be invoked asynchronously, even system calls that are not file descriptor based, (2) support in the operating system kernel is nonintrusive as code changes are not required for each system call, (3) processor efficiency is increased since mode switches are mostly avoided when issuing or executing asynchronous operations, and (4) enabling multi-core execution for event-driven programs is easier, given that a single user-mode execution context can generate enough requests to keep multiple processors/cores busy with kernel execution. We present libflexsc, an asynchronous system call and notification library suitable for building event-driven applications. Libflexsc makes use of exception-less system calls through our Linux kernel implementation, FlexSC. We describe the port of two popular event-driven servers, memcached and nginx, to libflexsc. We show that exception-less system calls increase the throughput of memcached by up to 35% and nginx by up to 120% as a result of improved processor efficiency.