Processes

Processes

1. Lifecycle

   • User causes process start
     • Single
     • Multi
       • Pseudo parallelism
         • Multiple processes appear to run concurrently
   • Process enters ready list
     • Moves to head
       • Next process to be executed
     • Ordered by scheduling algorithm
     • Highest priority nearest head
   • Program pauses
     • Program ends
       • Completion
       • Crash
         • Segmentation fault
       • Termination
         • Memory and resources released
     • Blocked
     • Pre-empted
   • Teardown
     • Process states saved
       • Saved in RAM / disk
         • 1st 1GB of memory reserved for kernel
   • Executes next process at head of ready list
     • Until block/pre-empt/crash/termination

2. Execution Rings

   • User mode
     • Restricted set of instructions
   • Kernel mode
     • Access to all hardware/memory

3. Process Management

   • Scheduling algorithms
     • Dictates order of ready list
     • Pre-Emptive
       • SJRF
       • RR
       • PQ
     • Non-Pre-emptive
       • FCFS
       • SJF
       • PP
   • Blocking
     • Moved to block list
       • Waits on user input / access to memory
       • Waits for I/O resource
   • Pre-emption
     • OS gives each process a quantum of CPU time
       • Amount of time until process is moved to tail of ready list
     • Context switching
       • Registers etc of current process saved
         • Machine register values
         • File table of open files
         • System call state
         • Accounting info
       • Registers etc of next process loaded

4. Creation

   • Forking
   • Program loaded
   • Memory allocated to code/data/stack
   • Program addresses mapped to actual addresses

5. Execution

   • Next instruction in EIP
   • Procedure call causes return add & params to be pushed onto stack
   • Entry point loaded into EIP

6. Threads

   • Multi-Threading
     • Better responsiveness - aspects of process can continue
     • Shares memory - reduces demand on resources
     • More efficient use of multiple CPUs

7. Process Control Block

   • Details of each process stored
     • PID
       • Uniquely identifies each process to the OS
   • Status
   • Scheduling parameters
   • Pointer to memory image