Merge pull request #2630 from karalabe/adaptive-qos-tuning

eth/downloader: adaptive quality of service tuning

3 files changed, 235 insertions, 48 deletions

diff --git a/eth/downloader/downloader.go b/eth/downloader/downloader.go
index 0bc8b4acf..92124cfeb 100644
--- a/eth/downloader/downloader.go
+++ b/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
+}
diff --git a/eth/downloader/downloader_test.go b/eth/downloader/downloader_test.go
index f3a0e38f1..a9c069a92 100644
--- a/eth/downloader/downloader_test.go
+++ b/eth/downloader/downloader_test.go
@@ -179,6 +179,12 @@ func newTester() *downloadTester {
 	return tester
 }
 
+// terminate aborts any operations on the embedded downloader and releases all
+// held resources.
+func (dl *downloadTester) terminate() {
+	dl.downloader.Terminate()
+}
+
 // sync starts synchronizing with a remote peer, blocking until it completes.
 func (dl *downloadTester) sync(id string, td *big.Int, mode SyncMode) error {
 	dl.lock.RLock()
@@ -740,6 +746,8 @@ func testCanonicalSynchronisation(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
 
 	// Synchronise with the peer and make sure all relevant data was retrieved
@@ -764,6 +772,8 @@ func testThrottling(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
 
 	// Wrap the importer to allow stepping
@@ -851,6 +861,8 @@ func testForkedSync(t *testing.T, protocol int, mode SyncMode) {
 	hashesA, hashesB, headersA, headersB, blocksA, blocksB, receiptsA, receiptsB := makeChainFork(common+fork, fork, genesis, nil, true)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("fork A", protocol, hashesA, headersA, blocksA, receiptsA)
 	tester.newPeer("fork B", protocol, hashesB, headersB, blocksB, receiptsB)
 
@@ -885,6 +897,8 @@ func testHeavyForkedSync(t *testing.T, protocol int, mode SyncMode) {
 	hashesA, hashesB, headersA, headersB, blocksA, blocksB, receiptsA, receiptsB := makeChainFork(common+fork, fork, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("light", protocol, hashesA, headersA, blocksA, receiptsA)
 	tester.newPeer("heavy", protocol, hashesB[fork/2:], headersB, blocksB, receiptsB)
 
@@ -934,6 +948,8 @@ func testBoundedForkedSync(t *testing.T, protocol int, mode SyncMode) {
 	hashesA, hashesB, headersA, headersB, blocksA, blocksB, receiptsA, receiptsB := makeChainFork(common+fork, fork, genesis, nil, true)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("original", protocol, hashesA, headersA, blocksA, receiptsA)
 	tester.newPeer("rewriter", protocol, hashesB, headersB, blocksB, receiptsB)
 
@@ -968,6 +984,8 @@ func testBoundedHeavyForkedSync(t *testing.T, protocol int, mode SyncMode) {
 	hashesA, hashesB, headersA, headersB, blocksA, blocksB, receiptsA, receiptsB := makeChainFork(common+fork, fork, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("original", protocol, hashesA, headersA, blocksA, receiptsA)
 	tester.newPeer("heavy-rewriter", protocol, hashesB[MaxForkAncestry-17:], headersB, blocksB, receiptsB) // Root the fork below the ancestor limit
 
@@ -987,7 +1005,9 @@ func testBoundedHeavyForkedSync(t *testing.T, protocol int, mode SyncMode) {
 // bodies.
 func TestInactiveDownloader62(t *testing.T) {
 	t.Parallel()
+
 	tester := newTester()
+	defer tester.terminate()
 
 	// Check that neither block headers nor bodies are accepted
 	if err := tester.downloader.DeliverHeaders("bad peer", []*types.Header{}); err != errNoSyncActive {
@@ -1002,7 +1022,9 @@ func TestInactiveDownloader62(t *testing.T) {
 // bodies and receipts.
 func TestInactiveDownloader63(t *testing.T) {
 	t.Parallel()
+
 	tester := newTester()
+	defer tester.terminate()
 
 	// Check that neither block headers nor bodies are accepted
 	if err := tester.downloader.DeliverHeaders("bad peer", []*types.Header{}); err != errNoSyncActive {
@@ -1039,6 +1061,8 @@ func testCancel(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
 
 	// Make sure canceling works with a pristine downloader
@@ -1074,6 +1098,8 @@ func testMultiSynchronisation(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	for i := 0; i < targetPeers; i++ {
 		id := fmt.Sprintf("peer #%d", i)
 		tester.newPeer(id, protocol, hashes[i*blockCacheLimit:], headers, blocks, receipts)
@@ -1103,6 +1129,8 @@ func testMultiProtoSync(t *testing.T, protocol int, mode SyncMode) {
 
