Chapter 2 Processes and Threads 2 1 Processes

Chapter 2 Processes and Threads 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 1

Agenda • • • 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 2

Process • The most central concept in any OS • An abstraction of a running program • Modern Computers – Can do more than one thing at the same time • Can run user programs • Can read disk and work with user terminal – In a multiprogramming system • More than one user program can be scheduled • Each may run for tens of msecs. 3

Processes The Process Model • Multiprogramming of four programs • Conceptual model of 4 independent, sequential processes • Only one program active at any instant 4

Process Creation Principal events that cause process creation 1. System initialization 2. Execution of a process creation system 3. User request to create a new process 4. Initiation of a batch job 5

Process Termination Conditions which terminate processes 1. Normal exit (voluntary) 2. Error exit (voluntary) 3. Fatal error (involuntary) 4. Killed by another process (involuntary) 6

Process Hierarchies • Parent creates a child process, child processes can create its own process • Forms a hierarchy – UNIX calls this a "process group" • Windows has no concept of process hierarchy – all processes are created equal 7

Process States (1) • Possible process states – running – blocked – ready • Transitions between states shown 8

Process States (2) • Lowest layer of process-structured OS – handles interrupts, scheduling • Above that layer are sequential processes 9

Implementation of Processes (1) Fields of a process table entry 10

Implementation of Processes (2) Skeleton of what lowest level of OS does when an interrupt occurs 11

Agenda • • • 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 12

Threads • In traditional OS, each process has – An address space – A single tread of control • In modern OS, each process may have – Multiple threads of control – Same address spare – Running in quasi-parallel • Thread or a Lightweight Process has – a program counter, registers, stack 13

Threads The Thread Model (1) (a) Three processes each with one thread (b) One process with three threads 14

The Thread Model (2) • Items shared by all threads in a process • Items private to each thread 15

The Thread Model (3) Each thread has its own stack 16

Thread Usage (1) A word processor with three threads 17

Thread Usage (2) A multithreaded Web server 18

Thread Usage (3) • Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread 19

Thread Usage (4) Three ways to construct a server 20

Implementing Threads in User Space A user-level threads package 21

Implementing Threads in the Kernel A threads package managed by the kernel 22

Hybrid Implementations Multiplexing user-level threads onto kernellevel threads 23

Scheduler Activations • Goal – mimic functionality of kernel threads – gain performance of user space threads • Avoids unnecessary user/kernel transitions • Kernel assigns virtual processors to each process – lets runtime system allocate threads to processors • Problem: – Fundamental reliance on kernel (lower layer) – Calling procedures in user space (higher layer) • Called upcall 24

Pop-Up Threads • Creation of a new thread when message arrives (a) before message arrives (b) after message arrives 25

Making Single-Threaded Code Multithreaded (1) Conflicts between threads over the use of a global variable 26

Making Single-Threaded Code Multithreaded (2) Threads can have private global variables 27

Agenda • • • 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 28

Interprocess Communication • Processes may need to communicate – E. g. , output of one goes to input of the other one • Issues – How to pass information • different address spaces – Making critical activities • Two process try to grab the last 1 MB of memory – Proper sequencing • One process is producing data for the other one 29

Interprocess Communication Race Conditions Two processes want to access shared memory at same time 30

Critical Regions (1) • Critical Regions or Critical Sections – • The part of the program where shared memory is accessed. Four conditions to provide mutual exclusion 1. No two processes simultaneously in critical region 2. No assumptions made about speeds or numbers of CPUs 3. No process running outside its critical region may block another process 4. No process must wait forever to enter its critical region 31

Critical Regions (2) Mutual exclusion using critical regions 32

Mutual Exclusion with Busy Waiting (0) • Mutual Exclusion – Only one process can be in the critical section. • Disabling Interrupts (HW Solution) – Dangerous: May lead to the end of the system • Lock Variables (SW Solution) – Fatal Flow: Similar to Spooler Directory • Strict Alternation (SW Solution) – Busy Waiting, Violating Condition 3 • Peterson’s Solution (SW Solution) – Busy Waiting • TSL Instruction (HW/SW Solution) – Busy Waiting, but Faster 33

Mutual Exclusion with Busy Waiting (1) Strict Alternation (SW Solution) Proposed solution to critical region problem (a) Process 0. (b) Process 1. 34

