Understanding Channels

Channels provide a simple mechanism for goroutines to communicate, and a powerful construct to build sophisticated concurrency patterns. We will delve into the inner workings of channels and channel operations, including how they're supported by the runtime scheduler.

Transcript

1. Understanding
   channels
   @kavya719
   

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2. kavya
   

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3. inner workings of
   channels
   

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4. goroutines
   to execute tasks independently,
   potentially in parallel.
   channels
   for communication, synchronization
   between goroutines.
   concurrency features
   

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5. func main() {
   tasks := getTasks()
   // Process each task.
   for _, task := range tasks {
   process(task)
   }

   

   ...
   }
   hellaTasks
   

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6. task queue
   func worker(ch) {
   for {

   // Get a task.
   task := process(task)
   }
   }
   func main() {
   // Buffered channel.
   ch := make(chan Task, 3)
   // Run fixed number of workers.
   for i := 0; i < numWorkers; i++ {
   go worker(ch)
   }
   // Send tasks to workers.
   hellaTasks := getTasks()
   for _, task := range hellaTasks {
   taskCh }
   ...
   }
   

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7. task queue
   func worker(ch) {
   for {

   // Receive task.
   task := process(task)
   }
   }
   func main() {
   // Buffered channel.
   ch := make(chan Task, 3)
   // Run fixed number of workers.
   for i := 0; i < numWorkers; i++ {
   go worker(ch)
   }
   // Send tasks to workers.
   hellaTasks := getTasks()
   for _, task := range hellaTasks {
   taskCh }
   ...
   }
   

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8. task queue
   func worker(ch) {
   for {

   // Receive task.
   task := process(task)
   }
   }
   func main() {
   // Buffered channel.
   ch := make(chan Task, 3)
   // Run fixed number of workers.
   for i := 0; i < numWorkers; i++ {
   go worker(ch)
   }
   // Send tasks to workers.
   hellaTasks := getTasks()
   for _, task := range hellaTasks {
   taskCh }
   ...
   }
   

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9. goroutine-safe.
   store and pass values between goroutines.
   provide FIFO semantics.
   can cause goroutines to block and unblock.
   channels are inherently interesting
   

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10. making channels
    the hchan struct
    stepping back:
    design considerations
    sends and receives
    goroutine scheduling
    

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11. making channels
    

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12. unbuffered channel
    ch := make(chan int)
    buffered channel
    ch := make(chan Task, 3)
    make chan
    

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13. goroutine-safe
    stores up to capacity elements,

    and provides FIFO semantics
    sends values between goroutines
    can cause them to block, unblock
    buffered channel
    ch := make(chan Task, 3)
    

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14. buf
    lock
    ...
    recvx
    circular queue
    mutex
    send index
    receive index
    sendx
    hchan
    buffered channel
    ch := make(chan Task, 3)
    

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15. buf
    lock
    ...
    recvx
    circular queue
    mutex
    send index
    receive index
    sendx
    hchan
    0
    0
    2 1 0
    empty
    buffered channel
    ch := make(chan Task, 3)
    

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16. buf
    lock
    ...
    recvx
    circular queue
    mutex
    send index
    receive index
    sendx
    hchan
    0
    1
    buffered channel
    ch := make(chan Task, 3)
    an enqueue
    2 1 0
    

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17. buf
    lock
    ...
    recvx
    circular queue
    mutex
    send index
    receive index
    sendx
    hchan
    0
    0
    buffered channel
    ch := make(chan Task, 3)
    two more;
    so, full
    2 1 0
    

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18. buf
    lock
    ...
    recvx
    circular queue
    mutex
    send index
    receive index
    hchan
    sendx
    1
    0
    buffered channel
    ch := make(chan Task, 3)
    a dequeue
    2 1 0
    

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19. make chan
    heap
    stack
    high addr
    low addr
    program memory
    buffered channel
    ch := make(chan Task, 3)
    allocates an hchan struct on the heap.
    initializes it.
    returns a pointer to it.
    

