Window-Constrained Process Scheduling for Linux Systems

This paper describes our experience using Dynamic Window-Constrained Scheduling (DWCS) (13, 14,12,11) to schedule processes (and threads) on available CPUs in a Linux system. We describe the implementation of a kernel-loadable module that replaces the default Linux scheduler. Each process scheduled using DWCS has a request period of T time units and a 'window-constraint', x/y .T he end of one request period and the start of the next denotes a deadline by which time a process must be serviced for one quantum,of K time units. Under these constraints,DWCS attempts to guarantee that each process receives at least (yx)K units of CPU time every window of yT time units. That is,DWCS tries to guarantee that each process misses no more than x deadlines every window of y requests for service, and each time a process is serviced it receives K units of CPU time. DWCS can produce a feasible schedule under certain constraints,when the least upper bound on resource utilization (8) does not exceed 100% and each process is serviced in fixed size time slots (12). We show that DWCS is capable of successfully scheduling CPU- and I/O-bound processes in Linux more than 99% of the time,when a feasible schedule is theoretically possible. Unlike in the theoretical case,interrupt handling,context-switching,scheduling latency and unpredictable management of other resources besides the CPU affect the predictable scheduling of processes. We discuss several approaches that we are considering,to account for system overheads and provide predictable real-time scheduling to processes in Linux.