module 2.0: processes

K. Salah Operating Systems 1

Module 2.0: Processes

• Process Concept

• Trace of Processes

• Process Context

• Context Switching

• Threads

– ULT

– KLT


Process

• Also called a task.

• Useful and Important Concept:Process = program in execution

• A process is not the same as a program. Program is a passive entity, whereas process is active. Process consists of an executable program, associated data, and execution context.

• Modern (multiprogramming) operating systems are structured around the concept of a process.

• Multiprogramming OS supports execution of many concurrent processes. OS issues tend to revolve around management of processes:

– How are processes created/destroyed?– How to manage resource requirements of a process during its execution:

cpu time, memory, I/O, communication, ... ?– How to avoid interference between processes?– How to achieve cooperation and communication between processes?


Program Creation

• Program (say, C program) is edited

• It is compiled into assembly language, which may consist of several modules.

• Assembly language modules are assembled into machine language.

• External references (i.e., to procedures and data in another module) are resolved. This is called linking, which creates a load module.

• Load or image module is stored as a file in file system and may be executed at a later time by loading into memory to be executed.


Process creation and termination

• Consider a simple disk operating system (like MS-DOS, typically supports only one process at a time)

• User types command like “run foo” at Keyboard (I/O device driver for keyboard, screen)

• Command is parsed by command shell

• Executable program file (load module) “foo” is located on disk (file system, I/O device driver for disk)

• Contents are loaded into memory and control transferred ==> process comes alive! (device driver for disk, relocating loader, memory management)

• During execution, process may call OS to perform I/O: console, disk, printer, etc. (system call interface, I/O device drivers)

• When process terminates, memory is reclaimed. (memory management)


Trace of Processes


Trace of processes (cont.)


Process Context

• The context (or image) of a process can be described by– contents of main memory– contents of CPU registers– other info (open files, I/O in progress, etc.)

• Main memory -- three logically distinct regions of memory:– code region: contains executable code (typically read-only)– data region: storage area for dynamically allocated data structure,

e.g., lists, trees (typically heap data structure) – stack region: run-time stack of activation records

• CPU registers: general registers, PC, SP, PSW, segmentation registers

• Other info:– open files table, status of ongoing I/O– process status (running, ready, blocked), user id, ...


Multiprogramming/Timesharing Systems

• They provide interleaved execution of several processes to give an illusion of many simultaneously executing processes.

• Computers can be a single-processor or multi-processor machine.

• The OS must keep track of the state for each active process and make sure that the correct information is properly installed when a process is given control of the CPU.


Multiprogramming (multiple processes)

• For each process, the O.S. maintains a data structure, called the process control block (PCB). The PCB provides a way of accessing all information relevant to a process:

– This data is either contained directly in the PCB, or else the PCB contains pointers to other system tables.

• Processes (PCBs) are manipulated by two main components of the process subsystem in order to achieve the effects of multiprogramming:

– Scheduler: determines the order by which processes will gain access to the CPU. Efficiency and fair-play are issues here.

– Dispatcher: actually allocates CPU to process next in line as determined by the scheduler.


The Process Control Block (PCB)

• The PCB typically contains the following types of information:

• Process status (or state): new, ready to run, user running, kernel running, waiting, halted

– Program counter: where in program the process is executing

– CPU registers: contents of general-purpose register stack pointer, PSW, index registers

– Memory Management info: segment base and limit registers, page table, location of pages on disk, process size

– User ID, Group ID, Process ID, Parent PID, ...– Event Descriptor: when process is in the

“sleep” or waiting state– Scheduling info: process priority, size of CPU

quantum, length of current CPU burst


PCB (cont.)

– List of pending signals

– Accounting info: process execution time, resource utilization

– Real and Effective User IDs: determine various privileges allowed the process such as file access rights

– More timers: record time process has spent executing in user and Kernel mode

– Array indicating how process wishes to react to signals

– System call info: arguments, return value, error field for current system call

– Pending I/O operation info: amount of data to transfer, addr in user memory, file offset, ...

– Current directory and root: file system environment of process

– Open file table: records files process has open


Process States & Transitions


Context Switching

• Let us first review the user/system mode distinction.

• When system starts (during system bootstrapping or boot) it is in system mode.

• This is process 0 in V.2, which creates process 1 (init). This process execs /etc/init and is then executing in user mode.

