Multiprocessor and Real-Time Scheduling

Multiprocessor and Real-Time Scheduling Chapter 10

Classifications of Multiprocessor Systems • Loosely coupled multiprocessor • Each processor has its own memory and I/O channels • Functionally specialized processors • Such as I/O processor • Controlled by a master processor • Tightly coupled multiprocessing • Processors share main memory • Controlled by operating system

Scheduling and Synchronization • Scheduling concurrent processes has to take into account the synchronization of processes. • Scheduling decisions for one process may affect another process if the two processes are synchronized. • Synchronization granularity means the frequency of synchronization between processes in a system

Types of synchronization granularity • Fine– parallelism inherent in a single instruction stream • Medium– parallel processing or multitasking within a single application • Coarse– multiprocessing of concurrent processes in a multiprogramming environment • Very coarse– distributed processing across network nodes to form a single computing environment • Independent– multiple unrelated processes

Independent Parallelism • Separate application or job • No synchronization • Same service as a multiprogrammed uniprocessor • Example: time-sharing system

Coarse and Very Coarse-Grained Parallelism • Synchronization among processes at a very gross level (e.g. at the beginning and at the end) • Good for concurrent processes running on a multiprogrammed uniprocessor • Can by supported on a multiprocessor with little change

Medium-Grained Parallelism • Parallel processing or multitasking within a single application • Single application is a collection of threads • Threads usually interact frequently

Fine-Grained Parallelism • Highly parallel applications • Specialized and fragmented area

Schedulingdesign issues • Assignment of processes to processors • Use of multiprogramming on individual processors • Actual dispatching of a process • The scheduling depends on • degree of granularity • number of processors available

Issue #1Assignment of Processes to Processors • Treat processors as a pooled resource and assign process to processors on demand • Static or dynamic assignment? • Master/slave or peer architecture?

Static vs. dynamic assignment • Static: dedicate short-term queue for each processor • Advantages: Less overhead • Disadvantages: Processor could be idle while another processor has a backlog • Dynamic: use a global queue and schedule processes to any available processor

Type of architecture - master/slave or peer? • Master/slave architecture • Key kernel functions always run on a particular processor • Master is responsible for scheduling • Advantages: simple approach • Disadvantages • Failure of master brings down whole system • Master can become a performance bottleneck

Type of architecture - master/slave or peer? • Peer architecture • Operating system can execute on any processor • Each processor does self-scheduling • Disadvantages • Complicates the operating system • Make sure two processors do not choose the same process

Issue #2:Multiprogramming on a single processor • Depends on the synchronization granularity • A. Course-grained parallelism: • processor utilization considerations • Each individual processor should be able to switch among a number of processes • B. Medium-grained parallelism: • application performance considerations: • An application that consists of a number of threads may run poorly unless all of its threads are available to run simultaneously

Issue #3:Process dispatching • Uniprocessor systems: • sophisticated scheduling algorithms • Multiprocessor systems • processes: simple algorithms work best (see Fig.10.1) • threads: the main consideration is the synchronization between threads

Process Scheduling • Basic method used: • dynamic assignment • Single queue for all processes • Multiple queues are used for priorities • All queues feed to the common pool of processors • specific scheduling discipline is much less important with multiprocessorswith one processor (see Fig.10.1)

Threads • An application can be a set of threads that cooperate and execute concurrently in the same address space • Threads running on separate processors yield a dramatic gain in performance

Multiprocessor Thread Scheduling • Load sharing • Processes are not assigned to a particular processor • Gang scheduling • Simultaneous scheduling of threads that make up a single process

Multiprocessor Thread Scheduling • Dedicated processor assignment • Threads are assigned to a specific processor • Dynamic scheduling • Number of threads can be altered during course of execution

Load Sharing • One of the most commonly used schemes in current multiprocessors, despite the potential disadvantages • Load is distributed evenly across the processors • No centralized scheduler required • Use global queues

Versions of Load Sharing • First come first served (FCFS) • Smallest number of threads first: • priority queue, with highest priority given to threads from jobs with the smallest number of unscheduled threads. • Preemptive smallest number of threads first: • An arriving job with a smaller number of threads than an executing job will preempt threads belonging to the scheduled job.

