Chapter 2.1 : Processes

1
Chapter 2.1 Processes

• Process concept
• Process scheduling
• Interprocess communication
• Threads

2
What is a Process?

• A process (task) is the activity resulting from
  the execution of a program with its data on a
  sequential processor
• A process is an abstract form of a program

3
Sequential Execution

• Programs are run one after the other (run to
  completion)

4
Concurrent Execution

• All programs execute concurrently (share the time
  of the CPU)

5
How to Implement Concurrency?

• Separate a program into code, data and
  environment (Process table) segments
• Process table contains all the necessary data to
  stop and restart a process at any time

6
Process Table (PCB - Process Control Block)

• Contents of registers
• Program counter
• PSW (Program Status Word)
• Process state
• Pointers to code and data
• Process identifier (PID)
• Process priority
• File descriptors
• Pointer to the parent of the process
• Pointers to all children of the process
• The processor it is running on

7
Switching Processes

• Save the real world of the currently active
  process in process table
• Restore the real world from the process table
  of the selected process (the one to be executed
  next)

8
System Process Tables
Process Tables
Code
Process
Data
Stack
9
Shared Code

• The shared code must be re-entrant (ie., it must
  not modify itself)

10
Process States
PREEMPT
Create
Ready
SCHEDULE
CONTINUE
Blocked
SUSPEND
Terminate
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Process States

• Create
• A process is created by an existing process
  (parent/child) a process is created by a
  service, or as a new batch job, or an interactive
  logon
• Terminate
• Normal completion (exit) External completion
  (forced completion by operator, by parent)
  Internal completion (error, time overrun)
• Schedule
• A process is selected out of the ready queue (is
  scheduled) and dispatched to the running state
• Preempt
• The running process is preempted because some
  other process of higher priority has become ready
  (or it yields)

12
Process States

• Suspend
• A process starts a time consuming IO operation,
  or is swapped out, or makes a spontaneous request
  to sleep, or is blocked in a synchronization
  operation. When a process is suspended, it is on
  some queue, namely the queue of all the processes
  waiting for the same resource. Processes during
  their life move from queue to queue, with stays
  in the running state
• Continue
• The inverse of Suspension the resource requested
  becomes available (timer, IO completes,
  unblocking)

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How to Create New Processes? Fork Exec

• fork
• create a new process that is a copy of current
  one
• exec
• change the program that a process is executing
• Some systems combine the two into one

14
A Simple Shell

• void main()
• int child
• if ( (child fork())-1 )
• return( -1 )
• else
• if ( child0 )
• printf( "This is the child executing\n")
• exit(1)
• printf( "This is the parent\n" )

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Parent Child Relationship

• root is the parent
• p1, p2 and p3 are children of root process
• p4 and p5 are children of p3