What is runtime and how does the scheduler work?

The runtime in Go is the environment in which Go programs are executed. It includes:

• Memory management: Garbage collector automatically manages memory allocation and deallocation.
• Scheduler: Manages the execution of goroutines on operating system threads.
• System calls: Provides an interface for interacting with the operating system.
• Synchronization primitives: Channels, mutexes, and other mechanisms for coordinating goroutines.

The Go scheduler implements the M:N model (multiple goroutines on multiple OS threads). Its main components:

• G (Goroutine): A lightweight thread of execution, essentially an abstraction over a function.
• M (Machine): An OS thread on which Go programs run.
• P (Processor): A logical processor associated with an OS thread. It holds a local runnable queue of goroutines.

Scheduler working principle:

1. Goroutines (G) are created and added to a global or local runnable queue.
2. OS threads (M) are associated with logical processors (P). The default number of P equals the number of CPU cores (GOMAXPROCS).
3. P takes a goroutine from its local queue. If the local queue is empty, P tries to "steal" goroutines from another P's queue or from the global queue.
4. M executes the goroutine.
5. When a goroutine blocks (e.g., reading from a channel with no data or performing a system call), M detaches from P, and P becomes available to run another goroutine. The blocked goroutine is put into waiting. When the block is released, the goroutine becomes runnable again and is re-queued.
6. If M blocks in a system call, it does not hold P. A new M can be created or an existing one can be used so that P can continue executing other goroutines.

This approach allows efficient use of CPU resources and easy scaling of parallel tasks, hiding the complexity of OS thread management from the developer.

// Example of creating a goroutine
func myFunc() {
	// Goroutine code
}

func main() {
	go myFunc() // Launch goroutine
	// Main program continues execution in parallel
}