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dial_queue.go
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dial_queue.go
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package dht
import (
"context"
"math"
"sync"
"time"
"github.com/libp2p/go-libp2p-core/peer"
queue "github.com/libp2p/go-libp2p-peerstore/queue"
)
const (
// DefaultDialQueueMinParallelism is the default value for the minimum number of worker dial goroutines that will
// be alive at any time.
DefaultDialQueueMinParallelism = 6
// DefaultDialQueueMaxParallelism is the default value for the maximum number of worker dial goroutines that can
// be alive at any time.
DefaultDialQueueMaxParallelism = 20
// DefaultDialQueueMaxIdle is the default value for the period that a worker dial goroutine waits before signalling
// a worker pool downscaling.
DefaultDialQueueMaxIdle = 5 * time.Second
// DefaultDialQueueScalingMutePeriod is the default value for the amount of time to ignore further worker pool
// scaling events, after one is processed. Its role is to reduce jitter.
DefaultDialQueueScalingMutePeriod = 1 * time.Second
// DefaultDialQueueScalingFactor is the default factor by which the current number of workers will be multiplied
// or divided when upscaling and downscaling events occur, respectively.
DefaultDialQueueScalingFactor = 1.5
)
type dialQueue struct {
*dqParams
nWorkers uint
out *queue.ChanQueue
startOnce sync.Once
waitingCh chan waitingCh
dieCh chan struct{}
growCh chan struct{}
shrinkCh chan struct{}
}
type dqParams struct {
ctx context.Context
target string
dialFn func(context.Context, peer.ID) error
in *queue.ChanQueue
config dqConfig
}
type dqConfig struct {
// minParallelism is the minimum number of worker dial goroutines that will be alive at any time.
minParallelism uint
// maxParallelism is the maximum number of worker dial goroutines that can be alive at any time.
maxParallelism uint
// scalingFactor is the factor by which the current number of workers will be multiplied or divided when upscaling
// and downscaling events occur, respectively.
scalingFactor float64
// mutePeriod is the amount of time to ignore further worker pool scaling events, after one is processed.
// Its role is to reduce jitter.
mutePeriod time.Duration
// maxIdle is the period that a worker dial goroutine waits before signalling a worker pool downscaling.
maxIdle time.Duration
}
// dqDefaultConfig returns the default configuration for dial queues. See const documentation to learn the default values.
func dqDefaultConfig() dqConfig {
return dqConfig{
minParallelism: DefaultDialQueueMinParallelism,
maxParallelism: DefaultDialQueueMaxParallelism,
scalingFactor: DefaultDialQueueScalingFactor,
maxIdle: DefaultDialQueueMaxIdle,
mutePeriod: DefaultDialQueueScalingMutePeriod,
}
}
type waitingCh struct {
ch chan<- peer.ID
ts time.Time
}
// newDialQueue returns an _unstarted_ adaptive dial queue that spawns a dynamically sized set of goroutines to
// preemptively stage dials for later handoff to the DHT protocol for RPC. It identifies backpressure on both
// ends (dial consumers and dial producers), and takes compensating action by adjusting the worker pool. To
// activate the dial queue, call Start().
//
// Why? Dialing is expensive. It's orders of magnitude slower than running an RPC on an already-established
// connection, as it requires establishing a TCP connection, multistream handshake, crypto handshake, mux handshake,
// and protocol negotiation.
//
// We start with config.minParallelism number of workers, and scale up and down based on demand and supply of
// dialled peers.
//
// The following events trigger scaling:
// - we scale up when we can't immediately return a successful dial to a new consumer.
// - we scale down when we've been idle for a while waiting for new dial attempts.
// - we scale down when we complete a dial and realise nobody was waiting for it.
//
// Dialler throttling (e.g. FD limit exceeded) is a concern, as we can easily spin up more workers to compensate, and
// end up adding fuel to the fire. Since we have no deterministic way to detect this for now, we hard-limit concurrency
// to config.maxParallelism.
func newDialQueue(params *dqParams) (*dialQueue, error) {
dq := &dialQueue{
dqParams: params,
out: queue.NewChanQueue(params.ctx, queue.NewXORDistancePQ(params.target)),
growCh: make(chan struct{}, 1),
shrinkCh: make(chan struct{}, 1),
waitingCh: make(chan waitingCh),
dieCh: make(chan struct{}, params.config.maxParallelism),
}
return dq, nil
}
// Start initiates action on this dial queue. It should only be called once; subsequent calls are ignored.
func (dq *dialQueue) Start() {
dq.startOnce.Do(func() {
go dq.control()
})
}
func (dq *dialQueue) control() {
var (
dialled <-chan peer.ID
waiting []waitingCh
lastScalingEvt = time.Now()
)
defer func() {
for _, w := range waiting {
close(w.ch)
}
waiting = nil
}()
// start workers
tgt := int(dq.dqParams.config.minParallelism)
for i := 0; i < tgt; i++ {
go dq.worker()
}
dq.nWorkers = uint(tgt)
// control workers
for {
// First process any backlog of dial jobs and waiters -- making progress is the priority.
// This block is copied below; couldn't find a more concise way of doing this.
select {
case <-dq.ctx.Done():
return
case w := <-dq.waitingCh:
waiting = append(waiting, w)
dialled = dq.out.DeqChan
continue // onto the top.
case p, ok := <-dialled:
if !ok {
return // we're done if the ChanQueue is closed, which happens when the context is closed.
