Processes and operating systems. Metrics. Rate Monotonic Scheduling. Scheduling policies: Scheduling modeling assumptions. Interprocess communication.

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1 Processes and operating systems Scheduling policies: RMS; EDF. Scheduling modeling assumptions. Interprocess communication. Metrics How do we evaluate a scheduling policy: Ability to satisfy all deadlines. CPU utilization---percentage of time devoted to useful work. Scheduling overhead---time required to make scheduling decision and switch context. Rate Monotonic Scheduling RMS (Liu and Layland): widely-used, analyzable scheduling policy. Analysis is known as Rate Monotonic Analysis (RMA). 1

2 RMA model All process run on single CPU. Zero context switch time. No data dependencies between processes. Process execution time is constant. Deadline is at end of period. Highest-priority ready process runs. Process parameters T i is computation time of process i; τ i is period of process i, (i.e. how often the process is executed). period τ i Pi computation time T i Rate-monotonic analysis Response time: time required to finish process. Critical instant: scheduling state that gives worst response time. Critical instant occurs when all higherpriority processes are ready to execute. 2

3 Critical instant interfering processes critical instant P3 P4 P3 RMS priorities Optimal (fixed) priority assignment: shortest-period process gets highest priority; priority inversely proportional to period; break ties arbitrarily. No fixed-priority scheme does better. RMS example period period and both ready time 3

4 RMS CPU utilization Utilization for n processes is Σ i T i / τ i Schedulable Bound U <= n(2 1/n 1) RMS does not allow the system to use 100% of the CPU cycles. As number of tasks approaches infinity, maximum utilization approaches 69%. RMS CPU utilization, cont d. RMS cannot asymptotically guarantee use 100% of CPU, even with zero context switch overhead. Must keep idle cycles available to handle worstcase scenario. However, RMS guarantees all processes will always meet their deadlines. Predictable behavior during transient overload. RMS implementation Efficient implementation: scan processes; choose highest-priority active process. 4

5 Earliest-deadline-first scheduling EDF: dynamic priority scheduling scheme. Process closest to its deadline has highest priority. Requires recalculating processes at every timer interrupt. EDF example Earliest Deadline Only Deadline Two Two tasks tasks ready ready Two tasks ready has a period of 4. has a period of 5. EDF analysis EDF can use 100% of CPU. EDF is optimal dynamic priority algorithm. U <= 1 Not predictable during transient overload. 5

6 EDF implementation On each timer interrupt: compute time to deadline; choose process closest to deadline. Generally considered too expensive to use in practice. Fixing scheduling problems What if your set of processes is unschedulable? Change deadlines in requirements. Reduce execution times of processes. Get a faster CPU. Priority inversion Priority inversion: low-priority process keeps high-priority process from running. Improper use of system resources can cause scheduling problems: Low-priority process grabs I/O device. High-priority device needs I/O device, but can t get it until low-priority process is done. Can cause deadlock. 6

7 Solving priority inversion Give priorities to system resources. Have process inherit the priority of a resource that it requests. Low-priority process inherits priority of device if higher. Data dependencies Data dependencies allow us to improve utilization. Restrict combination of processes that can run simultaneously. and can t run simultaneously. Context-switching time Non-zero context switch time can push limits of a tight schedule. Hard to calculate effects---depends on order of context switches. (easy in RMA) In practice, OS context switch overhead is small. 7

8 What about sporadic tasks? Interrupts take time away from processes. Perform minimum work possible in the interrupt handler. OS intr OS P3 Device processing structure Interrupt service routine (ISR) performs minimal I/O. Get register values, put register values. Interrupt service process/thread performs most of device function. POSIX scheduling policies SCHED_FIFO: RMS SCHED_RR: round-robin within a priority level, processes are timesliced in round-robin fashion SCHED_OTHER: undefined scheduling policy used to mix non-real-time and realtime processes. 8

9 Interprocess communication OS provides interprocess communication mechanisms: various efficiencies; communication power. Signals A Unix mechanism for simple communication between processes. Analogous to an interrupt---forces execution of a process at a given location. But a signal is caused by one process with a function call. No data---can only pass type of signal. POSIX signals Must declare a signal handler for the process using sigaction(). Handler is called when signal is received. A signal can be sent with sigqueue(): sigqueue(destpid,sigrtmax-1,sval) 9

10 POSIX signal types SIGABRT: abort SIGTERM: terminate process SIGFPE: floating point exception SIGILL: illegal instruction SIGKILL: unavoidable process termination SIGUSR1, SIGUSR2: user defined Signals in UML More general than Unix signal---may carry arbitrary data: <<signal>> asig p : integer <<send>> someclass sigbehavior() POSIX shared memory POSIX supports counting semaphores with _POSIX_SEMAPHORES option. Semaphore with N resources will not block until N processes hold the semaphore. Semaphores are given name: /sem1 P() is sem_wait(), V() is sem_post(). 10

11 POSIX message-based communication Unix pipe supports messages between processes. Parent process uses pipe() to create a pipe. Pipe is created before child is created so that pipe ID can be passed to child. POSIX pipe example /* create the pipe */ if (pipe(pipe_ends) < 0) { perror( pipe ); break; } /* create the process */ childid = fork(); if (childid == 0) { /* child reads from pipe_ends[1] */ childargs[0] = pipe_ends[1]; execv( mychild,childargs); perror( execv ); exit(1); } else { /* parent writes to pipe_ends[0] */ } Evaluating performance May want to test: context switch time assumptions; scheduling policy. Can use OS simulator to exercise process set, trace system behavior. 11

12 Processes and caches Processes can cause additional caching problems. Even if individual processes are wellbehaved, processes may interfere with each other. Worst-case execution time with bad behavior is usually much worse than execution time with good cache behavior. 12

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