MODERN OPERATING SYSTEMS Third Edition ….ppt

This offends the religious zealots because it violates layering Printer daemon prints processes and spooler directory contains files Run jobs according to their priority Can be static or can do it dynamically Typically combine RR with priority. Each priority class uses RR inside Priority Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Priority Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Highest priority gets one quantum, second highest gets 2….. If highest finishes during quantum, great. Otherwise bump it to second highest priority and so on into the night Consequently, shortest (high priority) jobs get out of town first They announce themselves-no previous knowledge assumed! Multiple Queues with Priority Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Cool idea if you know the remaining times exponential smoothing can be used to estimate a jobs’ run time aT0 + (1-a)T1 where T0 and T1 are successive runs of the same job Shortest Process Next Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Hold lottery for cpu time several times a second Can enforce priorities by allowing more tickets for “more important” processes Lottery Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Hard real time vs soft real time Hard: robot control in a factory Soft: CD player Events can be periodic or aperiodic Algorithms can be static (know run times in advance) or dynamic (run time decisions) Real Time Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639 Kernel picks process (left) Kernel picks thread (right) Thread Scheduling Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All r