eth/downloader: adaptive quality of service tuning

eth/downloader/downloader.go

@@ -54,14 +54,14 @@ 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)
+	rttMinEstimate   = 2 * time.Second  // Minimum round-trip time to target for download requests
-	headerTTL        = 3 * time.Second      // [eth/62] Time it takes for a header request to time out
+	rttMaxEstimate   = 20 * time.Second // Maximum rount-trip time to target for download requests
-	bodyTargetRTT    = 3 * time.Second / 2  // [eth/62] Target time for completing a block body retrieval request
+	rttMinConfidence = 0.1              // Worse confidence factor in our estimated RTT value
-	bodyTTL          = 3 * bodyTargetRTT    // [eth/62] Maximum time allowance before a block body request is considered expired
+	rttTTLScaling    = 3                // Constant scaling factor for RTT -> TTL conversion
-	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
+	qosTuningPeers   = 5    // Number of peers to tune based on (best peers)
-	stateTargetRTT   = 2 * time.Second / 2  // [eth/63] Target time for completing a state trie retrieval request
+	qosConfidenceCap = 10   // Number of peers above which not to modify RTT confidence
-	stateTTL         = 3 * stateTargetRTT   // [eth/63] Maximum time allowance before a node data request is considered expired
+	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)
@@ -110,7 +110,9 @@ type Downloader struct {
 	queue *queue   // Scheduler for selecting the hashes to download
 	peers *peerSet // Set of active peers from which download can proceed
 
-	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)
+	interrupt     int32  // Atomic boolean to signal termination
 
 	// Statistics
 	syncStatsChainOrigin uint64       // Origin block number where syncing started at
@@ -169,11 +171,13 @@ 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,
@@ -201,6 +205,8 @@ func New(stateDb ethdb.Database, mux *event.TypeMux, hasHeader headerCheckFn, ha
 		stateWakeCh:      make(chan bool, 1),
 		headerProcCh:     make(chan []*types.Header, 1),
 	}
+	go dl.qosTuner()
+	return dl
 }
 
 // Progress retrieves the synchronisation boundaries, specifically the origin
@@ -247,6 +253,8 @@ 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
 }
 
@@ -915,7 +923,7 @@ 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
 				}
@@ -956,7 +964,7 @@ 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:
@@ -1024,7 +1032,7 @@ 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 {
@@ -1101,7 +1109,7 @@ 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
@@ -1182,7 +1190,7 @@ 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)
@@ -1290,13 +1298,13 @@ 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,
@@ -1320,9 +1328,9 @@ 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,
@@ -1344,9 +1352,9 @@ 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,
@@ -1396,13 +1404,13 @@ 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,
@@ -1864,3 +1872,82 @@ 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 atomic.LoadInt32(&d.interrupt) == 0 {
+		// Gather all the active RTTs and set the target RTT to the median
+		rtts := d.peers.RTTs()
+
+		var rtt time.Duration
+		if qosTuningPeers <= len(rtts) {
+			rtt = rtts[qosTuningPeers/2] // Median of our tuning peers
+		} else if len(rtts) > 0 {
+			rtt = rtts[len(rtts)/2] // Median of our connected peers (maintain even like this some baseline qos)
+		} else {
+			rtt = rttMaxEstimate // No peers, reset to some sane default
+		}
+		if rtt < rttMinEstimate {
+			rtt = rttMinEstimate
+		}
+		if rtt > rttMaxEstimate {
+			rtt = rttMaxEstimate
+		}
+		rtt = time.Duration(float64(1-qosTuningImpact)*float64(atomic.LoadUint64(&d.rttEstimate)) + qosTuningImpact*float64(rtt))
+		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())
+		time.Sleep(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
+	)
+	return time.Duration(rttTTLScaling) * time.Duration(float64(rtt)/conf)
+}

eth/downloader/downloader_test.go

@@ -1734,7 +1734,7 @@ func testDeliverHeadersHang(t *testing.T, protocol int, mode SyncMode) {
 			impl := tester.peerGetAbsHeadersFn("peer", 0)
 			go impl(from, count, skip, reverse)
 			// None of the extra deliveries should block.
-			timeout := time.After(5 * time.Second)
+			timeout := time.After(15 * time.Second)
 			for i := 0; i < cap(deliveriesDone); i++ {
 				select {
 				case <-deliveriesDone:

eth/downloader/peer.go

@@ -31,8 +31,8 @@ import (
 )
 
 const (
-	maxLackingHashes = 4096 // Maximum number of entries allowed on the list or lacking items
+	maxLackingHashes  = 4096 // Maximum number of entries allowed on the list or lacking items
-	throughputImpact = 0.1  // The impact a single measurement has on a peer's final throughput value.
+	measurementImpact = 0.1  // The impact a single measurement has on a peer's final throughput value.
 )
 
 // Hash and block fetchers belonging to eth/61 and below
@@ -68,6 +68,8 @@ type peer struct {
 	receiptThroughput float64 // Number of receipts measured to be retrievable per second
 	stateThroughput   float64 // Number of node data pieces measured to be retrievable per second
 
