Processes and Threads
What are processes? How does the operating system manage them?
Nick Urbanik
nicku@vtc.edu.hk Department of Information and Communications Technology

Copyright Conditions: Open Publication License

(see http://www.opencontent.org/openpub/)

OSSI — ver. 1.5

Processes - p. 1/66

What is a process?
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A process is a program in execution Each process has a process ID In Linux, $ ps ax prints one line for each process. A program can be executed a number of times simultaneously. N Each is a separate process.

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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What is a process? — 2
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A process includes current values of: N Program counter N Registers N Variables A process also has: N The program code N It’s own address space, independent of other processes N A user that owns it N A group owner N An environment and a command line This information is stored in a process control block, or task descriptor or process descriptor N a data structure in the OS, in the process table N See slides starting at §34.

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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What is a thread?
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A thread is a lightweight process N Takes less CPU power to start, stop Part of a single process Shares address space with other threads in the same process Threads can share data more easily than processes Sharing data requires synchronisation, i.e., locking — see slide 61. This shared memory space can lead to complications in programming:
“Threads often prevent abstraction. In order to prevent deadlock. you often need to know how and if the library you are using uses threads in order to avoid deadlock problems. Similarly, the use of threads in a library could be affected by the use of threads at the application layer.” – David Korn

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

See page 180, ESR in references, §66.

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Program counter
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The code of a process occupies memory The Program counter (PC) is a CPU register PC holds a memory address. . . . . . of the next instruction to be fetched and executed

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Environment of a process
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The environment is a set of names and values Examples: PATH=/usr/bin:/bin:/usr/X11R6/bin HOME=/home/nicku SHELL=/bin/bash In Linux shell, can see environment by typing: $ set

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Permissions of a Process
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A process executes with the permissions of its owner N The owner is the user that starts the process A Linux process can execute with permissions of another user or group If it executes as the owner of the program instead of the owner of the process, it is called set user ID Similarly for set group ID programs

Introduction What is a process? What is a process? — 2 What is a thread? Program counter Environment of a process Permissions of a Process Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Multitasking
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Our lab PCs have one main CPU N But multiprocessor machines are becoming increasingly common N Linux 2.6.x kernel scales to 16 CPUs How execute many processes “at the same time”?

Introduction Multitasking Multitasking Multitasking — 2 Multitasking — 3 Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Multitasking — 2
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CPU rapidly switches between processes that are “ready to run” Really: only one process runs at a time Change of process called a context switch N See slide §36 With Linux: see how many context switches/second using vmstat under “system” in column “cs”

Introduction Multitasking Multitasking Multitasking — 2 Multitasking — 3 Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Multitasking — 3
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This diagram shows how the scheduler gives a “turn” on the CPU to each of four processes that are ready to run

Introduction Multitasking Multitasking Multitasking — 2 Multitasking — 3

process D C B A

CPU executes process

Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition

time context switches
OSSI — ver. 1.5

Summary and References

Processes - p. 10/66

Birth of a Process
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In Linux, a process is born from a fork() system call N A system call is a function call to an operating system service provided by the kernel Each process has a parent The parent process calls fork() The child inherits (but cannot change) the parent environment, open files Child is identical to parent, except for return value of fork(). N Parent gets child’s process ID (PID) N Child gets 0

Introduction Multitasking Start of Process Birth of a Process Process tree Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Process tree
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Processes may have parents and children Gives a family tree In Linux, see this with commands: $ pstree or $ ps axf

Introduction Multitasking Start of Process Birth of a Process Process tree Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Scheduler
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OS decides when to run each process that is ready to run (“runable”) The part of OS that decides this is the scheduler Scheduler aims to: N Maximise CPU usage N Maximise process completion N Minimise process execution time N Minimise waiting time for ready processes N Minimise response time

Introduction Multitasking Start of Process Scheduler Scheduler When to Switch Processes? Scheduling statistics: vmstat Interrupts Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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When to Switch Processes?
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The scheduler may change a process between executing (or running) and ready to run when any of these events happen: N clock interrupt N I/O interrupt N Memory fault N trap caused by error or exception N system call See slide §17 showing the running and ready to run process states.