Mutual Exclusion with Busy Waiting (2) Peterson’s Solution (SW Solution) 35

Mutual Exclusion with Busy Waiting (3) Using TSL Instruction (HW/SW Solution) Entering and leaving a critical region using the TSL instruction 36

Mutual Exclusion with Sleep and Wakeup (0) Producer-Consumer Problem • Priority Inversion Problem – busy waiting with priority! • Solution with Fatal Race Condition – Waking up a consumer that is not asleep yet! • Semaphore – An integer that counts the number of wake-up calls. • Mutexes – Binary semaphores, good for mutual exclusion. • Monitors – Easier to program (Synchronized in Java). • Message Passing – N messages used for communication coordination. • Barriers – Synchronization of N processes/threads. 37

Mutual Exclusion with Sleep and Wakeup (1) Fatal Race Condition 38

Mutual Exclusion with Sleep and Wakeup (2) Using Semaphores 39

Mutual Exclusion with Sleep and Wakeup (3) Using Mutexes Implementation of mutex_lock and mutex_unlock 40

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (1) Example of a monitor 41

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (2) • Outline of producer-consumer problem with monitors – only one monitor procedure active at one time – buffer has N slots 42

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (3) Solution to producer-consumer problem in Java (part 1) 43

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (4) Solution to producer-consumer problem in Java (part 2) 44

Mutual Exclusion with Sleep and Wakeup (5) Message Passing The producer-consumer problem with N messages 45

Barriers • Use of a barrier – processes approaching a barrier – all processes but one blocked at barrier – last process arrives, all are let through 46

Agenda • • • 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 47

Dining Philosophers (1) • • Philosophers eat/think Eating needs 2 forks Pick one fork at a time How to prevent deadlock 48

Dining Philosophers (2) A nonsolution to the dining philosophers problem 49

Dining Philosophers (3) Solution to dining philosophers problem (part 1) 50

Dining Philosophers (4) Solution to dining philosophers problem (part 2) 51

The Readers and Writers Problem A solution to the readers and writers problem 52

The Sleeping Barber Problem (1) 53

The Sleeping Barber Problem (2) Solution to sleeping barber problem. 54

Agenda • • • 2. 1 Processes 2. 2 Threads 2. 3 Interprocess communication 2. 4 Classical IPC problems 2. 5 Scheduling 55

Scheduling • Scheduler – The part of the OS that make the choice of which process to run next. • Scheduling Algorithm – The algorithm used for scheduling 56

Scheduling Introduction to Scheduling (1) • Bursts of CPU usage alternate with periods of I/O wait – a CPU-bound process: spends most of its time on computing – an I/O bound process: spends most of its time waiting for I/O 57

Introduction to Scheduling (2) System Algorithm Goals 58

Scheduling Algorithm Goals • Throughput – The number of jobs per hour that the system completes. • Turnaround time – The statically average time from the moment that a batch job is submitted until the moment it is completed. 59

Scheduling in Batch Systems (1) • First-Come First-Served • Shortest Job First (non-preemptive) – An example of shortest job first scheduling • Shortest Remaining Time First (preemptive) • Three-Level Scheduling 60

Scheduling in Batch Systems (2) Three level scheduling 61

Scheduling in Interactive Systems (1) • Round Robin Scheduling – list of runnable processes after B uses up its quantum • • • Priority Scheduling Multiple Queues Shortest Process Next Guaranteed Scheduling Lottery Scheduling Fair-Share Scheduling 62

Scheduling in Interactive Systems (2) A scheduling algorithm with four priority classes 63

Scheduling in Real-Time Systems Schedulable real-time system • Given – m periodic events – event i occurs within period Pi and requires Ci seconds • Then the load can only be handled if 64

Policy versus Mechanism • Separate what is allowed to be done with how it is done – a process knows which of its children threads are important and need priority • Scheduling algorithm parameterized – mechanism in the kernel • Parameters filled in by user processes – policy set by user process 65

Thread Scheduling (1) Possible scheduling of user-level threads • 50 -msec process quantum • threads run 5 msec/CPU burst 66

Thread Scheduling (2) Possible scheduling of kernel-level threads • 50 -msec process quantum • threads run 5 msec/CPU burst 67