When a new G is created or an existing G becomes runnable, it is pushed onto a list of runnable goroutines of current P. When P finishes executing G, it first tries to pop a G from own list of runnable goroutines; if the list is empty, P chooses a random victim (another P) and tries to steal a half of runnable goroutines from it.

Syscalls/M Parking and Unparking

When an M creates a new G, it must ensure that there is another M to execute the G (if not all M’s are already busy). Similarly, when an M enters syscall, it must ensure that there is another M to execute Go code. There are two options, we can either promptly block and unblock M’s, or employ some spinning. Here is inherent conflict between performance and burning unnecessary CPU cycles. The idea is to use spinning and do burn CPU cycles. However, it should not affect programs running with GOMAXPROCS=1 (command line utilities, appengine, etc). Spinning is two-level: (1) an idle M with an associated P spins looking for new G’s, (2) an M w/o an associated P spins waiting for available P’s. There are at most GOMAXPROCS spinning M’s (both (1) and (2)). Idle M’s of type (1) do not block while there are idle M’s of type (2). When a new G is spawned, or M enters syscall, or M transitions from idle to busy, it ensures that there is at least 1 spinning M (or all P’s are busy). This ensures that there are no runnable G’s that can be otherwise running; and avoids excessive M blocking/unblocking at the same time. Spinning is mostly passive (yield to OS, sched_yield()), but may include a little bit of active spinning (loop burnging CPU) (requires investigation and tuning).


There are two different run queues in the Go scheduler: the Global Run Queue (GRQ) and the Local Run Queue (LRQ). Each P is given a LRQ that manages the Goroutines assigned to be executed within the context of a P. These Goroutines take turns being context-switched on and off the M assigned to that P. The GRQ is for Goroutines that have not been assigned to a P yet. There is a process to move Goroutines from the GRQ to a LRQ that we will discuss later.