V2: server option, stream ID bitshift

server.go

@@ -131,6 +131,7 @@ type serverOptions struct {
 	connectionTimeout     time.Duration
 	maxHeaderListSize     *uint32
 	headerTableSize       *uint32
+	numStreamWorkers      uint32
 }
 
 var defaultServerOptions = serverOptions{
@@ -388,6 +389,22 @@ func HeaderTableSize(s uint32) ServerOption {
 	})
 }
 
+// NumStreamWorkers returns a ServerOption that sets the number of worker
+// goroutines that should be used to process incoming streams. Setting this to
+// zero (default) will disable workers and spawn a new goroutine for each
+// stream.
+//
+// This API is EXPERIMENTAL.
+func NumStreamWorkers(numStreamWorkers uint32) ServerOption {
+	// TODO: If/when this API gets stabilized (i.e. stream workers become the
+	// only way streams are processed), change the behavior of the zero value to
+	// a sane default. Preliminary experiments suggest that a value equal to the
+	// number of CPUs available is most performant; requires thorough testing.
+	return newFuncServerOption(func(o *serverOptions) {
+		o.numStreamWorkers = numStreamWorkers
+	})
+}
+
 // NewServer creates a gRPC server which has no service registered and has not
 // started to accept requests yet.
 func NewServer(opt ...ServerOption) *Server {
@@ -712,17 +729,6 @@ func (s *Server) newHTTP2Transport(c net.Conn, authInfo credentials.AuthInfo) tr
 	return st
 }
 
-func floorCPUCount() uint32 {
-	n := uint32(runtime.NumCPU())
-	for i := uint32(1 << 31); i >= 2; i >>= 1 {
-		if n&i > 0 {
-			return i
-		}
-	}
-
-	return 1
-}
-
 // workerStackReset defines how often the stack must be reset. Every N
 // requests, by spawning a new goroutine in its place, a worker can reset its
 // stack so that large stacks don't live in memory forever. 2^16 should allow
@@ -749,34 +755,36 @@ func (s *Server) streamWorker(st transport.ServerTransport, wg *sync.WaitGroup,
 	}
 }
 
-// numWorkers defines the number of stream handling workers. After experiments
-// with different CPU counts, using the floor of the number of CPUs available
-// was found to be the number optimal for performance across the board (QPS,
-// latency).
-var numWorkers = floorCPUCount()
-
-// workerMask is used to perform bitwise AND operations instead of expensive
-// module operations on integers.
-var workerMask = numWorkers - 1
-
 func (s *Server) serveStreams(st transport.ServerTransport) {
 	defer st.Close()
 	var wg sync.WaitGroup
 
-	streamChannels := make([]chan *transport.Stream, numWorkers)
-	for i := range streamChannels {
-		streamChannels[i] = make(chan *transport.Stream)
-		go s.streamWorker(st, &wg, streamChannels[i])
+	var streamChannels []chan *transport.Stream
+	if s.opts.numStreamWorkers > 0 {
+		streamChannels = make([]chan *transport.Stream, s.opts.numStreamWorkers)
+		for i := range streamChannels {
+			streamChannels[i] = make(chan *transport.Stream)
+			go s.streamWorker(st, &wg, streamChannels[i])
+		}
 	}
 
+	var streamChannelCounter uint32
 	st.HandleStreams(func(stream *transport.Stream) {
 		wg.Add(1)
-		select {
-		case streamChannels[stream.ID()&workerMask] <- stream:
-		default:
+		if s.opts.numStreamWorkers > 0 {
+			select {
+			case streamChannels[atomic.AddUint32(&streamChannelCounter, 1)%s.opts.numStreamWorkers] <- stream:
+			default:
+				// If all stream workers are busy, fallback to default code path.
+				go func() {
+					s.handleStream(st, stream, s.traceInfo(st, stream))
+					wg.Done()
+				}()
+			}
+		} else {
 			go func() {
+				defer wg.Done()
 				s.handleStream(st, stream, s.traceInfo(st, stream))
-				wg.Done()
 			}()
 		}
 	}, func(ctx context.Context, method string) context.Context {
@@ -788,8 +796,10 @@ func (s *Server) serveStreams(st transport.ServerTransport) {
 	})
 	wg.Wait()
 
-	for _, ch := range streamChannels {
-		close(ch)
+	if s.opts.numStreamWorkers > 0 {
+		for _, ch := range streamChannels {
+			close(ch)
+		}
 	}
 }