Download Module 4: Processes

Module 2.0: Processes
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Process Concept
Trace of Processes
Process Context
Context Switching
Threads
– ULT
– KLT
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Operating Systems
Process
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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?
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Operating Systems
Program Creation
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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.
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Process creation and termination
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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)
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Trace of Processes
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Trace of processes (cont.)
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Trace of processes (cont.)
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Trace of processes (cont.)
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Process Context
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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, ...
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Operating Systems
Multiprogramming/Timesharing Systems
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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.
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Operating Systems
Multiprogramming (multiple processes)
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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.
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Operating Systems
The Process Control Block (PCB)
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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
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Operating Systems
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
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Operating Systems
Process States & Transitions
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Context Switching
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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.
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Operating Systems
A Tree of Processes On A Typical UNIX System
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When to context switch
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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.
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Operating Systems
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 run
4 copy info from register save area in PCB of new process into CPU registers
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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.
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Operating Systems
MULTIPROGRAMMING Through
CONTEXT SWITCHING
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Operating Systems
Introduction to Threads
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Multitasking OS can do more than one thing concurrently
by running more than a single process
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Processes can do several things concurrently be running
more than a single thread.
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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
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Operating Systems
Processes and Threads
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A typical process consists of
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a running program
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a bundle of resources (file descriptor table, address space)
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A thread, called a lightweight process, has its own
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stack
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CPU Registers
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state
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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
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Operating Systems
Processes vs. Threads
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Single Threaded and Multithreaded Process Models
 Thread Control Block contains a
register image, thread priority and
thread state information.
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Benefits of Threads vs Processes
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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
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Operating Systems
Threads States
Three key states: running, ready, blocked
Termination of a process, terminates all threads within the
process
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Operating Systems
User-Level Threads (ULT)
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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
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Operating Systems
Threads library
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Contains code for:
– creating and destroying threads
– passing messages and data between threads
– scheduling thread execution
– saving and restoring thread contexts
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Operating Systems
Kernel activity for ULTs
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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
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Operating Systems
Advantages and inconveniences of ULT
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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
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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
Operating Systems
Kernel-Level Threads (KLT)
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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
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Operating Systems
Advantages and inconveniences of KLT
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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
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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
Operating Systems
Combined ULT/KLT Approaches
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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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