A Comparison Study of Process Scheduling in FreeBSD, Linux and Win2k

1. A Comparison Study of Process Scheduling in FreeBSD, Linux and Win2k Introduction Comparison of scheduling based on some parameters On Uniprocessor system – most of it On Multiprocessor system Problems addressed by the schedulers

2. Introduction • Objectives • OSs analyzed • FreeBSD 4.4 – pre SMPng • Linux 2.4.18 – pre Preemtible kernel support • Windows 2000 • Convention • Unit of work scheduled is Process

3. Comparison of scheduling on an uniprocessor system • Responsibility • Win2k – Kernel Dispatcher, running in the context of user process • Linux, FreeBSD – Scheduler process • Process types considered • I/O and interactive processes • CPU bound • Real-time processes – Not in FreeBSD

4. 16 – 31 1 - 15 50 – 127 0 - 49 >= 1000 0 to 999 Comparison of scheduling on an uniprocessor system • Scheduling instances • How scheduler picks-up a process • Process priorities Win2k FreeBSD Linux Kernel level priorities Kernel level priorities Real-time priorities User level priorities User level priorities Non real-time priorities

5. Comparison of scheduling on an uniprocessor system • How scheduler picks-up a process • Win2k picks a process with higher priority to run or if there are multiple processes with same priority then it runs them one by one for a quantum each • Linux picks a process with higher Goodness value • FreeBSD picks a process with lowest priority value • Scheduler data structure • Win2k, FreeBSD uses multilevel priority queue • Linux uses linked list of runnable processes • Wait queue

6. Comparison of scheduling on an uniprocessor system • Quantum of process execution • Trade-off in quantum • Win2k - configurable but it’s 20ms for Professional and 120ms on uniprocessor server • FreeBSD - it’s 100ms • Linux • No quantum for real-time FIFO processes • Epoch • Priority and quantum are related • Quantum = Quantum/2 + priority, for next epoch • Low priority process will never get to run in Win2k and FreeBSD but not the case in Linux • Same priority processes share the quantum in FreeBSD and Linux but will for quantum each in Win2k

7. Comparison of scheduling on an uniprocessor system • Priority boosting • Win2k, priority boosting for I/O and interactive processes (1-15) • E.g. 1 for disk I/O, 6 – keyboard • FreeBSD, priority is boosted based on CPU usage • When a process is awakened in kernel level then it’s assigned a kernel level priority ( 0 – 49) • User level dynamic priority (50 – 127) is calculated by, Priority=50 + accumulated clock ticks on previous execution/4 + 2*nice Accumulated clocks ticks = (2*load)/(2*load+1) * Accumulated clock ticks + nice • If a process is waiting on I/O for more than a sec. Accumulated clock ticks = ((2*load)/(2*load+1))# of seconds waiting * Accumulated clock ticks • Linux, Priority + quantum left in previous epoch + 1

8. Scheduling analysis on FreeBSD

9. Scheduling analysis on Win2k

10. Scheduling analysis on Win2k

11. Scheduling analysis on Win2k

12. Multiprocessor scheduling & problems addressed • Comparison of multiprocessor scheduling • Win2k, Scheduler chooses a processor based on whether it ran the scheduled process before (cache reuse) or it’s the ideal processor of the process • Linux • Boosts priority by 15 if the process was last running on the processor • Scheduling based on Cache Flush Time • Problems addressed by the schedulers • Win2k – priority inversion • FreeBSD – thrashing, solved by high-level scheduler • Linux – Quantum sharing between parent and child processes during forking