The base entity in computer programming is the process or task. The parallelism can be achieved by executing multiple processes on different processors. Distributed systems are managed by distributed operating systems that represent the extension for multiprocessor architectures of multitasking and multiprogramming operating systems.

Felician ALECU

English

Research Papers in Economics

Page 2  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
The Impact of Parallel Processing on Operating 
Systems 
 
 
Felician ALECU, PhD, University Lecturer 
Department of Economic Informatics 
Academy of Economic Studies, Bucharest, Romania 
E-mail: alecu[at]ase[dot]ro; Web Page: http://alecu.ase.ro 
 
 
Abstract:  The base entity in computer programming is the process or 
task. The parallelism can be achieved by executing multiple 
processes  on  different  processors.  Distributed  systems  are 
managed  by  distributed  operating  systems  that  represent 
the  extension  for  multiprocessor  architectures  of 
multitasking and multiprogramming operating systems. 
Keywords:   parallel processing, operating systems, process, thread 
 
 
1. Introduction 
A  parallel  computer  is  a  set  of  processors  that  are  able  to  work 
cooperatively to solve a computational problem. Based on this definition, 
a  parallel  computer  could  be  a  supercomputer  with  hundreds  or 
thousands of processors or could be a network of workstations.  
Concurrency  becomes  a  fundamental  requirement  for  algorithms 
and programs. A program has to be able to use a variable number of 
processors  and  also  has  to  be  able  to  run  on  multiple  processors 
computers architectures. 
According  to  Tanenbaum,  a  distributed  system  is  a  set  of 
independent computers that appear to the user like a single one. So, the Page 3  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
computers  have  to  be  independent  and  the  software  has  to  hide 
individual  computers  to  the  users.  MIMD  computers  and  workstations 
connected through LAN and are examples of distributed systems.  
The  main  difference  between  parallel  systems  and  distributed 
systems is the way in which these systems are used. A parallel system 
uses a set of processing units to solve a single problem. A distributed 
system is used by many users together.  
 
2. Processes and Threads 
  The process (or task) is the base entity in computers programming. 
The concept is taken from uniprocessor systems sequential programming. 
A process is a dynamic entity and represents an execution instance of a 
code segment. A process has its own status and data. The parallelism can 
be achieved by executing multiple processes on different processors.  
  Processes  can  be  found  in  all  operating  systems 
(multiprogramming, multitasking, parallel and distributed). 
  When using a RUN command on an executable file, a process is 
created by the operating system. The process will receive a process ID – 
a  unique  number  that  will  identify  the  process  in  the  system.  Every 
process  has  its  own  virtual  addresses  space  representing  the  range  of 
addresses that can be accessed. A process cannot access other process 
addresses space. 
The addresses space contains program text segment, data segment 
and  stack  segment.  The  text  segment  consists  of  program  executable 
text and is shared by multiple processes. A process can read from text 
segment  but  is  not  allowed  to  write  or  to  change  the  content  of  this 
segment.  The  next  segment  is  data  segment.  It  stores  static  and 
dynamic  allocated  data.  This  segment  is  not  shared  and  cannot  be 
accessed  by  other  processes.  The  stack  segment  contains  the  process 
stack and is not shared. 
  The  process  execution  context  is  formed  from  process  segments 
and  process  resources  (opened  files,  synchronization  objects,  working 
folder)  
  A  lot  of  operating  systems  are  implementing  a  virtual  memory Page 4  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
mechanism for loading just a part of a process addresses space into the 
physical  memory.  The  virtual  memory  mechanism  can  use  paging, 
swapping or a combination of these methods. 
 
2.1. Process States 
From the operating system point of view, a process can be in one 
of the following states: 
Dormant     the process is not known by the operating system 
  because  it  was  not  created  yet.  Every  program 
  not executed is in dormant state. 
Ready to run    all resources needed are allocated to the process 
 except  processor.  The  operating  system 
 scheduler selects one ready to run process at a t
 ime and executes it. 
Execution      the  process  has  allocated  all  resources  needed, 
    including processor. Only one process at a time 
    can be in this state. The executed process can  
    request a service from the operating system like 
    an input/output operation or a synchronizing  
    operation. In such a case, the operating system 
    suspends current process. 
Suspended     a suspended process is waiting for an event, like 
    a synchronization signal. Suspended processes  
    are not competing for execution until the event is 
    arising. At this moment, the operating system  
    changes the process state to ready to run. The  
    scheduler will select the process and will execute 
    it imediatelly. 
 
