Traffic Shaping System Calls Using Threading by Appointment

Threading by Appointment (TAP) is a concurrent programming model that combines automatic stack management (thread-based) with system call queueing (event-driven). However, unlike conventional threads, TAP threads invoke system calls by appointment only, and, unlike events, appointments have a duration, which is determined by the TAP runtime system transparently through a POSIX-compliant interface. The TAP mechanism essentially implements system call queueing using a given TAP policy that consists of a strategy to make appointments, i.e., enqueue system calls, and a logical clock to begin and end appointments, i.e., dequeue system calls. We propose a TAP policy that resembles traffic shaping in network routers, where system calls are treated as network packets. The policy distinguishes system calls for network and disk I/O, and gives priority to system calls invoked by short-running, interactive threads rather than long-running, bulk threads. We have implemented a high-performance, user-space TAP library and benchmarked the library using a multi-threaded, POSIX-compliant web server.