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Multiprocessor and Real-Time Scheduling

Bina RamamurthyChapter 10CSE421

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Introduction

When a computer system contains more than a single processor, several new issues are introduced into the design of scheduling functions. We will examine these issues and the details of scheduling algorithms for tightly coupled multi-processor systems.

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Topics for discussionGranularity of computationDesign issues

Assignment of processes to processors Multiprogramming on individual processors Actual dispatching of a process

Real-time systems Characteristics Real-time scheduling Rate-monotonic scheduling (RMS)

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Granularity

The main purpose of having multiple processors is to realize____________________?

Applications exhibit parallelism at various levels. There are at least five categories of parallelism that differ in the degree of granularity.

Fine grain parallelism: Parallelism inherent in a single inst. stream. usually does not involve the OS but done at compilation stage. High data dependency ==> high frequency of synch. reqd.

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Granularity Medium grain: Potential parallelism of an application can

be implemented by multiple threads in a single process. High degree of coordination among the threads. Usually programmer specifies the parallelism in the design.

Coarse grain: Multiprocessing of concurrent processes in a multiprogramming environment :Example : Prj2.

Very coarse grain: When the synchronization needed is not so frequent the processes can be distributed among network nodes to form a single computing environment. Example: CORBA standard for distributed system

Independent parallelism: Multiple unrelated processes.

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Design Issues

Assignment of processes to processors.

Use of multiprogramming on individual processors.

Actual dispatching of a process.

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Process/processor Assignment

Master/slave assignment: Kernel functions always run on a particular processor. Other processors execute user processes. Advantage: Resource conflict resolution

simplified since single processor has control. Problem: Failure of master processor? Master

processor does the scheduling ==> bottleneck. Peer assignment: OS can execute on any

processor. Each processor does its own scheduling from the pool of available processes. This is similar to Solaris or NT symmetric multiprocessing (SMP).

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Multiprogramming at each processor

Completion time and other application-related performance metrics are much more important than processor utilization in multi-processor environment.

For example, a multi-threaded application may require all its threads be assigned to different processors for good performance.

Static or dynamic allocation of processes.

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Process dispatching

After assignment, deciding who is selected from among the pool of waiting processes --- process dispatching.

Single processor multiprogramming strategies may be counter-productive here.

Priorities and process history may not be sufficient.

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Process scheduling

Single queue of processes or if multiple priority is used, multiple priority queues, all feeding into a common pool of processors.

Multi-server queuing model: multiple-queue/single queue , multiple server system. Study the system for 1, 2, and n processors.

Inference: Specific scheduling policy does not have much effect as the processor number increases.

Conclusion: Use FCFS with priority levels.

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Thread scheduling

An application can be implemented as a set of threads that cooperate and execute concurrently in the same address space. Criteria: When related threads run in parallel perf. improves.

Load sharing: pool of threads, pool of processors. Gang scheduling: Bunch of related threads

scheduled together. Dedicated processor assignment: Each program

gets as many processors as there are parallel threads. Dynamic scheduling: More like demand scheduling.

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Real-time systems

Real-time computing is an important emerging discipline in CS and CE.

Control of lab experiments, robotics, process control, telecommunication etc.

It is a type of computing where correctness of the computation depends not only on the logical results but also on the time at which the results are produced.

Hard real-time systems: Must meet deadline. Ex: Space shuttle rendezvous with other space station.

Soft real-time system: Deadlines are there but not mandatory. Results are discarded if the deadline is not met.

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Characteristics of Real-Time (RT) systems

DeterminismResponsivenessUser controlReliabilityFail-soft operation

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Deterministic Response

External event and timings dictate the request of service.

OS’s response depends on the speed at which it can respond to interrupts and on whether the system has sufficient capacity to handle requests.

Determinism is concerned with how long the OS delays before acknowledging an interrupt.

In non-RT this delay may be in the order of 10’s and 100’s of millisecs, whereas in an RT it may have an upper-bound of few microsec to 1millisec.

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RT .. Responsiveness

Responsiveness is the time for servicing the interrupt once it has been acknowledged.

Comprises: Time to transfer control, (and context switch)

and execute the ISR Time to handle nested interrupts, the

interrupts that should be serviced when executing this ISR. Higher priority Interrupts.

response time = F(responsiveness, determinism)

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RT .. User Control

User control : User has a much broader control in RT-OS than in regular OS.

PriorityHard or soft deadlinesDeadlinesMemory Managemenmt: paging or swappingName the processes to be resident in memoryScheduling policies

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RT .. Reliability

Reliability: A processor failure in a non-RT may result in reduced level of service. But in an RT it may be catastrophic : life and death, financial loss, equipment damage.

Fail-soft operation: Ability of the system to fail in such a way preserve as much capability and data as possible.

In the event of a failure, immediate detection and correction is important.

Notify user processes to rollback. Apply compensation.

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Requirements of RT

Fast context switchMinimal functionality (small size) Ability to respond to interrupts quickly

(Special interrupts handlers)Multitasking with signals and alarmsSpecial storage to accumulate data

fastPreemptive scheduling

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Requirements of RT (contd.)

Priority levelsMinimizing interrupt disablesShort-term scheduler (“omni-potent”)Time monitorGoal: Complete all hard real-time tasks

by dead-line. Complete as many soft real-time tasks as possible by their deadline.

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Preemption and short-term scheduling

Fig.10.4

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RT scheduling

Static table-driven approach For periodic tasks. Input for analysis consists of : periodic

arrival time, execution time, ending time, priorities.

Inflexible to dynamic changes. General policy: earliest deadline first.

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RT scheduling

Static priority-driven preemptive scheduling For use with non-RT systems: Priority

based preemptive scheduling. Priority assignment based on real-time

constraints. Example: Rate monotonic algorithm

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RT scheduling

Dynamic planning-based scheduling After the task arrives before execution begins,

a schedule is prepared that includes the new as well as the existing tasks.

If the new one can go without affecting the existing schedules than nothing is revised.

Else schedules are revised to accommodate the new task.

Remember that sometimes new tasks may be rejected if deadlines cannot be met.

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RT scheduling

Dynamic best-effort scheduling: used in most commercial RTs of today tasks are aperiodic, no static scheduling

is possible some short-term scheduling such as

shortest deadline first is used. Until the task completes we do not

know whether it has met the deadline.

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Deadline scheduling

Characteristics considered Aperiodic tasks Ready time Starting deadline Completion deadline Processing time Resource requirements Priority

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Scheduling tasks with completion deadline

Fig. 10.5, Table 10.2

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Scheduling tasks with starting deadline

Fig. 10.6, Table 10.3

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Rate Monotonic Scheduling

Fig. 10.7, Fig. 10.8, Table 10.4

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Summary

We studied issues and algorithms inMulti-processor schedulingThread scheduling Real-time scheduling