The  operating  system  can  switch  between  two  processes.  The 
operation  is  named  process  switch  and  is  requested  by  the  operating 
system  only.  Process  switch  is  a  complex  operation  with  a  significant 
overhead that can affect system performance.  
 Page 5  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
2.2. Threads 
  A thread is a software method used to reduce the process switch 
overhead  that  affects  the  system  performance.  A  thread  is  a  lighter 
process with a simplified status. In multithreading systems, a thread is 
the base scheduling entity. It has its own stack and hardware status. The 
thread switch implies saving and retrieving of hardware status and stack. 
The  thread  switch  between  threads  of  the  same  process  is  fast  and 
efficient because all other resources, except processor, are managed by 
the parent process. Unfortunately, the thread switch between threads of 
different processes involves process switch overhead. 
  Another  execution  entity  used  by  operating  system  derived  from 
UNIX  is  the  lightweight  process.  A  lightweight  process  has  a  minimal 
state  (hardware  status  and  stack)  and  shares  all  parent  process 
resources  except  processor.  Lightweight  processes  are  implemented  in 
IRIX  operating  system  for  Silicon  Graphics  multiprocessor  workstations 
and  in  DYNIX  operating  system  running  on  Sequent  Symmetry 
multiprocessors.  
 
2.3. Processes and Threads Creation 
  A process can be created dynamically by another running process 
using the fork operating system function. A new child process is created 
when invoking fork function. 
  The fork operation is used to divide a program in two sequences 
that  can  be  run  in  parallel.  The  child  process  will  execute  a  program 
segment and its parent will execute the other segment. Child and parent 
processes are executed from the same text segment. The child process, 
at creation time, receives a copy of parent data segment.   
  Using the exec operating function, it is possible to change the text 
segment of the child process. The parent and child processes will have 
different text and data segments.  
  The  parent  process  can  be  forced  to  wait  for  child  process 
execution end using the join function.  
  In UNIX, a process is identified by a process identifier (PID). The 
process identifier is a unique number in the system. Two processes will Page 6  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
have different values for process identifier. 
  Fork function creates a new process in UNIX operating system. The 
child process receives a copy of parent memory and shared access to all 
parent  opened  files.  So,  the  child  process  will  have  its  own  data  and 
stack segments but will be executed from parent text segment.  
  Exec function can be used in order to change the text segment for 
the  child  process.  In  such  a  way,  parent  and  child  processes  will  be 
executed from different text segments. Exec function arguments are the 
file  that  will  be  executed  by  the  child  process  and  command  line 
arguments of this file. 
  Wait function forces the parent to wait for child processes execution 
end. In some UNIX systems, fork and exec functions are grouped    
  Windows operating systems are thread oriented and they support 
multiprocessor shared memory architecture.   The elementary allocation 
entity  in  Windows  is  the  thread.  Every  process  contains  at  least  one 
thread (named main thread) and can create new threads that will share 
parent address space and resources (files, synchronization objects). 
  Programmers  can  access  operating  system  functions  through  API 
interface  (Application  Programming  Interface).  A  process  can  create  a 
new  child  process  using  CreateProcess  function.  The  result  of  using 
CreateProcess  function  is  equivalent  with  using  fork  and  exec 
combination in UNIX: a new process with a new text segment is created. 
The executable file that will be executed by the child process is passed to 
the function as parameter with command line arguments, if any.  
  If  the  child  process  successfully  created,  the  function  returns 
boolean value TRUE, otherwise the returned value will be FALSE. A new 
thread  can  be  created  using  CreateThread  function.  Before  using  the 
function  we  have  to  define  the  function  to  be  executed  by  the  new 
thread. This function will be passed as a parameter to CreateThread. 
 
