eth/downloader: adaptive quality of service tuning

eth/downloader/downloader.go

@@ -54,14 +54,15 @@ var (
 	blockTargetRTT = 3 * time.Second / 2 // [eth/61] Target time for completing a block retrieval request
 	blockTTL       = 3 * blockTargetRTT  // [eth/61] Maximum time allowance before a block request is considered expired
 
-	headerTargetRTT  = time.Second          // [eth/62] Target time for completing a header retrieval request (only for measurements for now)
-	headerTTL        = 3 * time.Second      // [eth/62] Time it takes for a header request to time out
-	bodyTargetRTT    = 3 * time.Second / 2  // [eth/62] Target time for completing a block body retrieval request
-	bodyTTL          = 3 * bodyTargetRTT    // [eth/62] Maximum time allowance before a block body request is considered expired
-	receiptTargetRTT = 3 * time.Second / 2  // [eth/63] Target time for completing a receipt retrieval request
-	receiptTTL       = 3 * receiptTargetRTT // [eth/63] Maximum time allowance before a receipt request is considered expired
-	stateTargetRTT   = 2 * time.Second / 2  // [eth/63] Target time for completing a state trie retrieval request
-	stateTTL         = 3 * stateTargetRTT   // [eth/63] Maximum time allowance before a node data request is considered expired
+	rttMinEstimate   = 2 * time.Second  // Minimum round-trip time to target for download requests
+	rttMaxEstimate   = 20 * time.Second // Maximum rount-trip time to target for download requests
+	rttMinConfidence = 0.1              // Worse confidence factor in our estimated RTT value
+	ttlScaling       = 3                // Constant scaling factor for RTT -> TTL conversion
+	ttlLimit         = time.Minute      // Maximum TTL allowance to prevent reaching crazy timeouts
+
+	qosTuningPeers   = 5    // Number of peers to tune based on (best peers)
+	qosConfidenceCap = 10   // Number of peers above which not to modify RTT confidence
+	qosTuningImpact  = 0.25 // Impact that a new tuning target has on the previous value
 
 	maxQueuedHashes   = 32 * 1024 // [eth/61] Maximum number of hashes to queue for import (DOS protection)
 	maxQueuedHeaders  = 32 * 1024 // [eth/62] Maximum number of headers to queue for import (DOS protection)
@@ -113,7 +114,8 @@ type Downloader struct {
 	fsPivotLock  *types.Header // Pivot header on critical section entry (cannot change between retries)
 	fsPivotFails int           // Number of fast sync failures in the critical section
 
-	interrupt int32 // Atomic boolean to signal termination
+	rttEstimate   uint64 // Round trip time to target for download requests
+	rttConfidence uint64 // Confidence in the estimated RTT (unit: millionths to allow atomic ops)
 
 	// Statistics
 	syncStatsChainOrigin uint64       // Origin block number where syncing started at
@@ -159,6 +161,9 @@ type Downloader struct {
 	cancelCh   chan struct{} // Channel to cancel mid-flight syncs
 	cancelLock sync.RWMutex  // Lock to protect the cancel channel in delivers
 
+	quitCh   chan struct{} // Quit channel to signal termination
+	quitLock sync.RWMutex  // Lock to prevent double closes
+
 	// Testing hooks
 	syncInitHook     func(uint64, uint64)  // Method to call upon initiating a new sync run
 	bodyFetchHook    func([]*types.Header) // Method to call upon starting a block body fetch
@@ -172,11 +177,13 @@ func New(stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, ha
 	headFastBlock headFastBlockRetrievalFn, commitHeadBlock headBlockCommitterFn, getTd tdRetrievalFn, insertHeaders headerChainInsertFn,
 	insertBlocks blockChainInsertFn, insertReceipts receiptChainInsertFn, rollback chainRollbackFn, dropPeer peerDropFn) *Downloader {
 
