OS thread or machine

Initially, the OS threads or machines are created by and managed by the OS. Later on, the scheduler can request for more OS threads or machines to be created or destroyed. It is the actual resource upon which a goroutine will be executed. It also maintains information about the main goroutine, the G currently being run on it, thread local storage (tls), and so on:

// Denoted as M in runtime 
type m struct { 
    g0               *g         // goroutine with scheduling stack 
    tls               [6]uintptr // thread-local storage (for x86 extern register) 
    curg            *g         // current running goroutine 
    p                 puintptr   // attached p for executing go code (nil if not executing go code) 
    id                 int32 
    createstack [32]uintptr // stack that created this thread. 
    spinning      bool        // m is out of work and is actively looking for work 
 
    // ... 
}