Introduction Multitasking Start of Process Scheduler Scheduler When to Switch Processes? Scheduling statistics: vmstat Interrupts Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Scheduling statistics: vmstat
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The “system” columns give statistics about scheduling: N “cs” — number of context switches per second N “in” — number of interrupts per second See slide §36, man vmstat

Introduction Multitasking Start of Process Scheduler Scheduler When to Switch Processes? Scheduling statistics: vmstat Interrupts Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Interrupts
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Will discuss interrupts in more detail when we cover I/O An interrupt is an event (usually) caused by hardware that causes: N Saving some CPU registers N Execution of interrupt handler N Restoration of CPU registers An opportunity for scheduling

Introduction Multitasking Start of Process Scheduler Scheduler When to Switch Processes? Scheduling statistics: vmstat Interrupts Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Process States
Introduction Multitasking Start of Process

waiting for input

Running scheduler chooses this process scheduler chooses another process Ready input available

Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

Blocked

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What is Most Common State?
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Now, my computer has 160 processes. How many are running, how many are ready to run, how many are blocked? What do you expect is most common state?

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Most Processes are Blocked
9:41am up 44 days, 20:12, 1 user, load average: 2.02, 2.06, 2.13 160 processes: 145 sleeping, 2 running, 13 zombie, 0 stopped
Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Here you see that most are sleeping, waiting for input! Most processes are “I/O bound”; they spend most time waiting for input or waiting for output to complete With one CPU, only one process can actually be running at one time However, surprisingly few processes are ready to run The load average is the average number of processes that are in the ready to run state. In output from the top program above, see over last 60 seconds, there are 2.02 processes on average in RTR state

OSSI — ver. 1.5

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Linux Process States
stopped
Introduction Multitasking Start of Process

running state

Scheduler Process States Process States What is Most Common State?

creation

ready to run

scheduling wait for event event

executing

zombie

Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks

signal or event

System Calls

uninterruptible

IPC Threads Race Condition Summary and References

interruptible

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Linux Process States — 2
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Running — actually contains two states: N executing, or N ready to execute Interruptable — a blocked state N waiting for event, such as: I end of an I/O operation, I availability of a resource, or I a signal from another process Uninterruptable — another blocked state N waiting directly on hardware conditions N will not accept any signals (even SIGKILL)

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Linux Process States — 3
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Stopped — process is halted N can be restarted by another process N e.g., a debugger can put a process into stopped state Zombie — a process has terminated N but parent did not wait() for it

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Process States: vmstat
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The “procs” columns give info about process states: “r” — number of processes that are in the ready to run state “b” — number of processes that are in the uninterruptable blocked state

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Tools for monitoring processes
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Linux provides: vmstat N Good to monitor over time: $ vmstat 5 procinfo N Easier to understand than vmstat N Monitor over time with $ procinfo -f View processes with top — see slides 27 to §30 The system monitor sar shows data collected over time: See man sar; investigate sar -c and sar -q See the utilities in the procps software package. You can list them with $ rpm -ql procps w ps slabtop top pkill watch free uptime snice pmap pgrep vmstat tload skill

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Monitoring processes in Win 2000
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Windows 2000 provides a tool: Start → Administrative Tools → Performance. Can use this to monitor various statistics

Introduction Multitasking Start of Process Scheduler Process States Process States What is Most Common State? Most Processes are Blocked Linux Process States Linux Process States — 2 Linux Process States — 3 Process States: vmstat Tools for monitoring processes Monitoring processes in Win 2000 top Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Introduction Multitasking Start of Process

Process Monitoring with top

Scheduler Process States top Process Monitoring — top load average
top: process states