-	return &Downloader{
+	dl := &Downloader{
 		mode:             FullSync,
 		mux:              mux,
 		queue:            newQueue(stateDb),
 		peers:            newPeerSet(),
+		rttEstimate:      uint64(rttMaxEstimate),
+		rttConfidence:    uint64(1000000),
 		hasHeader:        hasHeader,
 		hasBlockAndState: hasBlockAndState,
 		getHeader:        getHeader,
@@ -203,7 +210,10 @@ func New(stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, ha
 		receiptWakeCh:    make(chan bool, 1),
 		stateWakeCh:      make(chan bool, 1),
 		headerProcCh:     make(chan []*types.Header, 1),
+		quitCh:           make(chan struct{}),
 	}
+	go dl.qosTuner()
+	return dl
 }
 
 // Progress retrieves the synchronisation boundaries, specifically the origin
@@ -250,6 +260,8 @@ func (d *Downloader) RegisterPeer(id string, version int, head common.Hash,
 		glog.V(logger.Error).Infoln("Register failed:", err)
 		return err
 	}
+	d.qosReduceConfidence()
+
 	return nil
 }
 
@@ -515,7 +527,16 @@ func (d *Downloader) cancel() {
 // Terminate interrupts the downloader, canceling all pending operations.
 // The downloader cannot be reused after calling Terminate.
 func (d *Downloader) Terminate() {
-	atomic.StoreInt32(&d.interrupt, 1)
+	// Close the termination channel (make sure double close is allowed)
+	d.quitLock.Lock()
+	select {
+	case <-d.quitCh:
+	default:
+		close(d.quitCh)
+	}
+	d.quitLock.Unlock()
+
+	// Cancel any pending download requests
 	d.cancel()
 }
 
@@ -932,7 +953,7 @@ func (d *Downloader) fetchBlocks61(from uint64) error {
 				// Reserve a chunk of hashes for a peer. A nil can mean either that
 				// no more hashes are available, or that the peer is known not to
 				// have them.
-				request := d.queue.ReserveBlocks(peer, peer.BlockCapacity())
+				request := d.queue.ReserveBlocks(peer, peer.BlockCapacity(blockTargetRTT))
 				if request == nil {
 					continue
 				}
@@ -973,7 +994,7 @@ func (d *Downloader) fetchHeight(p *peer) (*types.Header, error) {
 	// Request the advertised remote head block and wait for the response
 	go p.getRelHeaders(p.head, 1, 0, false)
 
-	timeout := time.After(headerTTL)
+	timeout := time.After(d.requestTTL())
 	for {
 		select {
 		case <-d.cancelCh:
@@ -1041,7 +1062,7 @@ func (d *Downloader) findAncestor(p *peer, height uint64) (uint64, error) {
 
 	// Wait for the remote response to the head fetch
 	number, hash := uint64(0), common.Hash{}
-	timeout := time.After(hashTTL)
+	timeout := time.After(d.requestTTL())
 
 	for finished := false; !finished; {
 		select {
@@ -1118,7 +1139,7 @@ func (d *Downloader) findAncestor(p *peer, height uint64) (uint64, error) {
 		// Split our chain interval in two, and request the hash to cross check
 		check := (start + end) / 2
 
-		timeout := time.After(hashTTL)
+		timeout := time.After(d.requestTTL())
 		go p.getAbsHeaders(uint64(check), 1, 0, false)
 
 		// Wait until a reply arrives to this request
@@ -1199,7 +1220,7 @@ func (d *Downloader) fetchHeaders(p *peer, from uint64) error {
 
 	getHeaders := func(from uint64) {
 		request = time.Now()
-		timeout.Reset(headerTTL)
+		timeout.Reset(d.requestTTL())
 
