eth/downloader: change intial download size (#21366)

This changes how the downloader works, a little bit. Previously, when block sync started,
we immediately started filling up to 8192 blocks. Usually this is fine, blocks are small
in the early numbers. The threshold then is lowered as we measure the size of the blocks
that are filled.

However, if the node is shut down and restarts syncing while we're in a heavy segment,
that might be bad. This PR introduces a more conservative initial threshold of 2K blocks
instead.

eth/downloader/queue.go

@@ -40,9 +40,10 @@ const (
 )
 
 var (
-	blockCacheItems      = 8192             // Maximum number of blocks to cache before throttling the download
-	blockCacheMemory     = 64 * 1024 * 1024 // Maximum amount of memory to use for block caching
-	blockCacheSizeWeight = 0.1              // Multiplier to approximate the average block size based on past ones
+	blockCacheMaxItems     = 8192             // Maximum number of blocks to cache before throttling the download
+	blockCacheInitialItems = 2048             // Initial number of blocks to start fetching, before we know the sizes of the blocks
+	blockCacheMemory       = 64 * 1024 * 1024 // Maximum amount of memory to use for block caching
+	blockCacheSizeWeight   = 0.1              // Multiplier to approximate the average block size based on past ones
 )
 
 var (
@@ -142,7 +143,7 @@ type queue struct {
 }
 
 // newQueue creates a new download queue for scheduling block retrieval.
-func newQueue(blockCacheLimit int) *queue {
+func newQueue(blockCacheLimit int, thresholdInitialSize int) *queue {
 	lock := new(sync.RWMutex)
 	q := &queue{
 		headerContCh:     make(chan bool),
@@ -151,12 +152,12 @@ func newQueue(blockCacheLimit int) *queue {
 		active:           sync.NewCond(lock),
 		lock:             lock,
 	}
-	q.Reset(blockCacheLimit)
+	q.Reset(blockCacheLimit, thresholdInitialSize)
 	return q
 }
 
 // Reset clears out the queue contents.
-func (q *queue) Reset(blockCacheLimit int) {
+func (q *queue) Reset(blockCacheLimit int, thresholdInitialSize int) {
 	q.lock.Lock()
 	defer q.lock.Unlock()
 
@@ -175,6 +176,7 @@ func (q *queue) Reset(blockCacheLimit int) {
 	q.receiptPendPool = make(map[string]*fetchRequest)
 
 	q.resultCache = newResultStore(blockCacheLimit)
+	q.resultCache.SetThrottleThreshold(uint64(thresholdInitialSize))
 }
 
 // Close marks the end of the sync, unblocking Results.