eth/downloader: clean up PR change. Add some docs where appropriate and remove dead code/parameters/constants

eth/downloader/downloader.go

@@ -206,7 +206,7 @@ func New(stateDb ethdb.Database, mux *event.TypeMux, chain BlockChain, dropPeer
 	dl := &Downloader{
 		stateDB:        stateDb,
 		mux:            mux,
-		queue:          newQueue(blockCacheMaxItems, blockCacheInitialItems),
+		queue:          newQueue(),
 		peers:          newPeerSet(),
 		blockchain:     chain,
 		dropPeer:       dropPeer,
@@ -352,7 +352,7 @@ func (d *Downloader) synchronise(mode SyncMode, beaconPing chan struct{}) error
 		}
 	}
 	// Reset the queue, peer set and wake channels to clean any internal leftover state
-	d.queue.Reset(blockCacheMaxItems, blockCacheInitialItems)
+	d.queue.Reset()
 	d.peers.Reset()
 
 	for _, ch := range []chan bool{d.queue.blockWakeCh, d.queue.receiptWakeCh} {

eth/downloader/queue.go

@@ -43,7 +43,6 @@ const (
 
 var (
 	blockCacheMaxItems = 8192               // Maximum number of blocks to cache before throttling the download
-	blockCacheInitialItems = 32                 // Initial number of blocks to start fetching, before we know the sizes of the blocks
 	blockCacheMemory   = 1024 * 1024 * 1024 // Maximum amount of memory to use for block caching
 )
 
@@ -159,7 +158,7 @@ type queue struct {
 }
 
 // newQueue creates a new download queue for scheduling block retrieval.
-func newQueue(blockCacheLimit int, thresholdInitialSize int) *queue {
+func newQueue() *queue {
 	lock := new(sync.RWMutex)
 	q := &queue{
 		headerContCh:     make(chan bool, 1),
@@ -170,12 +169,12 @@ func newQueue(blockCacheLimit int, thresholdInitialSize int) *queue {
 		active:           sync.NewCond(lock),
 		lock:             lock,
 	}
-	q.Reset(blockCacheLimit, thresholdInitialSize)
+	q.Reset()
 	return q
 }
 
 // Reset clears out the queue contents.
-func (q *queue) Reset(blockCacheLimit int, thresholdInitialSize int) {
+func (q *queue) Reset() {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
@@ -193,7 +192,7 @@ func (q *queue) Reset(blockCacheLimit int, thresholdInitialSize int) {
 	q.receiptTaskQueue.Reset()
 	q.receiptPendPool = make(map[string]*fetchRequest)
 
-	q.resultCache = newResultStore(blockCacheLimit)
+	q.resultCache = newResultStore()
 }
 
 // Close marks the end of the sync, unblocking Results.

eth/downloader/resultstore.go

@@ -43,10 +43,10 @@ type resultStore struct {
 	lock sync.RWMutex
 }
 
-func newResultStore(size int) *resultStore {
+func newResultStore() *resultStore {
 	return &resultStore{
 		resultOffset: 0,
-		items:        make([]*fetchResult, size),
+		items:        make([]*fetchResult, blockCacheMaxItems),
 	}
 }
 
@@ -99,8 +99,10 @@ func (r *resultStore) GetDeliverySlot(headerNumber uint64) (*fetchResult, bool,
 // blocks each contain a transaction filled with calldata that is all zeroes.
 // The size of a worst-case block is ~= gasUsed / 10
 func (r *resultStore) throttleThreshold(index int) bool {
-	// estimate the average block size of all scheduled blocks
-	estBlockSize := max((r.itemsGasUsed/(uint64(index)+1))/10, 524)
+	// estimate the average size of a worst-case block, given what we have already queued:
+	//   total gas used by in flight blocks / number of in-flight blocks / 10
+	var headerSize uint64 = 524
+	estBlockSize := max((r.itemsGasUsed/(uint64(index)+1))/10, headerSize)
 
 	throttleThreshold := min(uint64(len(r.items)), uint64(blockCacheMemory)/estBlockSize+1)
 	return index >= int(throttleThreshold)