Scheduling Algorithmic Research

Rami Abielmona94.571 (ELG 6171)

Monday March 27, 2000Prof. T. W. Pearce

Scheduling Algorithms Introduction

• Problem definition:– “One CPU with a number of processes. Only

one process can use the CPU at a time and each process is specialized in one, and one task. What’s the best way to organize the processes (schedule them) ?” [1]

– “How will the CPU time be divided among the processes and threads competing to use it ?” [2]

Scheduling AlgorithmsEmbedded OS Architecture

• Kernel:– The executor

• Executive:– The manager

• Application Programs:– The programmer tasks

• Real World Interfacing:– S/W handling the H/W

Scheduling AlgorithmsBasic Assumptions

• A pool of runnable processes are contending for one CPU;

• The processes are independent and compete for resources;

• The job of the scheduler is to distribute the scarce resource of the CPU to the different processes “fairly” and in an optimal way;

• The job of the dispatcher is to provide the mechanism for running the processes;

• The OS is a multitasking, but not a multiprocessor, one;

• Only CPU scheduling is considered (the lowest level of scheduling).

Scheduling AlgorithmsEvaluation Characteristics

CPU utilization Keep it as high aspossible

Throughput Number of processescompleted per unittime

Waiting time Amount of time spentready to run but notrunnin

Response time Amount of timebetween submission ofrequests and firstresponse to the request

Scheduler efficiency Minimize theoverhead

Turnaround time Mean time fromsubmission to

Scheduling AlgorithmsProcesses and Resources

• Resources:– Preemptible:

• Take resource away, use it for something else, then give it back. (e.g. processor or I/O channel)

– Non-preemptible:• Once give, it can’t be

reused until process gives it back. (e.g. file space or terminal)

• Processes:– IO bound:

• Perform lots of IO operations.

• IO burst ---- short CPU burst to process IO --- IO burst

– CPU bound:• Perform lots of

computation and do little IO

• CPU burst ----------- Small IO burst ----------- CPU burst

Scheduling AlgorithmsProcess State Transitions

• The states of a process, at any given time, is comprised of the following minimal set:

– Running:

• The CPU is currently executing the code belonging to the process.

– Ready:

• The process could be running, but another process has the CPU.

– Waiting:

• Before the process can run, some external event must occur.

Scheduling AlgorithmsTypes of Schedulers

• Long-term scheduler:– admits new processes to the system;

– required because each process needs a portion of the available memory for its code and data.

• Medium-term scheduler:– is not found in all systems;

– required to control the temporary removal from memory of a process when the latter is extractable.

• Short-term scheduler:– determines the assignment of the CPU to ready processes;

– required because of IO requests and completions.

Scheduling AlgorithmsThe Contestants (1)

• First-Come First-Serve (FCFS)– One ready queue;

– OS runs the process at head of the queue;

– New processes come in at the end of the queue;

– Running process does not give up the CPU until it terminates or it performs IO.

• Round Robin– Process runs for one time slice, then moved to back of the

queue;

– Each process gets equal share of the CPU.

Scheduling AlgorithmsThe Contestants (2)

• Shortest Time to Completion (STCF)– Process with shortest computation time left is picked;

– Varianted by preemption;

– Requires knowledge of the future.

• Exponential Queue (Multi-level Feedback)– Gives newly runnable processes a high priority and a very

short time slice;

– If process uses up the time slice without blocking then decrease priority by one and double time slice for next time;

– Solves both efficiency and response time problems.

