eth/downloader: add a basic block download congestion control

eth/downloader/peer.go

@@ -5,10 +5,14 @@ package downloader
 
 import (
 	"errors"
+	"math"
 	"sync"
 	"sync/atomic"
+	"time"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/logger"
+	"github.com/ethereum/go-ethereum/logger/glog"
 	"gopkg.in/fatih/set.v0"
 )
 
@@ -27,14 +31,15 @@ type peer struct {
 	head common.Hash // Hash of the peers latest known block
 
 	idle int32 // Current activity state of the peer (idle = 0, active = 1)
-	rep  int32 // Simple peer reputation (not used currently)
+	rep  int32 // Simple peer reputation
 
-	mu sync.RWMutex
+	capacity int32     // Number of blocks allowed to fetch per request
+	started  time.Time // Time instance when the last fetch was started
 
-	ignored *set.Set
+	ignored *set.Set // Set of hashes not to request (didn't have previously)
 
-	getHashes hashFetcherFn
-	getBlocks blockFetcherFn
+	getHashes hashFetcherFn  // Method to retrieve a batch of hashes (mockable for testing)
+	getBlocks blockFetcherFn // Method to retrieve a batch of blocks (mockable for testing)
 }
 
 // newPeer create a new downloader peer, with specific hash and block retrieval
@@ -43,6 +48,7 @@ func newPeer(id string, head common.Hash, getHashes hashFetcherFn, getBlocks blo
 	return &peer{
 		id:        id,
 		head:      head,
+		capacity:  1,
 		getHashes: getHashes,
 		getBlocks: getBlocks,
 		ignored:   set.New(),
@@ -52,6 +58,7 @@ func newPeer(id string, head common.Hash, getHashes hashFetcherFn, getBlocks blo
 // Reset clears the internal state of a peer entity.
 func (p *peer) Reset() {
 	atomic.StoreInt32(&p.idle, 0)
+	atomic.StoreInt32(&p.capacity, 1)
 	p.ignored.Clear()
 }
 
@@ -61,6 +68,8 @@ func (p *peer) Fetch(request *fetchRequest) error {
 	if !atomic.CompareAndSwapInt32(&p.idle, 0, 1) {
 		return errAlreadyFetching
 	}
+	p.started = time.Now()
+
 	// Convert the hash set to a retrievable slice
 	hashes := make([]common.Hash, 0, len(request.Hashes))
 	for hash, _ := range request.Hashes {
@@ -72,10 +81,36 @@ func (p *peer) Fetch(request *fetchRequest) error {
 }
 
 // SetIdle sets the peer to idle, allowing it to execute new retrieval requests.
+// Its block retrieval allowance will also be updated either up- or downwards,
+// depending on whether the previous fetch completed in time or not.
 func (p *peer) SetIdle() {
+	// Update the peer's download allowance based on previous performance
+	scale := 2.0
+	if time.Since(p.started) > blockSoftTTL {
+		scale = 0.5
+	}
+	for {
+		// Calculate the new download bandwidth allowance
+		prev := atomic.LoadInt32(&p.capacity)
+		next := int32(math.Max(1, math.Min(MaxBlockFetch, float64(prev)*scale)))
+		if scale < 1 {
+			glog.V(logger.Detail).Infof("%s: reducing block allowance from %d to %d", p.id, prev, next)
+		}
+		// Try to update the old value
+		if atomic.CompareAndSwapInt32(&p.capacity, prev, next) {
+			break
+		}
+	}
+	// Set the peer to idle to allow further block requests
 	atomic.StoreInt32(&p.idle, 0)
 }
 
+// Capacity retrieves the peers block download allowance based on its previously
+// discovered bandwidth capacity.
+func (p *peer) Capacity() int {
+	return int(atomic.LoadInt32(&p.capacity))
+}
+
 // Promote increases the peer's reputation.
 func (p *peer) Promote() {
 	atomic.AddInt32(&p.rep, 1)