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Scheduling For Distributed Systems

Classification – degree of couplingClassification – granularity

Local vs centralized schedulingMethods

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Motivation

Additional computing capability can be added more easily by adding than enlarging processors

Users and tasks are distributed, but in different ways

Many applications and languages are now threaded

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Classification – degree of coupling

DegreeOf coupling

Description

Loosely coupled

Autonomous systems with individual memory and I/O – networked systems

Functionally specialized

Separate specialized processors for I/O, networking, DSP, etc.

Tightly coupled

I/O and usually main memory are in common, CPU’s usually have semiprivate caches

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Classification – granularity

Grain size DescriptionSynch interva

l

fine Single instruction stream – domain of processor and compiler design

<20

medium Parallel processing or multiprocessing within application

20-200

coarse Multiprocessing of applications inside an operating system

200-2000

very coarse Distributed processing across network 2000-1M

independent

Multiple unrelated processes ∞

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What scheduling involves

Assigning processes to processorsstatic assignment – process runs there until completion

Common queue – time slot is the unit of assignment

Peer architecture – each processor selects process to run

Using multiprogramming on individual processorsPrimary issue is how threads are handled

Goal is best performance rather than 100% utilization

Actual dispatching of a processCoefficient of variance idea – variability of service times

Simpler scheduling can work for multiprocessors without causing bottlenecks

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Thread scheduling

Load sharingGlobal queue of ready threads – this is not load balancing

Gang schedulingRelated threads are scheduled to run on a set of processors at the

same time

Dedicated processor assignmentApplication is alloted a processor per thread until done

Dedication of processors reduces thread-switching overhead

Dynamic schedulingNumber of threads in a process can change while it runs

Scheduler allots processors on a demand basis (it can take them away)

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Real-time Scheduling

Classification – hard vs soft real-timeClassification – aperiodic/periodic

Responsiveness requirementsReliability requirements

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Real-time OS characteristics

Characteristic Comments

Determinism Predictable response time to event acknowledgement – latency much faster than non-RT OS

Responsiveness Predictable service time (after acknowledgement)

User control User needs to be able to assign task priorities

Reliability Governed by needs of system under control – can be life-threatening

Fail-soft operation Useful behavior (partial functioning) even after failure – crash survival

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RT scheduling approaches

Approach Comments

Static - table-driven Static analysis of tasks before run-time – result is schedule

Static – priority-driven

Static analysis determines priority table – result is priority list for standard scheduler

Dynamic - planning Feasilility is determined at run time – table-driven, arriving tasks are accepted only if they can be done

Dynamic – best effort

System tries to meet all deadlines – aborts those whose deadline has expired

* Tacit assumption – tasks are identified with deadlines and durations

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Deadline scheduling – explicitly

Deadline type Description

Starting When it needs to begin

Completion When it needs to finish

Processing time How long it takes

Resource requirements Resources needed (other than processor)

Priority Relative importance (hard RT tasks have priority over soft)

Subtask structure May have mandatory and optional parts

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Rate monotonic scheduling (RMS)

Basic ideaSuboptimal algorithm

High arrival rate = high priority

Empirical observationsRMS is almost as good as earliest-deadline scheduling

Soft-RT parts of tasks can be scheduled to use the CPU time not used by the hard-RT work

Stability is better than earliest-deadline

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UNIX SVR4 scheduling

Single priority queueThree priority classes

Real-time – 100-159

Kernel – 60-99

Time-shared – 0-59

Each level is actually a priority queue

Preemption is done when a high-priority RT process arrives

Time-sharing classPenalty box – priority is reduced if process uses all its time

Time allowed varies from10 ms – priority 59

100 ms – priority 0

Observations – system has much adaptability

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Linux RT scheduling

Basic approachLike non-RT Linux – except RT processes have highest priority

Three task queues SCHED_FIFO

SCHED_RR – also has time quota

SCHED_OTHER – executes only if no RT is waiting

Multiple priorities within each class

This description differs from present methods Continuous revision area of Linux

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Windows 2K Scheduling

Priority-driven preemptive scheduler2 bands

Real-timeAll threads have fixed priority

All threads in a given level are in a RR queue

16 levels (31-16)

OtherAll threads have variable priority

FIFO at each level – behavior-based priority modification

16 levels (15-0)

Single vs multiple processorSingle – highest priority thread

Multiple – n-1 highest priority threads each get own processor

Last processor is shared by all others