 	// Create peers of every type
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("peer 61", 61, hashes, nil, blocks, nil)
 	tester.newPeer("peer 62", 62, hashes, headers, blocks, nil)
 	tester.newPeer("peer 63", 63, hashes, headers, blocks, receipts)
@@ -1140,6 +1168,8 @@ func testEmptyShortCircuit(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
 
 	// Instrument the downloader to signal body requests
@@ -1193,6 +1223,7 @@ func testMissingHeaderAttack(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
 
 	// Attempt a full sync with an attacker feeding gapped headers
 	tester.newPeer("attack", protocol, hashes, headers, blocks, receipts)
@@ -1225,6 +1256,7 @@ func testShiftedHeaderAttack(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
 
 	// Attempt a full sync with an attacker feeding shifted headers
 	tester.newPeer("attack", protocol, hashes, headers, blocks, receipts)
@@ -1256,6 +1288,7 @@ func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 	hashes, headers, blocks, receipts := makeChain(targetBlocks, 0, genesis, nil, false)
 
 	tester := newTester()
+	defer tester.terminate()
 
 	// Attempt to sync with an attacker that feeds junk during the fast sync phase.
 	// This should result in the last fsHeaderSafetyNet headers being rolled back.
@@ -1347,9 +1380,11 @@ func testHighTDStarvationAttack(t *testing.T, protocol int, mode SyncMode) {
 	t.Parallel()
 
 	tester := newTester()
-	hashes, headers, blocks, receipts := makeChain(0, 0, genesis, nil, false)
+	defer tester.terminate()
 
+	hashes, headers, blocks, receipts := makeChain(0, 0, genesis, nil, false)
 	tester.newPeer("attack", protocol, []common.Hash{hashes[0]}, headers, blocks, receipts)
+
 	if err := tester.sync("attack", big.NewInt(1000000), mode); err != errStallingPeer {
 		t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errStallingPeer)
 	}
@@ -1392,6 +1427,8 @@ func testBlockHeaderAttackerDropping(t *testing.T, protocol int) {
 	}
 	// Run the tests and check disconnection status
 	tester := newTester()
+	defer tester.terminate()
+
 	for i, tt := range tests {
 		// Register a new peer and ensure it's presence
 		id := fmt.Sprintf("test %d", i)
@@ -1433,6 +1470,8 @@ func testSyncProgress(t *testing.T, protocol int, mode SyncMode) {
 	progress := make(chan struct{})
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.downloader.syncInitHook = func(origin, latest uint64) {
 		starting <- struct{}{}
 		<-progress
@@ -1505,6 +1544,8 @@ func testForkedSyncProgress(t *testing.T, protocol int, mode SyncMode) {
 	progress := make(chan struct{})
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.downloader.syncInitHook = func(origin, latest uint64) {
 		starting <- struct{}{}
 		<-progress
@@ -1580,6 +1621,8 @@ func testFailedSyncProgress(t *testing.T, protocol int, mode SyncMode) {
 	progress := make(chan struct{})
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.downloader.syncInitHook = func(origin, latest uint64) {
 		starting <- struct{}{}
 		<-progress
@@ -1656,6 +1699,8 @@ func testFakedSyncProgress(t *testing.T, protocol int, mode SyncMode) {
 	progress := make(chan struct{})
 
 	tester := newTester()
+	defer tester.terminate()
+
 	tester.downloader.syncInitHook = func(origin, latest uint64) {
 		starting <- struct{}{}
 		<-progress
@@ -1742,7 +1787,7 @@ 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:
@@ -1755,6 +1800,7 @@ func testDeliverHeadersHang(t *testing.T, protocol int, mode SyncMode) {
 		if err := tester.sync("peer", nil, mode); err != nil {
 			t.Errorf("sync failed: %v", err)
 		}
+		tester.terminate()
 	}
 }
 
@@ -1772,8 +1818,9 @@ func testFastCriticalRestarts(t *testing.T, protocol int) {
 
 	// Create a tester peer with the critical section state roots missing (force failures)
 	tester := newTester()
-	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
+	defer tester.terminate()
 