 		if skeleton {
 			glog.V(logger.Detail).Infof("%v: fetching %d skeleton headers from #%d", p, MaxHeaderFetch, from)
@@ -1311,13 +1332,13 @@ func (d *Downloader) fillHeaderSkeleton(from uint64, skeleton []*types.Header) (
 			pack := packet.(*headerPack)
 			return d.queue.DeliverHeaders(pack.peerId, pack.headers, d.headerProcCh)
 		}
-		expire   = func() map[string]int { return d.queue.ExpireHeaders(headerTTL) }
+		expire   = func() map[string]int { return d.queue.ExpireHeaders(d.requestTTL()) }
 		throttle = func() bool { return false }
 		reserve  = func(p *peer, count int) (*fetchRequest, bool, error) {
 			return d.queue.ReserveHeaders(p, count), false, nil
 		}
 		fetch    = func(p *peer, req *fetchRequest) error { return p.FetchHeaders(req.From, MaxHeaderFetch) }
-		capacity = func(p *peer) int { return p.HeaderCapacity() }
+		capacity = func(p *peer) int { return p.HeaderCapacity(d.requestRTT()) }
 		setIdle  = func(p *peer, accepted int) { p.SetHeadersIdle(accepted) }
 	)
 	err := d.fetchParts(errCancelHeaderFetch, d.headerCh, deliver, d.queue.headerContCh, expire,
@@ -1341,9 +1362,9 @@ func (d *Downloader) fetchBodies(from uint64) error {
 			pack := packet.(*bodyPack)
 			return d.queue.DeliverBodies(pack.peerId, pack.transactions, pack.uncles)
 		}
-		expire   = func() map[string]int { return d.queue.ExpireBodies(bodyTTL) }
+		expire   = func() map[string]int { return d.queue.ExpireBodies(d.requestTTL()) }
 		fetch    = func(p *peer, req *fetchRequest) error { return p.FetchBodies(req) }
-		capacity = func(p *peer) int { return p.BlockCapacity() }
+		capacity = func(p *peer) int { return p.BlockCapacity(d.requestRTT()) }
 		setIdle  = func(p *peer, accepted int) { p.SetBodiesIdle(accepted) }
 	)
 	err := d.fetchParts(errCancelBodyFetch, d.bodyCh, deliver, d.bodyWakeCh, expire,
@@ -1365,9 +1386,9 @@ func (d *Downloader) fetchReceipts(from uint64) error {
 			pack := packet.(*receiptPack)
 			return d.queue.DeliverReceipts(pack.peerId, pack.receipts)
 		}
-		expire   = func() map[string]int { return d.queue.ExpireReceipts(receiptTTL) }
+		expire   = func() map[string]int { return d.queue.ExpireReceipts(d.requestTTL()) }
 		fetch    = func(p *peer, req *fetchRequest) error { return p.FetchReceipts(req) }
-		capacity = func(p *peer) int { return p.ReceiptCapacity() }
+		capacity = func(p *peer) int { return p.ReceiptCapacity(d.requestRTT()) }
 		setIdle  = func(p *peer, accepted int) { p.SetReceiptsIdle(accepted) }
 	)
 	err := d.fetchParts(errCancelReceiptFetch, d.receiptCh, deliver, d.receiptWakeCh, expire,
@@ -1417,13 +1438,13 @@ func (d *Downloader) fetchNodeData() error {
 				}
 			})
 		}
-		expire   = func() map[string]int { return d.queue.ExpireNodeData(stateTTL) }
+		expire   = func() map[string]int { return d.queue.ExpireNodeData(d.requestTTL()) }
 		throttle = func() bool { return false }
 		reserve  = func(p *peer, count int) (*fetchRequest, bool, error) {
 			return d.queue.ReserveNodeData(p, count), false, nil
 		}
 		fetch    = func(p *peer, req *fetchRequest) error { return p.FetchNodeData(req) }
-		capacity = func(p *peer) int { return p.NodeDataCapacity() }
+		capacity = func(p *peer) int { return p.NodeDataCapacity(d.requestRTT()) }
 		setIdle  = func(p *peer, accepted int) { p.SetNodeDataIdle(accepted) }
 	)
