🪴 Quartz 4.0

Go Channels Internals

Oct 10, 20247 min read

Overview

At a high level a channel is a pointer to the struct with a buffer, two wait queues and a lock

The hchan Struct

A channel is a pointer to a hchan struct allocated on the heap:

type hchan struct {
	qcount   uint           // total data in the queue
	dataqsiz uint           // size of the circular queue
	buf      unsafe.Pointer // points to an array of dexataqsiz elements
	elemsize uint16
	closed   uint32
	timer    *timer // timer feeding this chan
	elemtype *_type // element type
	sendx    uint   // send index
	recvx    uint   // receive index
	recvq    waitq  // list of recv waiters
	sendq    waitq  // list of send waiters
 
	// lock protects all fields in hchan, as well as several
	// fields in sudogs blocked on this channel.
	//
	// Do not change another G's status while holding this lock
	// (in particular, do not ready a G), as this can deadlock
	// with stack shrinking.
	lock mutex
}
 
// waitq represents stores a taild and a head of the waiters list
type waitq struct {
	first *sudog
	last  *sudog
}

The main fields are:

  • buf a circular FIFO queue storing elements of the channel (sendx and recvx are indexes to head and tail)
  • recvq a linked list of goroutines represented by sudog struct waiting for receiving data from the channel
  • sendq a linked list of goroutines represented by sudog struct waiting to send data on the channel
  • lock a runtime version of a mutex protecting fields in hchan and sudog

Enqueueing an element to the buf causes a copy of the value to be added to the buf. Dequeuing an element from the buf causes a copy of the buf element to be assigned to the variable. It provides channels memory safety guarantees, as the only shared memory is a hchan protected by a lock

The sudog Struct

Represents a goroutine (pseudo-goroutine) stored in a wait list:

// sudog (pseudo-g) represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog to allocate and free them.
type sudog struct {
	// The following fields are protected by the hchan.lock of the
	// channel this sudog is blocking on. shrinkstack depends on
	// this for sudogs involved in channel ops.
 
	g *g
 
	next *sudog
	prev *sudog
	elem unsafe.Pointer // data element (may point to stack)
 
	// The following fields are never accessed concurrently.
	// For channels, waitlink is only accessed by g.
	// For semaphores, all fields (including the ones above)
	// are only accessed when holding a semaRoot lock.
 
	acquiretime int64
	releasetime int64
	ticket      uint32
 
	// isSelect indicates g is participating in a select, so
	// g.selectDone must be CAS'd to win the wake-up race.
	isSelect bool
 
	// success indicates whether communication over channel c
	// succeeded. It is true if the goroutine was awoken because a
	// value was delivered over channel c, and false if awoken
	// because c was closed.
	success bool
 
	// waiters is a count of semaRoot waiting list other than head of list,
	// clamped to a uint16 to fit in unused space.
	// Only meaningful at the head of the list.
	// (If we wanted to be overly clever, we could store a high 16 bits
	// in the second entry in the list.)
	waiters uint16
 
	parent   *sudog // semaRoot binary tree
	waitlink *sudog // g.waiting list or semaRoot
	waittail *sudog // semaRoot
	c        *hchan // channel
}

The main fields are:

  • g a pointer to the waiting goroutine
  • elem an element g is waiting to send/receive
  • next/prev pointers to the next/previous g in a wait list hchan.recvq/hchan.sendq

Channel Send

Send on a Channel

Actions when G sends on a non-full and non-empty channel:

  1. Acquire the lock
  2. Enqueue the element to the buf. It stores a copy of the element in the buf
  3. Release the lock

Send on a Full Channel

Actions when G sends on a full channel (blocking call):

  1. Acquire the lock
  2. Create a sudog for ourselves with element we are waiting to send and enqueue it to the sendq. A receiver will use it to resume us in the future
  3. Make a gopark call to the scheduler and release the lock
  4. Scheduler changes G state from running to waiting and removes association between G and M
  5. Scheduler dequeues the runnable G' from the run queue and schedules it onto M

Essentially a user-level context switch, we blocked the G but not the OS thread M that started running a G'

Send on an Empty Channel

Actions when G sends on an empty channel and there is a blocked receiver G':

  1. Acquire the lock
  2. Dequeue the sudog struct of the waiting receiver G' from the recvq
  3. Directly copy the element to the element field of the waiting receiver G' sudog. An optimization, when the blocked goroutine G' resumes it doesn’t have to acquire the lock and dequeue the element from buf
  4. Make a goready(G') call to the scheduler and release the lock
  5. Scheduler changes G' state from waiting to runnable and enqueues it to the run queue. It will be eventually scheduled since it’s on the run queue
  6. G continues executing

Receive from a Channel

Actions when G receives from a non-empty and non-full channel:

  1. Acquire the lock
  2. Dequeue the element from the buf. It copies the element in buf to variable in assignment
  3. Release the lock

Receive from an Empty Channel

Actions when G receives from an empty channel (blocking call):

  1. Acquire the lock
  2. Create a sudog for ourselves with address we are waiting to receive to and enqueue it to the recvq. A sender will use it to resume us in the future
  3. Make a gopark call to the scheduler and release the lock
  4. Scheduler changes G state from running to waiting and removes association between G and M
  5. Scheduler dequeues the runnable G' from the run queue and schedules it onto M

Receive from a Full Channel

Actions when G receives from a full channel and there is a blocked sender G':

  1. Acquire the lock
  2. Dequeue the element from the buf
  3. Dequeue the sudog struct of the waiting sender G' from the sendq
  4. Enqueue the element elem from the sudog into the buf. An optimization, when the blocked goroutine G' resumes it doesn’t have to acquire the lock to enqueue the element
  5. Make a goready(G') call to the scheduler and release the lock
  6. Scheduler changes G' state from waiting to runnable and enqueues it to the run queue. It will be eventually scheduled since it’s on the run queue
  7. G continues executing

Resume in unbuffered/synchronous Channels

Unbuffered channels always work as direct send case

  1. Receiver waiting → sender directly writes to receiver’s stack from sudog
  2. Sender waiting → receiver directly writes to sender’s stack from sudog

Select Statement

  1. All channels are locked
  2. A sudog is put in thesendq/recvq queues of all channels
  3. Channels unlocked, all the selecting G is paused
  4. CAS operation so there is only one winning case
  5. Resuming mirrors the pause sequence

References

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