}
w := waiting[0]
logger.Debugf("delivering dialled peer to DHT; took %dms.", time.Since(w.ts)/time.Millisecond)
w.ch <- p
close(w.ch)
waiting = waiting[1:]
if len(waiting) == 0 {
// no more waiters, so stop consuming dialled jobs.
dialled = nil
}
continue // onto the top.
default:
// there's nothing to process, so proceed onto the main select block.
}
select {
case <-dq.ctx.Done():
return
case w := <-dq.waitingCh:
waiting = append(waiting, w)
dialled = dq.out.DeqChan
case p, ok := <-dialled:
if !ok {
return // we're done if the ChanQueue is closed, which happens when the context is closed.
}
w := waiting[0]
logger.Debugf("delivering dialled peer to DHT; took %dms.", time.Since(w.ts)/time.Millisecond)
w.ch <- p
close(w.ch)
waiting = waiting[1:]
if len(waiting) == 0 {
// no more waiters, so stop consuming dialled jobs.
dialled = nil
}
case <-dq.growCh:
if time.Since(lastScalingEvt) < dq.config.mutePeriod {
continue
}
dq.grow()
lastScalingEvt = time.Now()
case <-dq.shrinkCh:
if time.Since(lastScalingEvt) < dq.config.mutePeriod {
continue
}
dq.shrink()
lastScalingEvt = time.Now()
}
}
}
func (dq *dialQueue) Consume() <-chan peer.ID {
ch := make(chan peer.ID, 1)
select {
case p, ok := <-dq.out.DeqChan:
// short circuit and return a dialled peer if it's immediately available, or abort if DeqChan is closed.
if ok {
ch <- p
}
close(ch)
return ch
case <-dq.ctx.Done():
// return a closed channel with no value if we're done.
close(ch)
return ch
default:
}
// we have no finished dials to return, trigger a scale up.
select {
case dq.growCh <- struct{}{}:
default:
}
// park the channel until a dialled peer becomes available.
select {
case dq.waitingCh <- waitingCh{ch, time.Now()}:
// all good
case <-dq.ctx.Done():
// return a closed channel with no value if we're done.
close(ch)
}
return ch
}
func (dq *dialQueue) grow() {
// no mutex needed as this is only called from the (single-threaded) control loop.
defer func(prev uint) {
if prev == dq.nWorkers {
return
}
logger.Debugf("grew dial worker pool: %d => %d", prev, dq.nWorkers)
}(dq.nWorkers)
if dq.nWorkers == dq.config.maxParallelism {
return
}
// choosing not to worry about uint wrapping beyond max value.
target := uint(math.Floor(float64(dq.nWorkers) * dq.config.scalingFactor))
if target > dq.config.maxParallelism {
target = dq.config.maxParallelism
}
for ; dq.nWorkers < target; dq.nWorkers++ {
go dq.worker()
}
}
func (dq *dialQueue) shrink() {
// no mutex needed as this is only called from the (single-threaded) control loop.
defer func(prev uint) {
if prev == dq.nWorkers {
return
}
logger.Debugf("shrunk dial worker pool: %d => %d", prev, dq.nWorkers)
}(dq.nWorkers)
if dq.nWorkers == dq.config.minParallelism {
return
}
target := uint(math.Floor(float64(dq.nWorkers) / dq.config.scalingFactor))
if target < dq.config.minParallelism {
target = dq.config.minParallelism
}
// send as many die signals as workers we have to prune.
for ; dq.nWorkers > target; dq.nWorkers-- {
select {
case dq.dieCh <- struct{}{}:
default:
logger.Debugf("too many die signals queued up.")
}
}
}
func (dq *dialQueue) worker() {
// This idle timer tracks if the environment is slow. If we're waiting to long to acquire a peer to dial,
// it means that the DHT query is progressing slow and we should shrink the worker pool.
idleTimer := time.NewTimer(24 * time.Hour) // placeholder init value which will be overridden immediately.
for {
// trap exit signals first.
select {
case <-dq.ctx.Done():
return
case <-dq.dieCh:
return
default:
}
idleTimer.Stop()
select {
case <-idleTimer.C:
default:
}
idleTimer.Reset(dq.config.maxIdle)
select {
case <-dq.dieCh:
return
case <-dq.ctx.Done():
return
case <-idleTimer.C:
// no new dial requests during our idle period; time to scale down.
case p, ok := <-dq.in.DeqChan:
if !ok {
return
}
t := time.Now()
if err := dq.dialFn(dq.ctx, p); err != nil {
logger.Debugf("discarding dialled peer because of error: %v", err)
continue
}
logger.Debugf("dialling %v took %dms (as observed by the dht subsystem).", p, time.Since(t)/time.Millisecond)
waiting := len(dq.waitingCh)
// by the time we're done dialling, it's possible that the context is closed, in which case there will
// be nobody listening on dq.out.EnqChan and we could block forever.
select {
case dq.out.EnqChan <- p:
case <-dq.ctx.Done():
return
}
if waiting > 0 {
// we have somebody to deliver this value to, so no need to shrink.
continue
}
}
// scaling down; control only arrives here if the idle timer fires, or if there are no goroutines
// waiting for the value we just produced.
select {
case dq.shrinkCh <- struct{}{}:
default:
}
}
}