Overview

What is a Thread?

• Thread is a basic unit of CPU utilization
• Comprised of a thread ID, program counter, register set, and stack.
• Shares code section, data section, and other OS resources (eg. open files, signals) with sibling threads

Motivation

• An application may be required to perform several similar tasks
• For example: a busy web server has many clients concurrently making requests
• Running as a single-threaded process only allows 1 client to be serviced at a time
• Solution: one process listens for requests, fork process for each incoming request
  • Not best solution
  • Process creation is time consuming and resource intensive
• Better solution: use multithreaded process instead
  • Server has one thread listening for requests
  • When server accepts a request, create a new thread to service that request

Benefits

• Responsiveness
  • Evident in interactive applications
  • User clicks a button to perform a heavy computation. One thread keeps the UI responsive while another thread performs the computation asynchronously
• Resource sharing
  • Threads share the resources of belonging process by default
  • Processes can only share resources through special techniques like shared memory, message passing
• Economy
  • Threads share resources of belonging process, reducing overhead
  • Thread creation takes less time and uses less memory than process creation
  • Context switching between threads is faster than processes
• Scalability
  • Scales well to multiprocessor system—threads can run in parallel on different CPUs
  • Single-threaded application in the same environment can still only run on 1 CPU

Multicore Programming

• Single-CPU systems evolved into multi-CPU systems
  • each CPU has 1 core
• Multi-CPU evolved into multicore systems
  • each processing chip has multiple cores
  • each core appears as a separate CPU to the OS