Concurrency Introduction

• Perform two activities at the same time

  • Conceptually different

• e.g. A program which executes a long-running task

  • Fetching data over the network
  • Performing a lengthy computation

• The program displays "feedback" to the user

  • "Hourglass" cursor
  • Progress counter

• The task and the feedback are performed at the same time

Concurrency and Operating Systems

• Standard in operating systems for many years
• Computers run many programs at the same time
  • Compiler generates code
  • Word processor waits for the next keystroke
  • Mail program fetches a new email

Hardware Concurrency

• Modern computers have multiple processors

  • Multiple CPU chips
  • CPU chips which contain multiple processor "cores"

• Different processors can perform different activities at the same time

• Even within the same program

• These are known as "hardware threads"

  • Each processor follows its own "thread of execution" through the code
  • 1 hardware thread <=> 1 processor core

Software Concurrency

• Modern operating systems support "software threading"
• A program can perform multiple activities at the same time
  • These activities are managed by the operating system
• Typically there are more software threads than hardware threads
  • Some threads may have to stop and wait for something
  • While they are waiting, another thread which is ready can run