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20. make chan
    heap
    allocates an hchan struct on the heap.
    initializes it.
    returns a pointer to it.
    ch
    buffered channel
    ch := make(chan Task, 3)
    This is why we can pass
    channels between functions,
    don’t need to pass 

    pointers to channels.
    

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21. sends and receives
    

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22. task queue
    func main() {
    ...
    for _, task := range tasks {
    taskCh }
    ...
    }
    func worker() {
    for {
    task := process(task)
    }
    }
    G2
    G1
    

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23. ch buf
    ...
    lock
    G1
    

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24. ch G1
    buf
    ...
    lock
    1. acquire
    

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25. ch buf
    ...
    lock
    2. enqueue
    task0
    copy
    G1
    

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26. ch buf
    ...
    lock
    3. release
    task0
    copy
    G1
    

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27. t := G2
    buf
    ...
    lock
    task0
    copy
    

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28. buf
    ...
    lock
    1. acquire
    task0
    copy
    t := G2
    

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29. buf
    ...
    lock
    2. dequeue
    task0
    copy’
    t := G2
    

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30. buf
    ...
    lock
    3. release
    task0
    copy’
    t := G2
    

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31. no shared memory

    (except hchan)
    copies
    

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32. “Do not communicate by sharing memory; 

    instead, share memory by communicating.”
    no shared memory

    (except hchan)
    copies
    

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33. buf
    ...
    lock
    G1 G2
    

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34. buf
    ...
    lock
    ch G1 G2
    

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35. buf
    ...
    lock
    ch ch G1 G2
    

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36. buf
    ...
    lock
    ch ch ch G1 G2
    

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37. buf
    ...
    lock
    ch ch ch ch G1 G2
    

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38. buf
    ...
    lock
    ch ch ch ch ruh-roh, channel is full!
    G1’s execution is paused,
    resumed after a receive.
    G1 G2
    

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39. buf
    ...
    lock
    ch ch ch ch ?
    ruh-roh, channel is full!
    G1’s execution is paused,
    resumed after a receive.
    G1 G2
    

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40. goroutines are user-space threads.
    created and managed by the Go runtime, not the OS.
    lightweight compared to OS threads.
    the runtime scheduler schedules them onto OS threads.
    g5
    g1 g4
    g3
    OS thread2
    M:N scheduling
    OS thread1
    g1
    g2 g6
    g2
    interlude: the runtime scheduler
    

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41. Go’s M:N scheduling is described using three structures:
    M: OS thread
    G: goroutine
    P: context for scheduling.
    M
    

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42. M
    G
    Go’s M:N scheduling is described using three structures:
    M: OS thread
    G: goroutine
    P: context for scheduling.
    

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43. M
    P
    G
    Go’s M:N scheduling is described using three structures:
    M: OS thread
    G: goroutine
    P: context for scheduling.
    

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44. Ps hold the runqueues.
    In order to run goroutines (G),

    a thread (M) must hold a context (P).
    M
    P
    runQ
    G
    G
    G
    runnable
    Go’s M:N scheduling is described using three structures:
    M: OS thread
    G: goroutine
    P: context for scheduling.
    }
    

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45. Ps hold the runqueues.
    In order to run goroutines (G),

    a thread (M) must hold a context (P).
    M
    P
    runQ
    G
    G
    G
    runnable
    current G
    running
    Go’s M:N scheduling is described using three structures:
    M: OS thread
    G: goroutine
    P: context for scheduling.
    }
    

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46. pausing goroutines
    ch send on a full channel
    }
    

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47. ch calls into the scheduler
    gopark
    M
    P
    G1
    G
    G
    runQ
    runnable
    current G
    running
    }
    G1
    

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48. M
    P
    G1
    G
    G
    runnable
    runQ
    current G
    waiting
    ch sets G1 to waiting
    calls into the scheduler
    gopark
    }
    G1
    

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49. ch sets G1 to waiting
    calls into the scheduler
    removes association
    between G1, M
    gopark
    M
    P
    G
    G
    G1
    waiting
    runQ
    runnable
    }
    G1
    

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50. G1
    waiting
    M
    P
    G
    G
    runQ
    runnable
    current G
    ch schedules a runnable G 

    from the runqueue
    sets G1 to waiting
    calls into the scheduler
    removes association
    between G1, M
    “returns” 

    to a
    different G
    }
    G1
    gopark
    

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51. This is neat.
    G1 is blocked as needed, but not the OS thread.
    