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Process Monitoring — top
08:12:13 up 1 day, 13:34, 8 users, load average: 0.16, 0.24, 0.49 111 processes: 109 sleeping, 1 running, 1 zombie, 0 stopped CPU states: cpu user nice system irq softirq iowait idle total 0.0% 0.0% 3.8% 0.0% 0.0% 0.0% 96.1% Mem: 255608k av, 245064k used, 10544k free, 0k shrd, 17044k buff 152460k active, 63236k inactive Swap: 1024120k av, 144800k used, 879320k free 122560k cached PID 1253 1769 23548 1 2 3 4 6 5 USER root nicku nicku root root root root root root PRI 15 16 16 16 15 15 34 15 15 NI SIZE RSS SHARE STAT %CPU %MEM 0 73996 13M 11108 S 2.9 5.5 0 2352 1588 1488 S 1.9 0.6 0 1256 1256 916 R 1.9 0.4 0 496 468 440 S 0.0 0.1 0 0 0 0 SW 0.0 0.0 0 0 0 0 SW 0.0 0.0 19 0 0 0 SWN 0.0 0.0 0 0 0 0 SW 0.0 0.0 0 0 0 0 SW 0.0 0.0 TIME CPU COMMAND 19:09 0X 2:10 0 magicdev 0:00 0 top 0:05 0 init 0:00 0 keventd 0:00 0 kapmd 0:00 0 ksoftirqd/0 0:00 0 bdflush 0:11 0 kswapd

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top: load average
08:12:13 up 1 day, 13:34, 8 users, load average: 0.16, 0.24, 0.49
Introduction Multitasking Start of Process Scheduler Process States top Process Monitoring — top load average
top: process states

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load average is measured over the last minute, five minutes, fifteen minutes Over that time is the average number of processes that are ready to run, but which are not executing A measure of how “busy” a computer is.

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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top: process states
111 processes: 109 sleeping, 1 running, 1 zombie, 0 stopped
Introduction Multitasking Start of Process Scheduler Process States top Process Monitoring — top load average
top: process states

sleeping I/O running zombie

Most processes (109/111) are sleeping, waiting for

This is the number of processes that are both ready to run and are executing

There is one process here that has terminated, but its parent did not wait() for it. I The wait() system calls are made by a parent process, to get the exit() status of its child(ren). I This call removes the process control block from the process table, and the child process does not exist any more. (§34) ¨  stopped When you press  Control-z © a shell, you will in increase this number by 1

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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top: Processes and Memory
PID USER 1253 root PRI 15 NI SIZE 0 73996 RSS SHARE STAT %CPU %MEM 13M 11108 S 2.9 5.5 TIME CPU COMMAND Introduction 19:09 0X
Multitasking Start of Process Scheduler Process States top Process Monitoring — top load average
top: process states

SIZE

This column is the total size of the process, including the part which is swapped (paged out) out to the swap partition or swap file Here we see that the process X uses a total of 73,996 Kb, i.e., 73,996 × 1024 bytes ≈ 72MB, where here 1MB = 220 bytes.

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

RSS

The resident set size is the total amount of RAM that a process uses, including memory shared with other processes. Here X uses a total of 13MB RAM, including RAM shared with other processes. The amount of shared memory is the amount of RAM that this process shares with other processes. Here X shares 11,108 KB with other processes.

SHARE

We can see that the total amount of RAM used exclusively by one process is rss − share. Here we see that X uses about 13 × 220 − 11,108 × 210 ≈ 2 MB
OSSI — ver. 1.5 Processes - p. 30/66

Virtual Memory: suspended processes
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With memory fully occupied by processes, could have all in blocked state! CPU could be completely idle, but other processes waiting for RAM Solution: virtual memory N will discuss details of VM in memory management lecture Part or all of process may be saved to swap partition or swap file

Introduction Multitasking Start of Process Scheduler Process States top Process Monitoring — top load average
top: process states

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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Suspended Processes
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Could add more states to process state table: N ready and suspended N blocked and suspended

Introduction Multitasking Start of Process Scheduler Process States top Process Monitoring — top load average
top: process states

top and memory Virtual Memory: suspended processes Suspended Processes Process Control Blocks System Calls IPC Threads Race Condition Summary and References

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Introduction Multitasking Start of Process

Process Control Blocks The Process Table Data structure in OS to hold information about a process

Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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OS Process Control Structures
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Every OS provides process tables to manage processes In this table, the entries are called process control blocks (PCBs), process descriptors or task descriptors. We will use the abbreviation PCB. There is one PCB for each process in Linux, PCB is called task_struct, defined in include/linux/sched.h N In a Fedora Core or Red Hat system, you will find it in the file /usr/src/linux-2.*/include/linux/sched.h if you have installed the kernel-source software package

