Demystifying Go's Scheduler: How Goroutines, M, P, and G Work Together

1. Basics

Go's runtime manages scheduling, garbage collection, and the execution environment for goroutines. The scheduler maps goroutines to OS threads. Each goroutine is represented by a G struct that tracks its stack and state. Logical processors, called P, hold queues of runnable Gs, and OS threads, called M, execute the Gs assigned to a P.

The number of logical processors can be set with runtime.GOMAXPROCS(numLogicalProcessors), after which the runtime handles scheduling automatically.

2. The Dance Between Ms, Ps & Gs

The interaction among Ms, Ps, and Gs is complex. A global run queue exists in the schedt structure, but each P maintains its own local run queue of Gs.

When a new goroutine is created (e.g., via go func()), it is placed on the P's queue. If a M finishes executing a G and its P's queue is empty, it randomly steals half of the runnable Gs from another P.

Blocking system calls are intercepted; the runtime detaches the M from its P and creates a new OS thread if needed. When the system call returns, the goroutine is placed back on a local run queue and the thread may become idle.

Network calls are handled similarly, using Go's integrated network poller.

If the current goroutine blocks, the runtime runs a different goroutine. Typical blocking events include:

Blocking system calls (e.g., opening a file)

Network input

Channel operations

Synchronization primitives from the sync package

3. Scheduler Tracing

Go allows tracing of the runtime scheduler via the GODEBUG environment variable.

$ GODEBUG=scheddetail=1,schedtrace=1000 ./program

Example output:

SCHED 0ms: gomaxprocs=8 idleprocs=7 threads=2 spinningthreads=0 idlethreads=0 runqueue=0 gcwaiting=0 nmidlelocked=0 stopwait=0 ...
  P0: status=1 schedtick=0 syscalltick=0 m=0 runqsize=0 gfreecnt=0
  P1: status=0 ...
  M0: p=0 curg=1 mallocing=0 throwing=0 preemptoff= locks=1 dying=0 helpgc=0 spinning=false blocked=false lockedg=1
  G1: status=8() m=0 lockedm=0

For most use‑cases you can simplify the command: $ GODEBUG=schedtrace=1000 ./program An advanced tool, go tool trace, provides a UI to explore program execution in detail.