CODEWITHSUNDEEP
multithreadingmultiprocessing
Benoit Sanchez
Benoit Sanchez

Reputation: 711

Context switch: what decides when?

I'm looking for some background explanations about context switch in modern personal computers with mainstream architecture (say x64).

While context switch is mainly done by the hardware, I wonder what in the computer decides of task scheduling and context switch when running multiple threads and/or multiple processes. Is it the CPU itself, the operating system, the compiler/virtual machine... ?

I'd like to have an idea of what strategies are used to decide when to switch. For example, if I start a hundred threads doing independent dummy additions in endless loops, when will the context switches happen?

Upvotes: 0

Views: 622

Answers (1)

sagneta
sagneta

Reputation: 1691

This is a complex subject that I can't do justice in a simple response here. But let me hit some high-points. I further am going to assume modern OS's like Windows or the various Unix derivatives and ignore embedded real-time systems.

The context switch is not performed in hardware. It is critical to understand this. It is performed in software via a OS subsystem known as the scheduler. The scheduler is a glorified interrupt controller that will fire many times a microsecond and decide what thread will execute next. The algorithms for doing so are numerous and the subject of many a PHD thesis. A good overview I found quickly is here: http://www.studytonight.com/operating-system/cpu-scheduling

Good Operating Systems books will go over this in detail. There are too many to note so pick your poison.

One last point, to grasp at a complete level how scheduling is performed it really helps to understand how virtual addressing schemes work as that is truly what differentiates processes from threads. Threads are what is critical in terms of the Scheduler put processes encapsulate threads and the virtual memory space.

I'm not sure this helps but I was at least able to correct one misconception and point you at a simple article on OS thread scheduling.

Upvotes: 4

Related Questions