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

OSSI — ver. 1.5

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What is in a PCB
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In slide §3, we saw that a PCB contains: N a process ID (PID) N process state (i.e., executing, ready to run, sleeping waiting for input, stopped, zombie) N program counter, the CPU register that holds the address of the next instruction to be fetched and executed N The value of other CPU registers the last time the program was switched out of executing by a context switch — see slide §36 N scheduling priority N the user that owns the process N the group that owns the process N pointers to the parent process, and child processes N Location of process’s data and program code in memory N List of allocated resources (including open files) PCB holds the values as they were when process was last switched out of executing by a context switch — see slide §36

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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Context Switch
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does a context switch when: N stop current process from executing, and N start the next ready to run process executing on CPU OS saves the execution context (see §37) to its PCB OS loads the ready process’s execution context from its
PCB

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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When does a context switch occur? N When a process blocks, i.e., goes to sleep, waiting for input or output (I / O), or N When the scheduler decides the process has had its turn of the CPU, and it’s time to schedule another ready-to-run process A context switch must be as fast as possible, or multitasking will be too slow N Very fast in Linux OS

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Execution Context
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Also called state of the process (but since this term has two meanings, we avoid that term here), process context or just context The execution context is all the data that the OS must save to stop one process from executing on a CPU, and load to start the next process running on a CPU This includes the content of all the CPU registers, the location of the code, . . . N Includes most of the contents of the process’s PCB.

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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Program Counter in PCB
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What value is in the program counter in the PCB? If it is not executing on the CPU, N The address of the next CPU instruction that will be fetched and executed the next time the program starts executing If it is executing on the CPU, N The address of the first CPU instruction that was fetched and executed when the process began executing at the last context switch (§36)

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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Process Control Blocks—Example
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The diagram in slide §40 shows three processes and their process control blocks. There are seven snapshots t0 , t1 , t2 , t3 , t4 , t5 and t6 at which the scheduler has changed process (there has been a context switch—§36) On this particular example CPU, all I / O instructions are 2 bytes long The diagram also shows the queue of processes in the: N Ready queue (processes that are ready to run, but do not have a CPU to execute on yet) N Blocked, or Wait queue, where the processes have been blocked because they are waiting for I / O to finish.

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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PCB Example: Diagram
CPU idle

t0

t1

t2

t3

t4

t5

t6

time

Ready Queue: Blocked Queue:

P3 P2

P3 P1

P1 P2

P2 P3

P3

PCB for P1

0xCAFE Running

0xC0DE Blocked

0xC0DE Ready

0xC0DE Running

Process 1 has terminated; It’s PCB has been freed

ents

PCB for P2

0xFACE Ready

0xFACE Running

0xFEED Blocked

0xFEED Ready

0xFEED Running

Process 2 has terminated PCB is freed

PCB for P3

0xDEAF Ready

0xDEAF Ready

0xDEAF Running

0xD1CE Blocked

0xD1CE Ready

0xD1CE Running

P3 has exited; PCB freed

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PCB Example — Continued
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In slide §40, N The times t0 , t1 , t2 , t3 , t4 , t5 and t6 are when the scheduler has selected another process to run. N Note that these time intervals are not equal, they are just the points at which a scheduling change has occurred. Each process has stopped at one stage to perform I / O N That is why each one is put on the wait queue once during its execution. Each process has performed I / O once

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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What is the address of I/O instructions?
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We are given that all I / O instructions in this particular example are two bytes long (slide §39) N We can see that when the process is sleeping (i.e., blocked), then the program counter points to the instruction after the I / O instruction N So for process P1, which blocks with program counter PC = C0DE16 , the I / O instruction is at address C0DE16 − 2 = C0DC16 N for process P2, which blocks with program counter PC = FEED16 , the I / O instruction is at address FEED16 − 2 = FEEB16 N for process P3, which blocks with program counter PC = D1CE16 , the I / O instruction is at address D1CE16 − 2 = D1CC16

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks OS Process Control Structures What is in a PCB Context Switch Execution Context Program Counter in PCB PCB Example PCB Example Diagram PCB Example — Continued Address of I/O instructions System Calls IPC Threads Race Condition Summary and References

