Learn what sync.Cond is

Overview

This repo is not a library. It is a reference implementation of "hint" for the programmers who are trying to understand how sync.Cond works, how it is used in the real world.

I'll leave a brief explanation in this README, but I would strongly recommend you to read my full article to get better understanding instead of just looking at this repository.

This repository contains a working code which implements a size-limited-queue. First, let me describe the spec of it:

• The queue can contain only int values.
• The queue supports Push and Pop. Like the common queue data structure, the order is FIFO.
• The queue supports size capacity feature. The queue can contain elements up to the capacity.
• When trying to push an element to the queue when the queue is full, it blocks until it gets at least a space.
• When trying to pop an element from the queue when the queue is empty, it blocks until it gets at least an element.

There are three implementations described below:

• single_thread_slqueue.go
• mutex_slqueue.go
• slqueue.go

slqueue.go has simpler implementation at the old revision. 176eb78

single_thread_slqueue.go

single_thread_slqueue.go

It works basically, but doesn't work correctly under multithread environment. The length check in Push / Pop and queue manipulation (append / moving the queue head to pop) must be atomic, but this implementation does not consider it. As a result,

• Queue capacity is 10, now the Queue length is 9
• Goroutine A and B tries to push an value
  • A checks the queue capacity, then it is 9
  • At almost the same time, B checks the queue capacity, then it is 9 because A still have not finished the queue manipulation (append)
  • A appends an element, now the queue length is 10
  • B appends an element, now the queue length is 11 <- violates the queue spec!

I implemented single_thread_slqueue.go to compare it with upcoming mutex_slqueue.go.

mutex_slqueue.go

mutex_slqueue.go

This uses sync.Mutex to make the queue length check and queue manipulation atomic. This actually works following all the spec, but the problem is its inefficiency.

func (s *MutexQueue) Push(i int) {
	s.mu.Lock()
	for len(s.queue) == s.capacity { // 1
		s.mu.Unlock()
		runtime.Gosched()
		s.mu.Lock()
	}

	s.queue = append(s.queue, i)
	s.mu.Unlock()
}

See 1 in the code. Because of the spec When trying to push an element to the queue when the queue is full, it blocks until it gets at least a space, it needs spin (for-loop) to achieve it. However, spin is usually inefficient. Using sync.Cond, it can be improved to get more efficient.

slqueue.go - simple

slqueue.go

This uses sync.Cond to make mutex implementation better. See the implementation:

func (s *SizeLimitedQueue) Push(i int) {
	s.cond.L.Lock()
	for len(s.queue) == s.capacity { // 1
		s.cond.Wait() // 2
	}

	s.queue = append(s.queue, i)

	s.cond.Broadcast() // 3
	s.cond.L.Unlock()
}

func (s *SizeLimitedQueue) Pop() int {
	s.cond.L.Lock()
	for len(s.queue) == 0 {
		s.cond.Wait()
	}

	ret := s.queue[0]
	s.queue = s.queue[1:]
	s.cond.Broadcast()
	s.cond.L.Unlock()

	return ret
}

When trying to Push, it first check the length (at 1) then if the condition is not met, call cond.Wait() (at 2) . In the wait, runtime will suspend the waiting goroutine and waits for the "notification" on the cond. When a goroutine calls cond.Broadcast(), the cond is notified - waiting goroutines are waken up then goes to the top of for-loop.

This has an advantage because it does not require spins.

slqueue.go - improved

slqueue.go

On the above slqueue.go, some optimizations are applied. They are described more specifically on my article.

For readers

I'm quite sure this README is not enough to understand sync.Cond. This is just a brief "summary".

To understand sync.Cond better, I'd say you have to understand a synchronization primitive "Condition Variable" in POSIX. sync.Cond is actually just a Go version of it.

I described what "Condition Variable" is, and the detailed description of above code, also some supplementary information about it in my article. I would recommend reading it with the actual source code in this repository.

If you find this repo helpful to learn sync.Cond, please leave a star!