3. Distributed operating systems 
  A distributed operating system is the extension for multiprocessor 
architectures of multitasking and multiprogramming operating systems. Page 7  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
  Multitasking  operating  systems  can  execute  concurrently  multiple 
processes  on  single  processor  computers  using  resources  sharing.  A 
running process is not allowed to access or to destroy another process 
data. 
Multiprogramming  operating  systems  supports  multitasking  and 
have the ability to protect memory and to control concurrent processes 
access to shared resources. A multiprogramming operating system is also 
a multitasking operating system. 
A distributed system is a set of computers that communicate and 
collaborate  each  other  using  software  and  hardware  interconnecting 
components.  Multiprocessors  (MIMD  computers  using  shared  memory 
architecture),  multicomputers  connected  through  static  or  dynamic 
interconnection  networks  (MIMD  computers  using  message  passing 
architecture) and workstations connected through local area network are 
examples of such distributed systems. 
A distributed system is managed by a distributed operating system. 
A  distributed  operating  system  manages  the  system  shared  resources 
used  by  multiple  processes,  the  process  scheduling  activity  (how 
processes are allocating on available processors), the communication and 
synchronization between running processes and so on. 
Multiprocessors  are  known  as  tightly  coupled  systems  and 
multicomputers as loosely coupled systems.  
The software for parallel computers could be also tightly coupled or 
loosely  coupled.  The  loosely  coupled  software  allows  computers  and 
users of a distributed system to be independent each other but having a 
limited possibility to cooperate. An example of such a system is a group 
of computers connected through a local network. Every computer has its 
own memory, hard disk. There are some shared resources such files and 
printers. If the interconnection network broke down, individual computers 
could  be  used  but  without  some  features  like  printing  to  a  non-local 
printer.  
At  the  opposite  side  is  tightly  coupled  software.  If  we  use  a 
multiprocessor  in  order  to  solve  an  intensive  computational  problem, 
every processor will operate with a data set and the final result will be 
obtained  combining  partial  results.  In  such  a  case,  an  interconnection 
network  malfunction  may  result  in  the  incapacity  of  the  computer  to Page 8  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
solve the problem.   
Combining loosely coupled software and tightly coupled hardware 
and  software  we  can  identify  four  distributed  operating  systems 
categories.  The  loosely  coupled  software  and  tightly  coupled  hardware 
case is not met in practice and therefore will not be covered below.  
 
3.1. Network Operating systems 
  Network operating systems represent the loosely coupled hardware 
and loosely coupled software case. A typical example of such a system is 
a set of workstations connected together through a local area network 
(LAN).  
Every workstation has its own operating system. Every user has its 
own workstation in exclusive use. A user can execute a login command in 
order to connect to another station and also can access a set of shared 
files maintained by a workstation named file server.  
There  are  only  a  few  processing  requirements  at  system  level. 
Processes executed over the network do not need to synchronize. 
The  network  operating  system  has  to  manage  individual 
workstations  and  file  servers  and  has  to  assure  the  communication 
between  them.  For  this  reason,  workstations  do  not  need  to  use  the 
same operating system.  
 
3.2. Real Distributed Operating Systems 
  A real distributed operating system is the case of tightly coupled 
software used on a loosely coupled hardware. 
  Because the interconnection network is transparent for the users, 
the set of computers appears like a single multitasking system and not 
like a set of independent computers.  
  The computers appear to the users like a virtual uniprocessor or 
like  a  single  system  image.  This  is  why  the  users  don’t  have  to  be 
concerned by the number of computers from the network.  No  system 
available on the market today entirely fulfills this requirement. 
  All computers from the network use the same operating system. Page 9  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
The same operating system kernel will run on every individual computer. 
The  local  kernel  will  manage  local  resources  like  virtual  memory  and 
process scheduling. 
A process has to be able to communicate with any other process, 
no matter if the second process is running on the same computer or on 
different  one.  This  is  why  the  operating  system  has  to  use  the  same 
system calls routines for all computers. The operating system has also to 
use  the  same  file  system  on  all  workstations  without  file  name  length 
limitations.  Also,  the  operating  system  has  to  implement  files  security 
and protection policies. 
 
3.3. Multiprocessing Operating Systems 
  Multiprocessing  operating  systems  represent  the  tightly  coupled 
software and tightly coupled hardware case.  
  A  multiprocessing  operating  system  is  acting  like  a  multitasking 
UNIX operating system but there are multiple processors in the system. 
  The main characteristic of multiprocessing operating systems is a 
single list of ready to run processes used. When a new process is ready 
to run it is added to the list located in the shared memory area. The list 
can be accessed by any process. When a processor is free, it extracts a 
process from the list and executes it.  
The  operating  system  has  to  implement  mutual  exclusion 
mechanisms (semaphores, monitors, locks or events using busy-wait or 
sleep-wait protocols) in order to protect the concurrent accesses to the 
list from shared memory. So, once a process has exclusive access to the 
ready  to  run  processes  list,  it  extracts  the  first  process  from  the  list, 
releases the list and executes the process. 
  The system also appears as a virtual uniprocessor for users and the 
same operating system is executed in every processing unit.  
 
 
 Page 10  Oeconomics of Knowledge, Volume 1, Issue 1, 3Q 2009 
 
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D. GROSS         
C.M. HARRIS 
Fundamentals of Queuing Theory, Wiley, New 
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A. SILBERSCHATZ  
P.B. GALVIN         
G. GAGNE 
Operating System Concepts, Wiley, 8 edition, 2009 
A. ROBBINS  UNIX in a Nutshell, O'Reilly Media, 2008 
J. BOYCE  Windows 7 Bible, Wiley, 2009 
   

Parallel Processing, Economic Publishing House,

The Impact of Parallel Processing on Operating Systems

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