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Introduction Multitasking Start of Process

Process System Calls How the OS controls processes How you use the OS to control processe

Scheduler Process States top Process Control Blocks System Calls System Calls IPC Threads Race Condition Summary and References

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Major process Control System Calls
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fork() — start a new process execve() — replace calling process with machine code from another program file wait(), waitpid() — parent process gets status of its’ child after the child has terminated, and cleans up the process table entry for the child (stops it being a zombie) exit() — terminate the current process

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls System Calls IPC Threads Race Condition Summary and References

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Introduction Multitasking Start of Process

IPC Inter Process Communication How Processes can Talk to Each Other

Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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Problem with Processes
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Communication! Processes cannot see the same variables Must use Inter Process Communication (IPC) IPC Techniques include: N pipes, and named pipes (FIFOs) N sockets N messages and message queues N shared memory regions All have some overhead

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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Interprocess Communication (IPC)
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Pipe — circular buffer, can be written by one process, read by another N related processes can use unnamed pipes I used in shell programming, e.g., the vertical bar ‘|’ in $ find /etc | xargs file N unrelated processes can use named pipes — sometimes called FIFOs Messages — POSIX provides system calls msgsnd() and msgrcv() N message is block of text with a type N each process has a message queue, like a mailbox N processes are suspended when attempt to read from empty queue, or write to full queue.

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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IPC — Shared Memory
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Shared Memory — a Common block of memory shared by many processes Fastest way of communicating Requires synchronisation (See slide 61)

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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IPC — Signals
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A process sends a signal to another process using the kill() system call signals are implemented as single bits in a field in the PCB, so cannot be queued A process may respond to a signal with: N a default action (usually process terminates) N a signal handler function (see trap in shell programming notes), or N ignore the signal (unless it is SIGKILL or SIGSTOP) A process cannot ignore, or handle a SIGSTOP or a SIGKILL signal. N A KILL signal will always terminate a process (unless it is in interruptible sleep) N A SIGSTOP signal will always send a process into the stopped state.

Some signals can be generated from the keyboard, i.e., ¨  ¨  Control-C © interrupt (SIGINT);  — Control-\ © quit —  ¨  Control-Z © stop (SIGSTOP) — (SIGQUIT), 

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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Signals and the Shell
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We can use the kill built in command to make the kill() system call to send a signal A shell script uses the trap built in command to handle a signal Ignoring the signals SIGINT, SIGQUIT and SIGTERM: trap "" INT QUIT TERM Handling the same signals by printing a message then exiting:
trap "echo ’Got a signal; exiting.’;exit 1" INT QUIT TERM

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Problem with Processes Interprocess Communication (IPC) IPC — Shared Memory IPC — Signals Signals and the Shell Threads Race Condition Summary and References

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Handling the same signals with a function call: signal_handler() { echo "Received a signal; terminating." rm -f $temp_file exit 1 } trap signal_handler INT QUIT TERM Sending a SIGKILL signal to process with PID 3233: $ kill -KILL 3233

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Introduction Multitasking Start of Process

Threads Lightweight processes that can talk to each other easily

Scheduler Process States top Process Control Blocks System Calls IPC Threads Threads and Processes Threads have own. . . Threads share a lot Problem with threads: Race Condition Summary and References

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Threads and Processes
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Threads in a process all share the same address space Communication easier Overhead less Problems of locking and deadlock a major issue

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Processes have separate address spaces Communication more indirect: IPC (Inter Process Communication) Overhead higher Less problem with shared resources (since fewer resources to share!)

