Goroutines切换时，CPU上下文会发生什么？

Question

If I correctly understand how goroutines work on top of system threads - they run from queue one by one. But does it mean that every goroutine loads\unloads it's context to CPU? If yes what's difference between system threads and goroutines?

The most significant problem is time-cost of context-switching. Is it correct?

What mechanism lays under detecting which data was requested by which goroutine? For example: I am sending request to DB from goroutine A and doesn't wait for response and at the same time occurred switch to a next goroutine. How system understands that a request came from A and not from B or C?

Answer 1

Goroutines, memory and OS threads

Go has a segmented stack that grows as needed. Go runtime does the scheduling, not the OS. The runtime multiplexes the goroutines onto a relatively small number of real OS threads.

Goroutines switch cost

Goroutines are scheduled cooperatively and when a switch occurs, only 3 registers need to be saved/restored - Program Counter, Stack Pointer and DX. From the OS's perspective Go program behaves as event-driven program.

Goroutines and CPU

You cannot directly control the number of threads that the runtime will create. It is possible to set the number of processor cores used by the program by setting the variable GOMAXPROCS with a call to runtime.GOMAXPROCS(n).

Program Counter

and a completely different story

In computing, a program is a specific set of ordered operations for a computer to perform. An instruction is an order given to a computer processor by a program. Within a computer, an address is a specific location in memory or storage. A program counter register is one of a small set of data holding places that the processor uses.

This is different story how programs work and communicate with each other and it doesn't directly related to a goroutines topic.

Sources:

• http://blog.nindalf.com/how-goroutines-work/
• https://gobyexample.com/goroutines
• http://tleyden.github.io/blog/2014/10/30/goroutines-vs-threads/
• http://whatis.techtarget.com/definition/program-counter