Concurrency

About

Concurrency is a simulation of running tasks at the "same time".

It happens when tasks run on the same core, or different virtual/logical cores.

Each CPU physical core has a control unit, that switches between the logical cores when needed. (Enabled by Hyper-Threading (HT) / Simultaneous Multi-Threading (SMT))

This way the CPU can utilize idle execution units more efficiently. (e.g, thread stalled when waiting for memory)

Also, the O.S scheduler doesn't distinguishes between physical or logical cores. It sees them all as physical independant cores, when it is assigning o.s threads to them.

Thread

A thread is like the smallest execution unit.

Each thread executes a sequence of intructions and have a context, that stays in the CPU registers while the thread is executing in the CPU.

The more threads try to concurrently execute, the slower it will be, because of all this context switching.

Time Sharing

Context Switch

Context switching is the switch of contexts, in such a speed, that it looks like they are running on the same time.

Context

A context is what defines the work environment of a process. (meaning its memory allocation basically)

The machine offers a pre-defined amount of time (time-slice) to each context to run.

Scheduler (O.S)

The o.s scheduler is the task that will manage the context switch, to remove, contexts that do not cooperate and hang on the CPU.