Operating Systems  

  Concurrent Processes 

 
 

 
  Activation of one or more processes at the same time.
  Two types of concurrency normally discussed :  
   Apparent or Pseudo Concurrency  : A single hardware processor
is switched between processes by interleaving.  
 
Over a period of time this gives the    illusion  of concurrent
execution of the processes.  
 
   Real Concurrency  : Multiple hardware processors are present in
the machine, and the OS can schedule tasks onto the processors.  
 
Usually, processors are much less than the number of processes.
 

 


  Example of Concurrent Activity 

 

   Processes   
 
Activities being performed :   
 
   Order Clerk   
   Bagger   
   Cook   
   Cashier   
   Customers   

 

   Shared Communication Areas   
 
   Order list turntable   
   Cooked Hamburger bin   
   Checkout counter   

 

  Real Time Operating Systems 

  

 

 
  System must respond to external stimuli within a
   finite  and    specified time .
     Correctness  depends not only on    logical result  BUT
also on    response time .
     Problem  :    Shedule all activities to meet some requirements .
     Hard Real Time  : Absolutely imperative that responses occur
within specified deadlines. Late result   failure.
     Soft Real Time  : Response times are important, but system will
still function if deadlines are missed.
     Embedded System  : Systems which are interfaced to some external
physical equipment, and perform monitoring and control.  
The computer is    PART  of the overall system, and hence Real Time.
 

Real Time systems are used for :  
   Hard  :     Nuclear Power Stations, Flight Control,  Missile Guidance   
   Soft  :     Train or Airline Reservation   
   Embedded  :    Washing Machine, Chemical plant control etc   
 
   Distinction between OS and R/T systems are quite blurred 

 

  Distributed and Parallel Operating Systems 

 

 

 
  Facilitate use of geographically distributed resources.
  Supports communication between a job or between different jobs.
     Information  sharing is of most significance.
  Support for parallel applications wishing to obtain speedup of
computationally intensive tasks. 
  Supports synchronisation of activities to coordinate sharing
of information.
  Use of primitives defined within OS, such as RPC (Remote Procedure Call)
to support distributed activities.
  Efficient Communication primitives between activities also 
supported.
 

 

   Move towards the development of Object Oriented (OO) Methodologies or  
   OO Frameworks to support distributed applications 

 

  Non Determinism in Operating Systems  

 

   Operating and Real Time systems are non-deterministic, in that
they must respond to events that occur in an unpredictable order, and
at unpredictable times. Such as results generated from I/O devices. 

 

 
  The way an Operating system switches between processes cannot be
pre-determined, since the events which cause the switches are not
deterministic.
  The interleaving of instructions, executed by a processor, from a 
set of processes is non-deterministic
 

 

The    Events or Interrupts  cause process switches, such as
   an I/O completion interrupt causing the OS to switch to an
I/O handling process .