Scheduling AlgorithmsThe Contestants (3) - Priorities

• Priority Systems– The highest priority ready process is selected

– In case of a tie, FCFS can be used

– Priorities could be assigned:• Externally (e.g. by a system manager)

• Internally (e.g. by some algorithm)

• Combination of external and internal

– Preemptive schemes:• Once a process starts executing, allow it to continue until it

voluntarily yields the CPU

– Non-preemptive schemes:• A running process may be forced to yield the CPU by an external

event rather than by its own action

Scheduling AlgorithmsFirst-Come First-Serve

• Non-preemptive FCFS (no priority scheme)– Simplest implementation of scheduling algorithms

– Used on timeshared systems (with timer interruption)

• Non-preemptive FCFS (with priority scheme)– Next highest priority process is picked when CPU is yielded

– Once process grabs CPU, former keeps latter until completion

– Rarely used in real-time systems

• Preemptive FCFS (with priority scheme)– Most popular FCFS algorithm

Scheduling AlgorithmsRound Robin

• Used mostly on timeshared systems

• Allows multiple users slices of the CPU on a “round robin” basis

• Majority of users have the same priority

• Not a popular scheme with dynamic priority systems

Scheduling AlgorithmsShortest Time to Completion

• Priorities are assigned in inverse order of time needed for completion of the entire job

• Minimizes average turnaround time

• Exponential averaging is used to estimate the process’ burst duration

• A job exceeding the resource estimation is aborted

• A job exceeding the time estimation is preempted

• Store estimated value in PCB for the current burst, and compare with actual value

Scheduling AlgorithmsExponential Queues

• Popular in interactive systems

• A single queue is maintained for each priority level

• A new process is added at the end of the highest priority queue

– It is alloted a single time quantum when it reaches the front

• If it yields the CPU within the time quantum, it is moved to the rear

• If not, it is placed at the rear of the next queue down

• Dispatcher selects the head of the highest priority queue

– A job that “succeeds” moves up

– A job that “fails” moves down

Scheduling AlgorithmsImplementation - Data Structures

• A queue of processes is implemented by linking PCB’s together using a linked list (with first and last node pointers)

• Since this project’s queues are known to be short, priority is implemented by using priority queues, and PriorityInsert() function calls

• Different queues are used to represent different states of processes (Ready, Suspended)

• Self-release of CPU– Internal signal

– Process completion

• Forced-release of CPU– Time slot expired

– External signal

Process ID

Status

Priority

Next Process

Scheduling AlgorithmsImplementation - Progress

• Used an OO template in order to easily and efficiently implement any necessary queue

• Used structurally defined functions to “simulate” the scheduler and dispatcher– fill_poolQ(), get_tasks(src,dest), sort(queue)

• Implemented FCFS, RR and STCF– RR was implemented to fairly compare schemes

• Theoretical work still needs to be done– comparison and evaluation

Scheduling AlgorithmsImplementation - Issues

• The underlying interrupt system basically readies the task for a switch, but does not perform the switch

• Process switches are directly handled by the scheduler

• This causes a delay from the time of readiness to the time of the switch, which is not tolerated for, let’s say, system exceptions

• The solution is to completely by-pass the scheduler (OS) and go directly to an ISR. [1]

• Each process is allocated its own private stack and workspace

• This is done to avoid different processes overwriting each other’s data and code

• This is based on a strict process model, where all heavyweight processes do not share resources

• Code that can be shared safely is called ‘re-entrant’ code [1]

Scheduling AlgorithmsImplementation - Analysis

• Direct analysis– Pick a task set and observe results

• Apply queueing theory to obtain results– Multi-level feedback queue scheme

• Simulations of scheme implementations– FCFS, RR, STCF

• Innovations and projections– “Lottery” scheduling and “own” algorithm

Scheduling AlgorithmsReferences

1) Cooling, J.E. Software Design for Real-Time Systems. Chapman & Hall, London, UK: 1995.

2) Stallings, William. Operating Systems: Internals and Design Principles. Upper Saddle River, NJ: Prentice Hall, 1998.

3) http://www.cs.wisc.edu/~bart/537/lecturenotes/s11.html - viewed on 03/24/2000

4) Savitzky, Stephen. Real-Time Microprocessor Systems. Van Nostrand Reinhold Company, N.Y.: 1985.

5) Undergraduate Operating System Course Notes (Ottawa University, 1998)