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52. This is neat.
    G1 is blocked as needed, but not the OS thread.
    …great, but how do we resume
    the blocked goroutine?
    after a channel receive, and
    the channel is no longer full
    

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53. the hchan struct stores waiting senders, receivers as well.
    ...
    hchan
    sendq
    recvq
    waiting senders
    waiting receivers
    buf
    lock
    G
    elem
    sudog
    ...
    waiting G
    elem to send/ recv
    resuming goroutines
    represents a G
    in a waiting list
    stored as a sudog
    

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54. G
    elem
    sudog
    ...
    G1
    task4
    ch G1
    creates a sudog for itself, puts it in the sendq
    happens before calling into the scheduler.
    

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55. ...
    hchan
    sendq
    recvq
    G
    elem
    sudog
    ...
    G1
    task4
    ch G1
    creates a sudog for itself, puts it in the sendq
    happens before calling into the scheduler.
    receiver
    uses it to
    resume G1.
    

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56. ...
    hchan
    sendq
    recvq
    G
    elem
    sudog
    ...
    G1
    task4
    ch G1
    creates a sudog for itself, puts it in the sendq
    happens before calling into the scheduler.
    receiver
    uses it to
    resume G1.
    

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57. sendq
    ...
    g
    elem
    ...
    buf
    lock
    task4
    task1
    G1
    t := full buffer
    waiting
    sender
    {
    {
    G2
    waiting to send
    task4
    ||
    

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58. sendq
    ...
    g
    elem
    ...
    buf
    lock
    task4
    t := G1
    task1
    dequeue G2
    

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59. t := task1
    sendq
    ...
    g
    elem
    ...
    buf
    lock
    task4
    G1
    pops off sudog
    G2
    

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60. task4
    task1
    sendq
    ...
    g
    elem
    ...
    buf
    lock
    G1
    t := enqueues the sudog’s elem:
    G2
    

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61. need to set G1 to runnable
    task1
    task4
    sendq
    ...
    g
    elem
    ...
    buf
    lock
    G1
    t := G2
    

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62. t := calls into the scheduler
    G1
    waiting
    M
    P
    G2
    G
    runQ
    current G
    G2
    goready

    (G1)
    

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63. goready

    (G1)
    t := sets G1 to runnable
    calls into the scheduler
    G1
    runnable
    M
    P
    G2
    G
    runQ
    current G
    G2
    

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64. t := sets G1 to runnable
    calls into the scheduler
    puts it on runqueue
    G1
    M
    P
    G2
    G
    runQ
    current G
    returns to G2
    G2
    }runnable
    goready

    (G1)
    

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65. sends and receives
    when the receiver
    comes first
    

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66. recvq
    ...
    buf
    lock
    t := G2
    receive on an empty channel:

    G2’s execution is paused
    resumed after a send.
    empty buffer
    {
    

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67. receive on an empty channel:

    G2’s execution is paused
    resumed after a send.
    recvq
    ...
    buf
    lock
    G2
    t := empty buffer
    {
    

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68. receive on an empty channel:

    G2’s execution is paused
    resumed after a send.
    recvq
    ...
    buf
    lock
    G2
    t := empty buffer
    {
    set up state for resumption, and pause
    put a sudog in the recvq gopark G2.
    

