CODEWITHSUNDEEP
goconcurrency
user2494770
user2494770

Reputation:

Is sync.Mutex.Lock a FIFO?

If many threads lock on a mutex do they queue up in a FIFO order or is there some amount of randomness to which goroutine acquires the lock when it becomes unlocked?

Upvotes: 8

Views: 3681

Answers (1)

user2494770
user2494770

Reputation:

From the source:

// Mutex fairness.
//
// Mutex can be in 2 modes of operations: normal and starvation.
// In normal mode waiters are queued in FIFO order, but a woken up waiter
// does not own the mutex and competes with new arriving goroutines over
// the ownership. New arriving goroutines have an advantage -- they are
// already running on CPU and there can be lots of them, so a woken up
// waiter has good chances of losing. In such case it is queued at front
// of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
// it switches mutex to the starvation mode.
//
// In starvation mode ownership of the mutex is directly handed off from
// the unlocking goroutine to the waiter at the front of the queue.
// New arriving goroutines don't try to acquire the mutex even if it appears
// to be unlocked, and don't try to spin. Instead they queue themselves at
// the tail of the wait queue.
//
// If a waiter receives ownership of the mutex and sees that either
// (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
// it switches mutex back to normal operation mode.
//
// Normal mode has considerably better performance as a goroutine can acquire
// a mutex several times in a row even if there are blocked waiters.
// Starvation mode is important to prevent pathological cases of tail latency.

So guaranteeing order across threads cannot be achieved by simply using a mutex.

Upvotes: 11

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