• Process 1, like any user process, continues executing in user mode until one of the following:– interrupt by an asynchronous device like timer, disk, or

terminal– the process makes a system call by executing an instruction

to cause a software interrupt

• Occurrence of such an event causes the CPU to switch to system mode and begin execution appropriate interrupt handler.


A Tree of Processes On A Typical UNIX System


When to context switch

• Typically, hardware automatically saves the user PC and PSW when interrupt occurs, and takes new PC from interrupt vector.

• Interrupt handler may simply perform its function and then return to the same process that was interrupted (restoring the PC and PSW from the stack).

• Alternatively, may no longer be appropriate to resume execution of process that was running because: process has used up its current CPU quantum process has requested I/O and must wait for results process has asked to be suspended (sleep) for some amount of time a signal or error requires process be destroyed (killed) a “higher priority” process should be given the CPU

• In such a situation, a context switch is performed to install appropriate info for running a new process.


Mechanics of a Context Switch

1 copy contents of CPU registers (general-purpose, SP, PC, PSW, etc.) into a save area in the PCB of running process

2 change status of running process from “running” to “waiting” (or “ready”)3 change a system variable running-process to point to the PCB of new

process to run4 copy info from register save area in PCB of new process into CPU registers

• Note: context switching is pure overhead and should be done as fast as possible often hardware-assisted - special instructions for steps 1 and 4 determining new process to run accomplished by consulting scheduler

queues step 4 will start execution of new process - known as dispatching.


MULTIPROGRAMMING Through CONTEXT SWITCHING


Introduction to Threads

Multitasking OS can do more than one thing concurrently by running more than a single process

Processes can do several things concurrently be running more than a single thread.

Each thread is a different stream of control that can execute its instructions independently.

A program (e.g. Browser) may consist of the following threads:

GUI thread I/O thread computation


Processes and Threads

A typical process consists of a running program a bundle of resources (file descriptor table, address space)

A thread, called a lightweight process, has its own stack CPU Registers state All the other resources are shared by all threads of that process.

These include: open files virtual address space (code and data segments). child processes


Processes vs. Threads


Single Threaded and Multithreaded Process Models

Thread Control Block contains a register image, thread priority and thread state information.


Benefits of Threads vs Processes

Takes less time to create a new thread than a process

Less time to terminate a thread than a process

Less time to switch between two threads within the same process

Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel. However, it is necessary to synchronize the activities of various threads so that they do not obtain inconsistent views of the data.

• Example 1: a file or web server on a LAN. The server needs to handle several file or web requests over a short period.

• Hence more efficient to create (and destroy) a single thread for each request.

• Example 2: one thread display menu and read user input while the other thread execute user commands


Threads States

Three key states: running, ready, blocked

Termination of a process, terminates all threads within the process


User-Level Threads (ULT)

• The kernel is not aware of the existence of threads

• All thread management is done by the application by using a thread library

• Thread switching does not require kernel mode privileges (no mode switch)

• Scheduling is application specific


Threads library

• Contains code for:

– creating and destroying threads

– passing messages and data between threads

– scheduling thread execution

– saving and restoring thread contexts


Kernel activity for ULTs

• The kernel is not aware of thread activity but it is still managing process activity

• When a thread makes a system call, the whole process will be blocked

• but for the thread library that thread is still in the running state

• So thread states are independent of process states


Advantages and inconveniences of ULT

• Advantages– Thread switching does not

involve the kernel: no mode switching

– Scheduling can be application specific: choose the best algorithm.

– ULTs can run on any OS. Only needs a thread library

• Inconveniences– Most system calls are

blocking and the kernel blocks processes. So all threads within the process will be unable to run

– The kernel can only assign processes to processors. Two threads within the same process cannot run simultaneously on two processors


Kernel-Level Threads (KLT)

• All thread management is done by kernel

• No thread library but an API (I.e. system calls) to the kernel thread facility

• Kernel maintains context information for the process and the threads

• Switching between threads requires the kernel

• Scheduling on a thread basis

• Ex: Windows NT and OS/2


Advantages and inconveniences of KLT

• Advantages– the kernel can simultaneously

schedule many threads of the same process on many processors

– blocking is done on a thread level

– kernel routines can be multithreaded

• Inconveniences– thread switching within the

same process involves the kernel. We have 2 mode switches per thread switch: user to kernel and kernel to user.

– this results in a significant slow down


Combined ULT/KLT Approaches

• Thread creation done in the user space

• Bulk of scheduling and synchronization of threads done in the user space

• The programmer may adjust the number of KLTs

• May combine the best of both approaches

• Example is Solaris

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