+	tester.newPeer("peer", protocol, hashes, headers, blocks, receipts)
 	for i := 0; i < fsPivotInterval; i++ {
 		tester.peerMissingStates["peer"][headers[hashes[fsMinFullBlocks+i]].Root] = true
 	}
@@ -1783,11 +1830,14 @@ func testFastCriticalRestarts(t *testing.T, protocol int) {
 		if err := tester.sync("peer", nil, FastSync); err == nil {
 			t.Fatalf("failing fast sync succeeded: %v", err)
 		}
+		time.Sleep(500 * time.Millisecond) // Make sure no in-flight requests remain
+
 		// If it's the first failure, pivot should be locked => reenable all others to detect pivot changes
 		if i == 0 {
+			tester.lock.Lock()
 			tester.peerMissingStates["peer"] = map[common.Hash]bool{tester.downloader.fsPivotLock.Root: true}
+			tester.lock.Unlock()
 		}
-		time.Sleep(100 * time.Millisecond) // Make sure no in-flight requests remain
 	}
 	// Retry limit exhausted, downloader will switch to full sync, should succeed
 	if err := tester.sync("peer", nil, FastSync); err != nil {
diff --git a/eth/downloader/peer.go b/eth/downloader/peer.go
index 6aab907d7..94d44fca4 100644
--- a/eth/downloader/peer.go
+++ b/eth/downloader/peer.go
@@ -23,6 +23,8 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"sort"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -31,8 +33,8 @@ import (
 )
 
 const (
-	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.
+	maxLackingHashes  = 4096 // Maximum number of entries allowed on the list or lacking items
+	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 +70,8 @@ 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 +294,47 @@ 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
-	*throughput = (1-throughputImpact)*(*throughput) + throughputImpact*measured
+	elapsed := time.Since(started) + 1 // +1 (ns) to ensure non-zero divisor
+	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)
@@ -361,13 +368,14 @@ func (p *peer) String() string {
 	p.lock.RLock()
 	defer p.lock.RUnlock()
 
-	return fmt.Sprintf("Peer %s [%s]", p.id,
-		fmt.Sprintf("headers %3.2f/s, ", p.headerThroughput)+
-			fmt.Sprintf("blocks %3.2f/s, ", p.blockThroughput)+
-			fmt.Sprintf("receipts %3.2f/s, ", p.receiptThroughput)+
-			fmt.Sprintf("states %3.2f/s, ", p.stateThroughput)+
-			fmt.Sprintf("lacking %4d", len(p.lacking)),
-	)
+	return fmt.Sprintf("Peer %s [%s]", p.id, strings.Join([]string{
+		fmt.Sprintf("hs %3.2f/s", p.headerThroughput),
+		fmt.Sprintf("bs %3.2f/s", p.blockThroughput),
+		fmt.Sprintf("rs %3.2f/s", p.receiptThroughput),
+		fmt.Sprintf("ss %3.2f/s", p.stateThroughput),
+		fmt.Sprintf("miss %4d", len(p.lacking)),
+		fmt.Sprintf("rtt %v", p.rtt),
+	}, ", "))
 }
 
 // peerSet represents the collection of active peer participating in the chain
@@ -402,6 +410,10 @@ func (ps *peerSet) Reset() {
 // average of all existing peers, to give it a realistic chance of being used
 // for data retrievals.
 func (ps *peerSet) Register(p *peer) error {
+	// Retrieve the current median RTT as a sane default
+	p.rtt = ps.medianRTT()
+
+	// Register the new peer with some meaningful defaults
 	ps.lock.Lock()
 	defer ps.lock.Unlock()
 
@@ -564,3 +576,34 @@ func (ps *peerSet) idlePeers(minProtocol, maxProtocol int, idleCheck func(*peer)
 	}
 	return idle, total
 }
+
+// medianRTT returns the median RTT of te peerset, considering only the tuning
+// peers if there are more peers available.
+func (ps *peerSet) medianRTT() time.Duration {
+	// Gather all the currnetly measured round trip times
+	ps.lock.RLock()
+	defer ps.lock.RUnlock()
+
+	rtts := make([]float64, 0, len(ps.peers))
+	for _, p := range ps.peers {
+		p.lock.RLock()
+		rtts = append(rtts, float64(p.rtt))
+		p.lock.RUnlock()
+	}
+	sort.Float64s(rtts)
+
+	median := rttMaxEstimate
+	if qosTuningPeers <= len(rtts) {
+		median = time.Duration(rtts[qosTuningPeers/2]) // Median of our tuning peers
+	} else if len(rtts) > 0 {
+		median = time.Duration(rtts[len(rtts)/2]) // Median of our connected peers (maintain even like this some baseline qos)
+	}
+	// Restrict the RTT into some QoS defaults, irrelevant of true RTT
+	if median < rttMinEstimate {
+		median = rttMinEstimate
+	}
+	if median > rttMaxEstimate {
+		median = rttMaxEstimate
+	}
+	return median
+}