Disadvantages of Load Sharing • The common queue needs mutual exclusion • May be a bottleneck when more than one processor looks for work at the same time • Preempted threads are unlikely to resume execution on the same processor • Cache use is less efficient • If all threads are in the global queue, all threads of a program will not gain access to the processors at the same time

Gang Scheduling • Simultaneous scheduling of threads that make up a single process • Useful for applications where performance severely degrades when any part of the application is not running • Threads often need to synchronize with each other

Gang scheduling • Processor allocation • N processors, M applications • each application - less than N threads • options: • each application is given 1/M of the available time on the N processors • the given time slice is proportional to the number of threads in each application

Scheduling Groups

Dedicated Processor Assignment • When application is scheduled, its threads are assigned to a set of processors, one-to-one, for the duration of the application. • Some processors may be idle • No multiprogramming of processors

Dedicated Processor Assignment • Motivation • In a highly parallel system processor utilization is no longer so important • Total avoidance of process switching

Dedicated Processor Assignment • Problem: • If the number of the active threads is greater than the number of the processors, there would be greater frequency of thread preemption and rescheduling

Dedicated Processor Assignment • Comparison with gang scheduling: • Similarities - threads are assigned to processors at the same time • Differences - in dedicated processor assignment threads do not change processors.

Gang scheduling and Dedicated Processor Assignment • More similar to memory assignment than to uniprocessor scheduling: • In memory scheduling, pages are assigned to processes • In Gang scheduling and dedicated processor assignment processors are assigned to processes.

Dynamic Scheduling • Both the operating system and the application are involved in making scheduling decisions. • The operating system is responsible for partitioning the processors among jobs. • Each job uses the processors currently in its partition to execute some subset of its runnable tasks by mapping these tasks to threads.

Dynamic Scheduling • On request for a processor • If there are idle processors, use them to satisfy the request. • Otherwise, if it is a new arrival, allocate it a single processor (by taking one away from any job currently allocated more than one processor.) • If any portion of the request cannot be satisfied, it remains outstanding

Upon release of one or more processors • Scan the current queue of unsatisfied requests for processors. • Assign a single processor to each job in the list that currently has no processors (i.e., to all waiting new arrivals). • Then scan the list again, allocating the rest of the processors on an FCFS basis.

Real-Time Systems • Correctness of the system depends not only on the logical result of the computation but also on the time at which the results are produced • Tasks or processes attempt to control or react to events that take place in the outside world • These events occur in “real time” and process must be able to keep up with them

Real-Time Systems • Control of laboratory experiments • Process control plants • Robotics • Air traffic control • Telecommunications • Military command and control systems

Types of Tasks • A. With respect to urgency • Hard real-time task • must meet its deadline • Soft real-time task • deadline is desirable but not mandatory

Types of Tasks • B. With respect to execution • Aperiodic task has a deadline by which it must finish or start, or both • Periodic task executes once per period T or exactly T units apart.

Characteristics of Real-Time Operating Systems • Areas of concern • Determinism • Responsiveness • User control • Reliability • Fail-soft operation

Characteristics of Real-Time Operating Systems • Determinism • Operations are performed at fixed, predetermined times or within predetermined time intervals • Concerned with how long the operating system delays before acknowledging an interrupt

Characteristics of Real-Time Operating Systems • Responsiveness • How long, after acknowledgment, it takes the operating system to service the interrupt • Includes amount of time to begin execution of the interrupt • Includes the amount of time to perform the interrupt

Characteristics of Real-Time Operating Systems • User control • User specifies priority • Specifies paging • What processes must always reside in main memory • Disks algorithms to use • Rights of processes

Characteristics of Real-Time Operating Systems • Reliability • Degradation of performance may have catastrophic consequences • fail-soft operation - the ability to fail in such a way as to preserve as much capability and data as possible • Attempt either to correct the problem or minimize its effects while continuing to run

Features of Real-Time Operating Systems • Small size (with its associated minimal functionality) • Fast process or thread switch • Ability to respond to external interrupts quickly • Use of special sequential files that can accumulate data at a fast rate

Features of Real-Time Operating Systems • Ability to respond to external interrupts quickly • Multitasking with interprocess communication tools such as semaphores, signals, and events • Preemptive scheduling based on priority • Minimization of intervals during which interrupts are disabled • Primitives to delay tasks for a fixed amount of time and to pause/resume tasks • Special alarms and time-outs

Real-Time Scheduling Approaches • When to dispatch • How to schedule • (1) whether a system performs schedulability analysis, • (2) if it does, whether it is done statically or dynamically, and • (3) whether the result of the analysis itself produces a schedule or plan according to which tasks are dispatched at run time. • Types of tasks periodic/aperiodic • deadline scheduling • rate monotonic scheduling

Scheduling of a Real-Time ProcessWhen to dispatch

Real-Time SchedulingHow to schedule • Static table-driven • Static priority-driven preemptive • Dynamic planning-based • Dynamic best effort

Multiprocessor and Real-Time Scheduling