Processes & Threads

Thread:

A thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler (typically as part of an operating system).

Scheduler:

scheduling is the method by which threads, processes or data flows are given access to system resources (e.g. processor time, communications bandwidth). This is usually done to load balance and share system resources effectively or achieve a target quality of service. The need for a scheduling algorithm arises from the requirement for most modern systems to perform multitasking (executing more than one process at a time) and multiplexing (transmit multiple data streams simultaneously across a single physical channel).

The scheduler is concerned mainly with:

• Throughput - The total number of processes that complete their execution per time unit.
• Latency, specifically:
  • Turnaround time - total time between submission of a process and its completion.
  • Response time - amount of time it takes from when a request was submitted until the first response is produced.
• Fairness - Equal CPU time to each process (or more generally appropriate times according to each process' priority and workload).
• Waiting Time - The time the process remains in the ready queue.

The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is a component of a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources. In particular, the threads of a process share the latter's instructions (its code) and its context (the values that its variables reference at any given moment).