Proportional Share Scheduling of Operating System Services for Real-Time Applications

While there is currently great interest in the problem of providing real-time services in general purpose operating systems, the issue of real-time scheduling of internal operating system activities has received relatively little attention. Without such real-time scheduling, the system is susceptible to conditions such as receive livelock — a situation in which an operating system spends all its time processing arriving network packets, and application processes, even if scheduled with a real-time scheduler, are starved. We investigate the problem of scheduling operating system activities such as network protocol processing in a proportional share manner. We describe a proportional share implementation of the FreeBSD operating system and demonstrate that it solves the receive livelock problem. Packets are processed within the operating system only at the cumulative rate at which the destination applications are prepared to receive them. If packets arrive at a faster rate then they are discarded after consuming minimal system resources. In this manner the performance of “well behaved” applications is unaffected by “misbehaving” applications. We demonstrate this effect by running a set of multimedia applications under a variety of network conditions on a set of increasingly sophisticated proportional share implementations of FreeBSD and comparing their performance. This work contributes to our knowledge of the engineering of proportional share real-time systems.