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Threads and Processes Threads have own. . . Threads share a lot Problem with threads: Race Condition Summary and References

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Threads have own. . .
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stack pointer register values scheduling properties, such as policy or priority set of signals they can each block or receive own stack data (local variables are local to thread)

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Threads and Processes Threads have own. . . Threads share a lot Problem with threads: Race Condition Summary and References

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Threads share a lot
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Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads. Two pointers having the same value point to the same data. A number of threads can read and write to the same memory locations, and so you need to explicitly synchronise access

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Threads and Processes Threads have own. . . Threads share a lot Problem with threads: Race Condition Summary and References

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Problem with threads:
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Avoid 2 or more threads writing or reading and writing same data at the same time Avoid data corruption Need to control access to data, devices, files Need locking Provide three methods of locking: N mutex (mutual exclusion) N semaphores N condition variables

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Threads and Processes Threads have own. . . Threads share a lot Problem with threads: Race Condition Summary and References

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Introduction Multitasking Start of Process

Race Condition

Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Race Conditions
race condition — where outcome of computation depends on sheduling I an error in coding I Example: two threads both access same list with code like this: if ( list.numitems > 0 ) { // Oh, dear, better not change to // other thread here! remove_item( list ); // not here! // ...and not here either: --list.numitems; }
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Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Critical Sections
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critical resource — a device, file or piece of data that cannot be shared critical section — part of program only one thread or process should access contains a critical resource N i.e., you lock data, not code All the code in the previous slide is a critical section Consider the code: very_important_count++; executed by two threads on a multiprocessor machine (SMP = symmetric multiprocessor)

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Race Condition — one possibility
Introduction

thread 1 read very_important_count (5) add 1 (6) write very_important_count (6)

thread 2

Multitasking Start of Process Scheduler Process States top Process Control Blocks

read very_important_count (6) add 1 (7) write very_important_count (7)

System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Example — another possibility
Introduction

thread 1 read very_important_count (5)

thread 2

Multitasking Start of Process Scheduler

read very_important_count (5) add 1 (6) add 1 (6) write very_important_count (6) write very_important_count (6)

Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Solution: Synchronisation
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Solution is to recognise critical sections use synchronisation, i.e., locking, to make sure only one thread or process can enter critical region at one time. Methods of synchronisation include: N file locking N semaphores N monitors N spinlocks N mutexes

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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File Locking
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For example, an flock() system call can be used to provide exclusive access to an open file The call is atomic N It either: I completely succeeds in locking access to the file, or I it fails to lock access to the file, because another thread or process holds the lock I No “half-locked” state N No race condition Alternatives can result in race conditions; for example: N thread/process 1 checks lockfile N thread/process 2 checks lockfile a very short time later N both processes think they have exclusive write access to the file N file is corrupted by two threads/processes writing to it at the same time

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Race Conditions Critical Sections Race Condition — one possibility Example — another possibility Solution: Synchronisation File Locking Summary and References

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Introduction Multitasking Start of Process

Summary and References

Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References Summary — Process States, Scheduling Summary — Processes and Threads References

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Summary — Process States, Scheduling
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Scheduler changes processes between ready to run and running states N context switch: when scheduler changes process or thread Most processes are blocked, i.e., sleeping: waiting for I / O N understand the process states N why a process moves from one state to another Communication between processes is not trivial; IPC methods include N pipes N shared memory N messages N signals N semaphores

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References Summary — Process States, Scheduling Summary — Processes and Threads References

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Summary — Processes and Threads
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With Linux and Unix, main process system calls are fork(), exec() and wait() Threads are lightweight processes N part of one process N share address space N can share data easily N sharing data requires synchronisation, i.e., locking

Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References Summary — Process States, Scheduling Summary — Processes and Threads References

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References
There are many good sources of information in the library and on the Web about processes and threads. Here are some I recommend: I Operating Systems: A Modern Perspective: Lab Update, 2nd Edition, Gary Nutt, Addison-Wesley, 2002. A nice text book that emphasises the practical (like I do!)
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Introduction Multitasking Start of Process Scheduler Process States top Process Control Blocks System Calls IPC Threads Race Condition Summary and References Summary — Process States, Scheduling Summary — Processes and Threads References

William Stallings, Operating Systems, Fourth Edition, Prentice Hall, 2001, chapters 3, 4 and 5 Deitel, Deitel and Choffnes, Operating Systems, Third Edition, Prentice Hall, 2004, ISBN 0-13-1182827-4, chapters 3, 4 and 5 Paul Rusty Russell, Unreliable Guide To Locking http://kernelnewbies.org/documents/kdoc/ kernel-locking/lklockingguide.html Eric S. Raymond, The Art of UNIX Programming, Addison-Wesley, 2004, ISBN 0-13-142901-9.

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