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69. recvq
    ...
    buf
    lock
    G
    elem
    ...
    G2
    t
    empty buffer
    waiting
    receiver
    {
    {
    G2
    t := set up state for resumption, and pause
    put a sudog in the recvq gopark G2.
    waiting to receive
    to t
    ||
    

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70. recvq
    ...
    buf
    lock
    elem
    ...
    G
    t
    G2
    ch G1
    

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71. recvq
    ...
    buf
    lock
    elem
    ...
    G
    t
    G2
    enqueue task in the buffer,
    goready(G2)
    could:
    ch G1
    

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72. recvq
    ...
    buf
    lock
    elem
    ...
    G
    t
    G2
    or we can be smarter.
    ch G1
    

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73. recvq
    ...
    buf
    lock
    elem
    ...
    G
    t
    G2
    ch G1
    or we can be smarter.
    the memory location
    for the receive
    

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74. recvq
    ...
    buf
    lock
    elem
    ...
    G
    t
    G2
    ch G1
    G1 writes to t directly.
    !
    the memory location
    for the receive
    

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75. }
    per-goroutine stacks heap
    stack
    G1 stack
    G2 stack
    t
    only operations in runtime where this happens.
    G1 writes to G2’s stack!
    direct send
    

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76. This is clever.
    On resuming, G2 does not need to acquire channel lock
    and manipulate the buffer.
    

    Also, one fewer memory copy.
    

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77. goroutine-safe
    hchan mutex
    store values, pass in FIFO.
    copying into and out of hchan buffer
    can cause goroutines to pause and resume.
    hchan sudog queues
    calls into the runtime scheduler 

    (gopark, goready)
    we now understand channels (sorta)…
    

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78. a note (or two)…
    unbuffered channels

    unbuffered channels always work like the “direct send” case:
    receiver first —> sender writes to receiver’s stack.
    sender first —> receiver receives directly from the sudog.

    select (general-case)
    all channels locked.
    a sudog is put in the sendq /recvq queues of all channels.
    channels unlocked, and the select-ing G is paused.
    CAS operation so there’s one winning case.
    resuming mirrors the pause sequence.
    

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79. stepping back…
    

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80. simplicity and performance
    

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81. queue with a lock preferred to lock-free implementation:
    “The performance improvement does not materialize from the air,
    it comes with code complexity increase.” — dvyokov
    simplicity
    

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82. calling into the runtime scheduler:
    OS thread remains unblocked.
    cross-goroutine stack reads and writes.
    goroutine wake-up path is lockless,
    potentially fewer memory copies
    need to account for
    memory management:
    

    garbage collection,
    stack-shrinking
    performance
    }
    

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83. simplicity and performance
    astute trade-offs
    between
    

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84. “The noblest pleasure is
    the joy of understanding.”
    - Leonardo da Vinci
    @kavya719
    speakerdeck.com/kavya719/understanding-channels
    

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85. Railgun: CloudFlare’s web proxy
    We chose to use Go because Railgun is inherently
    highly concurrent…
    Railgun makes extensive use of goroutines and channels.
    Docker: software container platform
    …Go is geared for distributed computing.
    It has many built-in features to support concurrency…
    Doozer: Heroku’s distributed data store
    Fortunately, Go’s concurrency primitives made the task
    much easier.
    Kubernetes: Google’s container orchestration platform
    Built in concurrency. Building distributed systems in Go is
    helped tremendously by being able to fan out …
    

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86. unbuffered channels
    unbuffered channel
    ch := make(chan int)
    That’s how unbuffered channels (always) work too.

    If receiver is first:
    > puts a sudog on the recvq and is paused.
    > subsequent sender will “send directly”, and resume the receiver.
    If sender is first: 

    > puts a sudog for itself on the sendq and is paused.

    > receiver then “receives directly” from the sudog and 

    resumes the sender.
    

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87. func worker() {
    for {
    select {
    case task := process(task)
    case cmd := execute(cmd)
    }
    }
    }
    var taskCh = make(chan Task, 3)
    var cmdCh = make(chan Command)
    selects
    

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