COMP3891

Processes and Threads

We often require an operating system to be able to:

• Interleave execution of several processes to maximise processor utilisation while providing reasonable repsonse time.
• Allocate resources to processes.
• Support interprocess communication and user creation of processes.

Overview of Process Model

Processes

• Also called “task” or “job”.
• Execution of an individual program.
• Owner of resources allocated for program execution.
• Encompasses one or more threads.

Threads

• Unit of execution.
• Can be traced
  • List the sequence of instructions that execute.
• Belongs to a process
  • Executes within it.

Multiprogramming

• One physical program counter switches between processes.
• Conceptual model of multiple independent, sequential processes (with a single thread each).
• Only one program active at any instant on CPU (pseudoparallelism).

Implementation of Processes

A processes’ information is stored in a process control block (PCB). PCBs together form a process table.

Example Process Table Entry Fields

|Process Management|Memory Management|File Management| |–|–|–| |Registers|Pointer to text segment|Root directory| |Program counter|Pointer to data segment|Working directory| |Program status word|Pointer to stack segment|File descriptors| |Stack pointer||File descriptors| |Process state||User ID| |Priority||Group ID| |Scheduling parameters||| |Process ID||| |Parent process||| |Process group||| |Signals||| |Time when process started||| |CPU time used||| |Children’s CPU time||| |Time of next alarm|||

Lifecycle of a Process

Process Creation Causes

• System initialisation:
  • Foreground processes (interactive programs).
  • Background processes.
    • Email server, web server, print server, etc.
    • Called a daemon (Unix) or service (Windows).
• Execeution of a process creation system by a running process.
  • New login shell for an incoming SSH connection.
• Users request to create a new process.
• Initiation of a batch job.

Process Termination Conditions

• Normal exit (voluntary).
• Error exit (voluntary).
• Fatal error (involuntary).
• Killed by another process (involuntary).

Process/Thread States

• Running (actually using CPU at that instant)
• Blocked (unable to run until some external event happens)
• Ready (runnable; temporarily stopped to let another process run)

Events That Cause Process Transitions

• Ready to Running:
  • All other processes have ran enough (scheduler decides to pick this process to run).
• Running to Ready:
  • Voluntary Yield().
  • End of timeslice (scheduler decides running process has run long enough).
• Running to Blocked:
  • Waiting for input:
    • File or network, etc.
  • Waiting for a timer (alarm signal).
  • Waiting for a resource to become available.
• Blocked to Ready:
  • Resource on which process was originally waiting on becomes available.

Scheduler

Scheduler decides which of the Ready processes to run. There are various algorithms for scheduling.

Two Queue Algorithm

• Queue for ready events and for blocked ev-ents.

• Avoids scanning list of processes to select one to make ready.

  

Multi Queue Algorithm

• Having one blocked queue is inefficient as one resource being blocked has no effect on another resource.

• Use a (shorter) blocked queue for each resource.

  

Thread Model

Thread vs Process

|Per process items|Per thread items| |-|-| |Address space|Program counter| |Global variables|Registers| |Open files|Stack (local variables)| |Child processes|State| |Pending alarms|| |Signals and signal handlers|| |Accounting information||

Thread Usage

|Model|Characteristics| |-|-| |Threads|Parallelism, blocking system calls| |Single-threaded process|No parallelism, blocking system calls| |Finite-state machine|Parallelism, nonblocking system calls, interrupts|

Why Threads?

• Simpler to program than state machine.
• Less resources than a complete process:
  • Cheaper to create and destroy.
  • Share resources (especially memory) between them.
• Performance: Threads waiting for I/O can be overlapped with computing threads.
  • Note: if all threads are compute bound, then no performance improvement on a uniprocessor.
• Threads can take advantage of parallelism available on machines with multiple processors.