Scheduling – part 2

Real-time Hard real-time – Mandatory response requirement Soft real-time – Requested response requirement Used in: – robotics, – aircraft control, – manufacturing

Other Schemes FCFS (FIFO) – single queue Round robin – equal time slice, but overhead for context switching – Time-sharing systems – Transaction processing Shortest Process Next (SPN) – non-preemptive – Exponential averaging to compute time left Shortest Remaining Time (SRT) – add-on to SPN Highest Response Ratio Next – Based on expected turnaround time – Accounts for aging

Queues & Lists Multiple lists/queues – Based on interactivity Highly interactive programs – Short quanta – Frequent access to CPU Moderately interactive – Longer quanta – Less frequent access to CPU Batch – Very long quanta – Very low access to CPU

Multiprocessor systems Multi-CPU vs. hyper-threading of cores Multi-CPU – Each CPU runs a separate process – Each CPU (if hyperthreaded) can run separate inst streams of ONE process Hyperthreading – Requires SMP support – Duplicates arch elements Control, MMU, status, general purpose, address, interrupt-mask & stack registers NOT the ALU

MP system considerations (continued) Load sharing Gang scheduling – groups of related threads scheduled on specific CPU’s Dedicated CPU assignment (1 CPU/thread) Dynamic scheduling - # threads varies, O/S manages thread placement

Development -1 MS-DOS – one process at a time, runs to completion Windows 3.1 – multi-programming (NO threads), non-preemptive (needed H/W) Windows 95 – added preemption (H/W available) Mac OS X (based on Mach from CMU) – Multi-level queue ( “bands”) Regular threads Elevated user threads KLT’s Real-time threads

Development -2 Linux – No multi-level feedback queue – Version differences 2.2 – added scheduling classes & real-time policies 2.4 – real-time scheduler + UNIX-based scheduler – O(n) scheduler – O(1) scheduler – Completely Fair Scheduler Symbian – 64 priority levels – Does not work on MP systems