refactor: send RequestNext in batches for chainsync pipelining (#843)

We now send pipelined RequestNext messages in batches instead of an
initial batch and subsequent single messages. This potentially improves
performance and reduces pressure on mini-protocol buffers

Author: agaffney

protocol/chainsync/client.go

@@ -28,12 +28,13 @@ import (
 // Client implements the ChainSync client
 type Client struct {
 	*protocol.Protocol
-	config                *Config
-	callbackContext       CallbackContext
-	busyMutex             sync.Mutex
-	readyForNextBlockChan chan bool
-	onceStart             sync.Once
-	onceStop              sync.Once
+	config                   *Config
+	callbackContext          CallbackContext
+	busyMutex                sync.Mutex
+	readyForNextBlockChan    chan bool
+	onceStart                sync.Once
+	onceStop                 sync.Once
+	syncPipelinedRequestNext int
 
 	// waitingForCurrentTipChan will process all the requests for the current tip until the channel
 	// is empty.
@@ -404,8 +405,8 @@ func (c *Client) Sync(intersectPoints []common.Point) error {
 	}
 
 	intersectResultChan, cancel := c.wantIntersectFound()
-	msg := NewMsgFindIntersect(intersectPoints)
-	if err := c.SendMessage(msg); err != nil {
+	msgFindIntersect := NewMsgFindIntersect(intersectPoints)
+	if err := c.SendMessage(msgFindIntersect); err != nil {
 		cancel()
 		return err
 	}
@@ -418,14 +419,14 @@ func (c *Client) Sync(intersectPoints []common.Point) error {
 		}
 	}
 
-	// Pipeline the initial block requests to speed things up a bit
-	// Using a value higher than 10 seems to cause problems with NtN
-	for i := 0; i <= c.config.PipelineLimit; i++ {
-		msg := NewMsgRequestNext()
-		if err := c.SendMessage(msg); err != nil {
-			return err
-		}
+	// Send initial RequestNext
+	msgRequestNext := NewMsgRequestNext()
+	if err := c.SendMessage(msgRequestNext); err != nil {
+		return err
 	}
+	// Reset pipelined message counter
+	c.syncPipelinedRequestNext = 0
+	// Start sync loop
 	go c.syncLoop()
 	return nil
 }
@@ -441,15 +442,23 @@ func (c *Client) syncLoop() {
 			return
 		}
 		c.busyMutex.Lock()
-		// Request the next block
-		// In practice we already have multiple block requests pipelined
-		// and this just adds another one to the pile
-		msg := NewMsgRequestNext()
-		if err := c.SendMessage(msg); err != nil {
-			c.SendError(err)
+		// Wait for next block if we have pipelined messages
+		if c.syncPipelinedRequestNext > 0 {
+			c.syncPipelinedRequestNext--
 			c.busyMutex.Unlock()
-			return
+			continue
+		}
+		// Request the next block(s)
+		msgCount := max(c.config.PipelineLimit, 1)
+		for i := 0; i < msgCount; i++ {
+			msg := NewMsgRequestNext()
+			if err := c.SendMessage(msg); err != nil {
+				c.SendError(err)
+				c.busyMutex.Unlock()
+				return
+			}
 		}
+		c.syncPipelinedRequestNext = msgCount - 1
 		c.busyMutex.Unlock()
 	}
 }