 	err := d.fetchParts(errCancelStateFetch, d.stateCh, deliver, d.stateWakeCh, expire,
@@ -1799,8 +1820,10 @@ func (d *Downloader) processContent() error {
 		}
 		for len(results) != 0 {
 			// Check for any termination requests
-			if atomic.LoadInt32(&d.interrupt) == 1 {
+			select {
+			case <-d.quitCh:
 				return errCancelContentProcessing
+			default:
 			}
 			// Retrieve the a batch of results to import
 			var (
@@ -1901,3 +1924,74 @@ func (d *Downloader) deliver(id string, destCh chan dataPack, packet dataPack, i
 		return errNoSyncActive
 	}
 }
+
+// qosTuner is the quality of service tuning loop that occasionally gathers the
+// peer latency statistics and updates the estimated request round trip time.
+func (d *Downloader) qosTuner() {
+	for {
+		// Retrieve the current median RTT and integrate into the previoust target RTT
+		rtt := time.Duration(float64(1-qosTuningImpact)*float64(atomic.LoadUint64(&d.rttEstimate)) + qosTuningImpact*float64(d.peers.medianRTT()))
+		atomic.StoreUint64(&d.rttEstimate, uint64(rtt))
+
+		// A new RTT cycle passed, increase our confidence in the estimated RTT
+		conf := atomic.LoadUint64(&d.rttConfidence)
+		conf = conf + (1000000-conf)/2
+		atomic.StoreUint64(&d.rttConfidence, conf)
+
+		// Log the new QoS values and sleep until the next RTT
+		glog.V(logger.Debug).Infof("Quality of service: rtt %v, conf %.3f, ttl %v", rtt, float64(conf)/1000000.0, d.requestTTL())
+		select {
+		case <-d.quitCh:
+			return
+		case <-time.After(rtt):
+		}
+	}
+}
+
+// qosReduceConfidence is meant to be called when a new peer joins the downloader's
+// peer set, needing to reduce the confidence we have in out QoS estimates.
+func (d *Downloader) qosReduceConfidence() {
+	// If we have a single peer, confidence is always 1
+	peers := uint64(d.peers.Len())
+	if peers == 1 {
+		atomic.StoreUint64(&d.rttConfidence, 1000000)
+		return
+	}
+	// If we have a ton of peers, don't drop confidence)
+	if peers >= uint64(qosConfidenceCap) {
+		return
+	}
+	// Otherwise drop the confidence factor
+	conf := atomic.LoadUint64(&d.rttConfidence) * (peers - 1) / peers
+	if float64(conf)/1000000 < rttMinConfidence {
+		conf = uint64(rttMinConfidence * 1000000)
+	}
+	atomic.StoreUint64(&d.rttConfidence, conf)
+
+	rtt := time.Duration(atomic.LoadUint64(&d.rttEstimate))
+	glog.V(logger.Debug).Infof("Quality of service: rtt %v, conf %.3f, ttl %v", rtt, float64(conf)/1000000.0, d.requestTTL())
+}
+
+// requestRTT returns the current target round trip time for a download request
+// to complete in.
+//
+// Note, the returned RTT is .9 of the actually estimated RTT. The reason is that
+// the downloader tries to adapt queries to the RTT, so multiple RTT values can
+// be adapted to, but smaller ones are preffered (stabler download stream).
+func (d *Downloader) requestRTT() time.Duration {
+	return time.Duration(atomic.LoadUint64(&d.rttEstimate)) * 9 / 10
+}
+
+// requestTTL returns the current timeout allowance for a single download request
+// to finish under.
+func (d *Downloader) requestTTL() time.Duration {
+	var (
+		rtt  = time.Duration(atomic.LoadUint64(&d.rttEstimate))
+		conf = float64(atomic.LoadUint64(&d.rttConfidence)) / 1000000.0
+	)
+	ttl := time.Duration(ttlScaling) * time.Duration(float64(rtt)/conf)
+	if ttl > ttlLimit {
+		ttl = ttlLimit
+	}
+	return ttl
+}