CE Operating Systems Lecture 7 Threads & Introduction to CPU Scheduling

Timetable change for this week only. Group 1 Tuesday 12-2pm K106 Group 2 Friday 11am-1pm in K006 Group 3 Thursday 11am-1pm in K006

Overview of lecture In this lecture we will be looking at What is a thread? thread types CPU/IO burst cycle CPU scheduling - preemptive & nonpreemptive dispatcher scheduling criteria First Come First Served (FCFS) algorithm Shortest Job First (SJF) algorithm

Threads

Threads - analogy Analogy: Process is like a manual of procedures (code), sets of files and paper (memory), and other resources. CPU is like a person who carries out (executes) the instructions in the manual of procedures CPU (person) may be ‘context switched’ from doing one task to doing another

Threads – analogy (Cont.) A thread consists of a bookmark in the manual of procedures (program counter value), and pad of paper that is used to hold information that is currently being used (register and stack values) it is possible for a single process to have a number of bookmarks in the manual with a pad of paper associated with each bookmark (a number of threads within a process)

Threads - analogy (Cont.) the person (CPU) could then switch between doing one thing in the manual of procedures (executing one thread) to doing another thing somewhere else (start executing another thread) This switching between threads is different from context switching between processes - it is quicker to switch between threads in a process

Threads A thread exists as the current execution state of a process consisting of: program counter, processor register values and stack space it is called a thread because of the analogy between a thread and a sequence of executed instructions (imagine drawing a line through each line of instructiuins in the manual of procedures (code) when it has been executed - you get a thread (line) through the manual (code)

Threads (Cont.) A thread is often called a lightweight process there can be multiple threads associated with a single process each thread in a process shares with other peer threads the following: code section, data section, operating-system resources all threads collectively form a task

Threads (Cont.) A traditional process is equal to a task with one thread i.e. processes used to only have a single thread Overhead of switching between processes is expensive especially with more complex operating systems - threads reduce switching overhead and improve granularity of concurrent operation

Example in use: In a multiple threaded task, while one server thread is blocked and waiting, a second thread in the same task can run. Cooperation of multiple threads in same job confers higher throughput and improved performance. Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism. Threads (Cont.)

Thread types 2 different thread types: Kernel-supported threads (e.g. Mach and OS/2) - kernel of O/S sees threads and manages switching between threads i.e. in terms of analogy boss (OS) tells person (CPU) which thread in process to do next.

Thread types (Cont.) User-level threads - supported above the kernel, via a set of library calls at the user level. Kernel only sees process as whole and is completely unaware of any threads i.e. in terms of analogy manual of prcedures (user code) tells person (CPU) to stop current thread and start another (using library call to switch threads)

Introduction to CPU Scheduling Topics: CPU-I/O burst cycle Preemptive, nonpreemptive dispatcher Scheduling Criteria Scheduling Algorithms -some this lecture, the rest next lecture. This lecture: First come first served (FCFS) Shortest Job First (SJF)

CPU-I/O Burst Cycle

CPU-I/O Burst Cycle (Cont.) CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. CPU burst is length of time process needs to use CPU before it next makes a system call (normally request for I/O). I/O burst is the length of time process spends waiting for I/O to complete.

Histogram of CPU-burst Times Typical CPU burst distribution

CPU Scheduler Allocates CPU to one of processes that are ready to execute (in ready queue) CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state (e.g. when I/O request) 2.Terminates 3.Switches from waiting to ready(e.g. on I/O completion) 4.Switches from running to ready state(e.g.Timer interrupt)

CPU Scheduler (Cont.) If scheduling occurs only when 1 and 2 happens it is called nonpreemptive - process keeps CPU until it voluntarily releases it (process termination or request for I/O) If scheduling also occurs when 3 & 4 happen it is called preemptive - CPU can be taken away from process by OS (external I/O interrupt or timer interrupt)

Dispatcher Dispatcher gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart that program (i.e. last action is to set program counter)

Dispatcher (Cont.) Dispatch latency – time it takes for the dispatcher to switch between processes and start new one running

Scheduling Criteria CPU utilisation i.e. CPU usage - to maximise Throughput = number of processes that complete their execution per time unit - to maximise Turnaround time = amount of time to execute a particular process - to minimise

Scheduling criteria (Cont.) Waiting time = amount of time a process has been waiting in the ready queue - to minimise Response time = amount of time it takes from when a job was submitted until it initiates its first response (output), not to time it completes output of its first response - to minimise

First-Come, First-Served (FCFS) Scheduling Schedule = order of arrival of process in ready queue Example:ProcessBurst Time P 1 24 P23 P23 P33 P33 Suppose that the processes arrive in the order: P 1, P 2, P 3.

FCFS Scheduling (Cont.) The Gantt Chart for the schedule then is: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1. The Gantt chart for the schedule is: Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 P1P1 P3P3 P2P

FCFS Scheduling (Cont.) waiting time usually not minimal and large variance in times Convoy effect – this is where short process may have a long wait before being scheduled onto CPU due to long process being ahead of them

Shortest-Job-First (SJF) Scheduling Each process has a next CPU burst - and this will have a length (duration). Use these lengths to schedule the process with the next shortest burst. Two schemes: 1. non-preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst.

SJF Scheduling (Cont.) 2. Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is known as Shortest-Remaining-Time-First (SRTF). SJF is optimal – gives minimum average waiting time for a given set of processes.

Process Arrival Time Burst Time P P P P SJF (non-preemptive) Average waiting time = ( )/4 = 4 Example of Non-Preemptive SJF P1P1 P3P3 P2P P4P4 812

Example of Preemptive SJF Process Arrival Time Burst Time P P P P SJF (preemptive) Average waiting time = ( )/4 = 3 P1P1 P3P3 P2P P4P4 57 P2P2 P1P1 16

Determining Length of Next CPU Burst Can only estimate the length. Can be done by using the length of previous CPU bursts, using exponential averaging (decaying average).

Determining Length of Next CPU Burst (Cont.)

Examples of Exponential Averaging  =0,  n+1 =  n last CPU burst does not count - only longer term history  =1,  n+1 = t n Only the actual last CPU burst counts.

Examples of Exponential Averaging (Cont.) If we expand the formula, we get:  n+1 =  t n +(1 -  )  t n-1 + … +(1 -  ) j  t n-j + … +(1 -  ) n+1  0 Since both  and (1 -  ) are less than or equal to 1, each successive term has less weight than its predecessor.

References Operating System Concepts. Chapter 4 & 5.