Go channels

For communication, synchronization between goroutines.

• https://www.youtube.com/watch?v=KBZlN0izeiY
• https://github.com/golang/go/blob/master/src/runtime/chan.go#L253

What happens when create a channel

// Buffered channel
ch := make(chan int, 3)
// UnBuffered channel
ch := make(chan int)

Allocate an hchan struct on the heap returns a pointer of it

Sends and receives

What happens when sending data to channel

1. Acquire the lock
2. Enqueue (A memory copy)
3. Release the lock

What happens when receiving data from channel

1. Aquire the lock
2. Dequeue (A memory copy)
3. Release the lock

Making a memory copy makes the goroutine memory safe. (No shared memory except for hchan)

Senders

If there are waiting receivers

• Dequeue receiver and pass value directly to receiver
• Golang calls goready to invoke the waiting receiver

if sg := c.recvq.dequeue(); sg != nil {
    // Found a waiting receiver. We pass the value we want to send
    // directly to the receiver, bypassing the channel buffer (if any).
    send(c, sg, ep, func() { unlock(&c.lock) }, 3)
    return true
}

If there are no waiting receivers

What happens when buffer is not full

Enqueue the value into buffer

if c.qcount < c.dataqsiz {
    // Space is available in the channel buffer. Enqueue the element to send.
    qp := chanbuf(c, c.sendx)
    if raceenabled {
        raceacquire(qp)
        racerelease(qp)
    }
    typedmemmove(c.elemtype, qp, ep)
    c.sendx++
    if c.sendx == c.dataqsiz {
        c.sendx = 0
    }
    c.qcount++
    unlock(&c.lock)
    return true
}

What happens when buffer is full

• Create sudog instance with current goroutine info and enqueue into sendq

gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
    mysg.releasetime = -1
}
// No stack splits between assigning elem and enqueuing mysg
// on gp.waiting where copystack can find it.
mysg.elem = ep
mysg.waitlink = nil
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.waiting = mysg
gp.param = nil
c.sendq.enqueue(mysg)

• Golang calls gopark to put current goroutine to be waiting
• Dequeue next goroutine from runQ to process

The goroutine is blocked, but no OS thread, so that OS thread can still process other goroutines

Receivers

If there are waiting senders

if sg := c.sendq.dequeue(); sg != nil {
    // Found a waiting sender. If buffer is size 0, receive value
    // directly from sender. Otherwise, receive from head of queue
    // and add sender's value to the tail of the queue (both map to
    // the same buffer slot because the queue is full).
    recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
    return true, true
}

• Dequeue the waiting sender
• Either pass the value directly to receiver OR enqueue the value to buffer, receiver reads the value from the head of buffer
• Golang calls goready to invoke the waiting sender

If there are no waiting senders

• If buffer is not empty, read from the buffer
• If buffer is empty, and no waiting senders
  • Create sudog instance with current goroutine info and enqueue into recvq
  • Golang calls gopark to put current goroutine to be waiting
  • Dequeue a goroutine from runQ to process