Introduction to Operating Systems

DLSU - CCS College of Computer Studies

 Process Concept
● An operating system executes a variety of programs
that run as a process.
● Process – a program in execution; process
execution must progress in sequential fashion. No
parallel execution of instructions of a single
process
● Multiple parts
○ The program code, also called text section
○ Current activity including program counter,
processor registers
○ Stack containing temporary data
■ Function parameters, return
addresses, local variables
○ Data section containing global variables
○ Heap containing memory dynamically
allocated during run time
 Process Concept
● Program is passive entity stored on disk
(executable file); process is active
○ Program becomes process when an
executable file is loaded into memory
● Execution of program started via GUI mouse clicks,
command line entry of its name, etc.
● One program can be several processes
○ Consider multiple users executing the same
program
Process In Memory
Memory Layout of a C Program
 Process State
● As a process executes, it changes state
○ New: The process is being created
○ Running: Instructions are being executed
○ Waiting: The process is waiting for some
event to occur
○ Ready: The process is waiting to be
assigned to a processor
○ Terminated: The process has finished
execution
Process State
Process Control Block
● Information associated with each process(also
called task control block)
○ Process state – running, waiting, etc.
○ Program counter – location of instruction to
next execute
○ CPU registers – contents of all
process-centric registers
○ CPU scheduling information- priorities,
scheduling queue pointers
○ Memory-management information – memory
allocated to the process
○ Accounting information – CPU used, clock
time elapsed since start, time limits
○ I/O status information – I/O devices
allocated to process, list of open files
 Threads
● So far, process has a single thread of execution
● Consider having multiple program counters per
process
○ Multiple locations can execute at once
● Multiple threads of control -> threads
● Must then have storage for thread details, multiple
program counters in PCB
Process Scheduling
● Process scheduler selects among available
processes for next execution on CPU core
● Goal -- Maximize CPU use, quickly switch
processes onto CPU core
● Maintains scheduling queues of processes
○ Ready queue – set of all processes
residing in main memory, ready and waiting
to execute
○ Wait queues – set of processes waiting for
an event (i.e., I/O)
○ Processes migrate among the various
queues
Ready and Wait queues
Ready and Wait queues
 Context Switch
● CPU switches from one process to another
Process Scheduling
● When CPU switches to another process, the system
must save the state of the old process and load
the saved state for the new process via a context
switch
● Context of a process represented in the PCB
● Context-switch time is pure overhead; the system
does no useful work while switching
○ The more complex the OS and the PCB 🡺
the longer the context switch
● Time dependent on hardware support
○ Some hardware provides multiple sets of
registers per CPU 🡺 multiple contexts
loaded at once
Multitasking in Mobile Systems
● Some mobile systems (e.g., early version of iOS) allow only one process
to run, others suspended
● Due to screen real estate, user interface limits iOS provides for a
○ Single foreground process- controlled via user interface
○ Multiple background processes– in memory, running, but
not on the display, and with limits
○ Limits include single, short task, receiving notification of
events, specific long-running tasks like audio playback
● Android runs foreground and background, with fewer limits
○ Background process uses a service to perform tasks
○ Service can keep running even if background process is
suspended
○ Service has no user interface, small memory use
 Process Creation
● Parent process create children processes, which, in turn create other
processes, forming a tree of processes
● Generally, process identified and managed via a process identifier
(pid)
● Resource sharing options
○ Parent and children share all resources
○ Children share subset of parent’s resources
○ Parent and child share no resources
● Execution options
○ Parent and children execute concurrently
○ Parent waits until children terminate
● Address space
○ Child duplicate of parent
○ Child has a program loaded into it
● UNIX
○ fork() system call creates new process
○ exec() system call used after a fork() to load a new
program
○ Parent process calls wait() waiting for the child to
terminate
Process Creation
 Process Termination
● Process executes last statement and then asks the operating system to
delete it using the exit() system call.
○ Returns status data from child to parent (via wait())
○ Process’ resources are deallocated by operating system
● Parent may terminate the execution of children processes using the
abort() system call. Some reasons for doing so:
○ Child has exceeded allocated resources
○ Task assigned to child is no longer required
○ The parent is exiting, and the operating systems does not
allow a child to continue if its parent terminates
 Process Termination
● Some operating systems do not allow child to exists if its parent has
terminated. If a process terminates, then all its children must also be
terminated.
○ cascading termination. All children, grandchildren, etc., are
terminated.
○ The termination is initiated by the operating system.
● The parent process may wait for termination of a child process by using
the wait()system call. The call returns status information and the pid of
the terminated process

pid = wait(&status);

● If no parent waiting (did not invoke wait()) process is a zombie

● If parent terminated without invoking wait(), process is an orphan
Termination - Android Process Importance Hierarchy

● Mobile operating systems often have to terminate processes to reclaim

system resources such as memory. From most to least important:
○ Foreground process
○ Visible process
○ Service process
○ Background process
○ Empty process
● Android will begin terminating processes that are least important.
Multiprocess Architecture – Chrome Browser
● Many web browsers ran as single process (some still do)
○ If one web site causes trouble, entire browser can hang or crash
● Google Chrome Browser is multiprocess with 3 different types of
processes:
○ Browser process manages user interface, disk and network I/O
○ Renderer process renders web pages, deals with HTML, Javascript.
A new renderer created for each website opened
■ Runs in sandbox restricting disk and network I/O,
minimizing effect of security exploits
○ Plug-in process for each type of plug-in
Interprocess Communication (IPC)
● Processes within a system may be independent or cooperating
● Cooperating process can affect or be affected by other processes,
including sharing data
● Reasons for cooperating processes:
○ Information sharing
○ Computation speedup
○ Modularity
○ Convenience
● Cooperating processes need interprocess communication (IPC)
● Two models of IPC
○ Shared memory
○ Message passing
Interprocess Communication (IPC)
 References
Silberschatz, A., Galvin, P. B., Gagne, G. (2014)
Operating Systems Concepts (10th ed.).
John-Wiley and Sons.