o perating s ystems and a rchitectures cs-m98 p art 7 s cheduling benjamin mora 1 swansea university...

OPERATING SYSTEMS AND ARCHITECTURES CS-M98 PART 7 SCHEDULING

Benjamin Mora 1Swansea University

Dr. Benjamin Mora

CONTENT

Main Principles behind Scheduling.

Scheduling Algorithms.

Comparison of Several Algorithms.

2Benjamin MoraSwansea University

MAIN PRINCIPLES


REMINDER: PROCESS STATES


SCHEDULING Scheduling consists of determining which processes are to

be run by the processor.Mainly for multitasked environments.Mainly Pre-emptive.

The scheduler is the part of the OS managing scheduling. The scheduling algorithm is the algorithm used by the

scheduler. Process/Context Switching consists of switching between

tasks.Consume resources


WHEN SCHEDULING When a process blocks (e.g. on I/O, a semaphore, or

voluntarily). When a process terminates. When a new process is created. When an interrupt occurs

e.g. blocking I/O operation now ready, so might want to activate waiting process.

End of a quantumcurrent process has had its time slot, OS will preempt it and run

another process. Note that some operating systems and some scheduling algorithms are non-preemptive, so this never happens.


DECISION MODE Non-Multitask (Non-preemptive)

Processes will be run sequentially.

Co-operative (Non-preemptive)The process blocks itself when needed (termination, I/O, OS

request).

PreemptiveThe OS interrupts Processes. The process can be interrupted by the OS.When quantum time lapsed.More overhead.More interactive.


DIFFERENCES BETWEEN PROCESSES Different processes: Different goals!

OS must have a good strategy.Adaptive mechanism.

CPU-bound processes.

Mainly computations, only a few I/O operations.More time can be allocated to these processes.

I/O-bound processes.

Most time waiting for events.


SCHEDULING ALGORITHM TYPE Different Operating Systems, different goals! Interactive Environment:

Interact with the user. Personal computers.

Preemptive scheduling.Event programming.

Batch Environment:Send jobs to the system.

Data centers, supercomputers, etc…Global performances improved.Can be cooperative, preemptive with long delays.


SUB-SCHEDULERS Long-Term Scheduler

Not Frequently Used, only on coarse-grained programs.Decides which programs to add to the pool.

Mid-Term Scheduler

For virtual memory management.

Short-Term Scheduler

Often called, makes fast decisions.Favor short tasks (real-time programs).


SCHEDULING ALGORITHM TYPEReal-Time Operating Systems:

Ensuring/Meeting/Predicting deadlines.Upper bound execution time.Required efficient programming methodologies!

Planes, nuclear power plants, consoles, multimedia systems, etc…

Process environment better known from the start.


SCHEDULING ALGORITHM TYPE Hard Real-Time. Deadlines must be met!

System failing otherwise Deterministic and reliable.

Though immediate detection & recovery must be implemented. Paging, Swapping & dynamic memory allocations usually not possible.

Proven/normalized/deterministic compilers. Interrupts minimized.

Planes, power plants, critical systems. Worst Case Executation Time must be known!

Soft Real-Time. Deadlines should be met! Relaxed constraints. Priority given to real-time processes. Results sometimes discarded if not on time. Multimedia systems. Consoles. Phones.


SCHEDULING ALGORITHM TYPEFirm Real-Time. Intermediate between soft and

hard

Deadlines should be met. Any failure will degrade performance. “A few” misses could be accepted.

Results are invalid if obtained after deadline! The opportunity is lost

Interactive systems E.g, RealTime financial systems. High-Frequency Trading

Loss of money


SCHEDULING OBJECTIVESFairness.

Make sure each process gets its fair share of the CPU, and no process suffers indefinite postponement.Priority settings possible.Balance resource usage.

Efficiency. Keep the CPU and the other parts of the system as busy

as possible.Reduced overhead.


SCHEDULING OBJECTIVESBatch Systems:

Maximize throughput/jobs per hour. Turnaround time. Minimize time between submission

and termination.Maximize CPU utilization.

Interactive systems:Minimize response time.Increase productivity.

Real-Time Systems:Quite specific to the context.


SCHEDULING ALGORITHMS


SCHEDULING ALGORITHMSFirst come, first served.Round Robin.Priority scheduling.Shortest process next.Shortest remaining time & Highest Response Ratio

Next.Multilevel Feedback queues.Early Unix Priority Scheduling.Fair Share Scheduling.