+	rtt time.Duration // Request round trip time to track responsiveness (QoS)
+
 	headerStarted  time.Time // Time instance when the last header fetch was started
 	blockStarted   time.Time // Time instance when the last block (body) fetch was started
 	receiptStarted time.Time // Time instance when the last receipt fetch was started
@@ -290,44 +292,47 @@ func (p *peer) setIdle(started time.Time, delivered int, throughput *float64, id
 		return
 	}
 	// Otherwise update the throughput with a new measurement
-	measured := float64(delivered) / (float64(time.Since(started)+1) / float64(time.Second)) // +1 (ns) to ensure non-zero divisor
+	elapsed := time.Since(started) + 1 // +1 (ns) to ensure non-zero divisor
-	*throughput = (1-throughputImpact)*(*throughput) + throughputImpact*measured
+	measured := float64(delivered) / (float64(elapsed) / float64(time.Second))
+
+	*throughput = (1-measurementImpact)*(*throughput) + measurementImpact*measured
+	p.rtt = time.Duration((1-measurementImpact)*float64(p.rtt) + measurementImpact*float64(elapsed))
 }
 
 // HeaderCapacity retrieves the peers header download allowance based on its
 // previously discovered throughput.
-func (p *peer) HeaderCapacity() int {
+func (p *peer) HeaderCapacity(targetRTT time.Duration) int {
 	p.lock.RLock()
 	defer p.lock.RUnlock()
 
-	return int(math.Max(1, math.Min(p.headerThroughput*float64(headerTargetRTT)/float64(time.Second), float64(MaxHeaderFetch))))
+	return int(math.Min(1+math.Max(1, p.headerThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxHeaderFetch)))
 }
 
 // BlockCapacity retrieves the peers block download allowance based on its
 // previously discovered throughput.
-func (p *peer) BlockCapacity() int {
+func (p *peer) BlockCapacity(targetRTT time.Duration) int {
 	p.lock.RLock()
 	defer p.lock.RUnlock()
 
-	return int(math.Max(1, math.Min(p.blockThroughput*float64(blockTargetRTT)/float64(time.Second), float64(MaxBlockFetch))))
+	return int(math.Min(1+math.Max(1, p.blockThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxBlockFetch)))
 }
 
 // ReceiptCapacity retrieves the peers receipt download allowance based on its
 // previously discovered throughput.
-func (p *peer) ReceiptCapacity() int {
+func (p *peer) ReceiptCapacity(targetRTT time.Duration) int {
 	p.lock.RLock()
 	defer p.lock.RUnlock()
 
-	return int(math.Max(1, math.Min(p.receiptThroughput*float64(receiptTargetRTT)/float64(time.Second), float64(MaxReceiptFetch))))
+	return int(math.Min(1+math.Max(1, p.receiptThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxReceiptFetch)))
 }
 
 // NodeDataCapacity retrieves the peers state download allowance based on its
 // previously discovered throughput.
-func (p *peer) NodeDataCapacity() int {
+func (p *peer) NodeDataCapacity(targetRTT time.Duration) int {
 	p.lock.RLock()
 	defer p.lock.RUnlock()
 
-	return int(math.Max(1, math.Min(p.stateThroughput*float64(stateTargetRTT)/float64(time.Second), float64(MaxStateFetch))))
+	return int(math.Min(1+math.Max(1, p.stateThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxStateFetch)))
 }
 
 // MarkLacking appends a new entity to the set of items (blocks, receipts, states)
@@ -362,11 +367,12 @@ func (p *peer) String() string {
 	defer p.lock.RUnlock()
 
 	return fmt.Sprintf("Peer %s [%s]", p.id,
-		fmt.Sprintf("headers %3.2f/s, ", p.headerThroughput)+
+		fmt.Sprintf("hs %3.2f/s, ", p.headerThroughput)+
-			fmt.Sprintf("blocks %3.2f/s, ", p.blockThroughput)+
+			fmt.Sprintf("bs %3.2f/s, ", p.blockThroughput)+
-			fmt.Sprintf("receipts %3.2f/s, ", p.receiptThroughput)+
+			fmt.Sprintf("rs %3.2f/s, ", p.receiptThroughput)+
-			fmt.Sprintf("states %3.2f/s, ", p.stateThroughput)+
+			fmt.Sprintf("ss %3.2f/s, ", p.stateThroughput)+
-			fmt.Sprintf("lacking %4d", len(p.lacking)),
+			fmt.Sprintf("miss %4d, ", len(p.lacking))+
+			fmt.Sprintf("rtt %v", p.rtt),
 	)
 }
 
@@ -410,6 +416,7 @@ func (ps *peerSet) Register(p *peer) error {
 	}
 	if len(ps.peers) > 0 {
 		p.headerThroughput, p.blockThroughput, p.receiptThroughput, p.stateThroughput = 0, 0, 0, 0
+		p.rtt = rttMaxEstimate
 
 		for _, peer := range ps.peers {
 			peer.lock.RLock()
@@ -564,3 +571,27 @@ func (ps *peerSet) idlePeers(minProtocol, maxProtocol int, idleCheck func(*peer)
 	}
 	return idle, total
 }
+
+// RTTs retrieves all the round trip times measured for the current set of peers,
+// sorted ascending.
+func (ps *peerSet) RTTs() []time.Duration {
+	// Gather all the currnetly measured round trip times
+	ps.lock.RLock()
+	defer ps.lock.RUnlock()
+
+	rtts := make([]time.Duration, 0, len(ps.peers))
+	for _, p := range ps.peers {
+		p.lock.RLock()
+		rtts = append(rtts, p.rtt)
+		p.lock.RUnlock()
+	}
+	// Sort into ascending order and return them
+	for i := 0; i < len(rtts); i++ {
+		for j := i + 1; j < len(rtts); j++ {
+			if rtts[i] > rtts[j] {
+				rtts[i], rtts[j] = rtts[j], rtts[i]
+			}
+		}
+	}
+	return rtts
+}