FIRST COME, FIRST SERVED FIFO list of ready processes. When a process terminates its execution, the next process

in the queue is executed. Nonpreemptive. High response time. Minimum overhead. Penalize short processes. No starvation.


ROUND ROBIN (TIME SLICING) FIFO list. Widely used.

SimpleFairpreemptive.minimum overhead.no starvation.good response time for short processes

A quantum of time is defined.Must be carefully chosen. Depends on the processor type.


P1

P2

P3

P4

FIFO List10 ms

10 ms

10 ms

10 ms

10 ms

10 ms

10 ms

PRIORITY SCHEDULINGAdding priorities to processes is sometimes

required.Highest priority processes are run.Several priority queues have to be handled.Round-Robin can be performed inside each queue.

May lead to starvation.The OS has to detect starvation.Priority may be decreased.


SHORTEST PROCESS NEXT Select the expected shortest process.

High throughput.High overhead.Better for short processes.

Non-preemptive.

Requires estimating the amount of time required by each process.Can be estimated by the user for batch jobs.Can be done based on statistics acquired from the running

processes.


RUNNING TIME PREDICTION

WithSn: Predicted evaluation of running time for the nth time the

process uses the CPU.Ti: Time used for the ith execution of the process.

To give more importance to the recent executions of the process, one can use:


nn

n

inn Sn

nTn

TS11

1

11

nnn STS )1(1

SHORTEST REMAINING TIME, HIGHEST RESPONSE RATIO NEXT

Based on the same idea as Short Process Next.

Shortest Remaining Time:Preemptive version of Short Process Next.

Highest Response Ratio Next:Non-Preemptive.Minimization of the normalized turn around time.Minimize starvation.Priority =1+ (waiting_time/estimated_run_time)


MULTILEVEL FEEDBACK QUEUES


MULTILEVEL FEEDBACK QUEUESBased on multi-level queues.

Lowest levels have increased priorities, but also shortest quantum times.

A new process enters the lowest level queue and may preempt the current running process.

All levels but the last one (round-robin) are usually first come, first served.


MULTILEVEL FEEDBACK QUEUESMain characteristics:

Preemptive.Throughput and response times OK, although not best.Possibly high overhead.May favor I/O bound processes.Possible starvation.


EARLY UNIX PRIORITY SCHEDULINGEvery active process has a scheduling priority.

The CPU is given to the process having the highest priority The lowest priority value in implementation!

User process priorities change every quantum.Cannot go beyond a specific priority.Not for super-user processes.


EARLY UNIX PRIORITY SCHEDULING Each process control block contains:

the CPU usage of the process. (cpu)a scheduling priority. (prio)

Rules: If a process does not use its entire time slice, the algorithm will

tend to run sooner.At every clock interupt, the CPU usage is incremented.At the end of every quantum (60 time units in example):

cpu = cpu / 2prio = cpu / 2 + BASE_LEVEL_PRIORITY;


EARLY UNIX PRIORITY SCHEDULING


SYSTEM VR4 AND SOLARIS


FAIR SHARE SCHEDULING Fixed distribution of CPU usage among users and groups.

A weight is given to every user/group. A grp_cpu variable must be added per user/group

After every quantum, for every process:


cpu = cpu / 2;grp_cpu = grp_cpu / 2;prio = cpu / 2 + weight*(grp_cpu/2)+ BASE_LEVEL_PRIORITY;

FAIR SHARE SCHEDULING : EXAMPLE


=

COMPLETELY FAIR SCHEDULER Linux 2.6.23.

Not O(1) Scheduler.

Schedule classes.

Different scheduling policies:

SCHED_NORMAL, SCHED_BATCH, SCHED_IDLE.Real-time scheduler: SCHED_RR and SCHED_FIFO.

Uses a time-ordered red-black tree.

Nano-second granularity.


COMPARISON OF SEVERAL ALGORITHMS


ASSUMPTIONS 5 Processes arrive at time 0.

The CPU ready queue is empty at time 0.

The time needed for context switches can be ignored.


ASSUMPTIONS In round robin scheduling, a process will release the CPU

voluntarily if it needs less than a quantum.

A newly arrived process is placed in the ready queue before a process whose quantum expires at the same time.


COMPARISON


CRITERIA / ANALYSISAverage waiting time.

Throughput.number of jobs finished in 30 ms.

Fairness/Efficiency:number of processes getting the CPU after 30 ms.


COMPARISON


o perating s ystems and a rchitectures cs-m98 p art 7 s cheduling benjamin mora 1 swansea university...

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