server.go: use worker goroutines for fewer stack allocations

Currently (go1.13.4), the default stack size for newly spawned
goroutines is 2048 bytes. This is insufficient when processing gRPC
requests as the we often require more than 4 KiB stacks. This causes the
Go runtime to call runtime.morestack at least twice per RPC, which
causes performance to suffer needlessly as stack reallocations require
all sorts of internal work such as changing pointers to point to new
addresses.

Since this stack growth is guaranteed to happen at least twice per RPC,
reusing goroutines gives us two wins:

  1. The stack is already grown to 8 KiB after the first RPC, so
     subsequent RPCs do not call runtime.morestack.
  2. We eliminate the need to spawn a new goroutine for each request
     (even though they're relatively inexpensive).

Performance improves across the board. The improvement is especially
visible in small, unary requests as the overhead of stack reallocation
is higher, percentage-wise. QPS is up anywhere between 3% and 5%
depending on the number of concurrent RPC requests in flight. Latency is
down ~3%. There is even a 1% decrease in memory footprint in some cases,
though that is an unintended, but happy coincidence.

unary-networkMode_none-bufConn_false-keepalive_false-benchTime_1m0s-trace_false-latency_0s-kbps_0-MTU_0-maxConcurrentCalls_8-reqSize_1B-respSize_1B-compressor_off-channelz_false-preloader_false
               Title       Before        After Percentage
            TotalOps      2613512      2701705     3.37%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op      8657.00      8654.17    -0.03%
           Allocs/op       173.37       173.28     0.00%
             ReqT/op    348468.27    360227.33     3.37%
            RespT/op    348468.27    360227.33     3.37%
            50th-Lat    174.601µs    167.378µs    -4.14%
            90th-Lat    233.132µs    229.087µs    -1.74%
            99th-Lat     438.98µs    441.857µs     0.66%
             Avg-Lat    183.263µs     177.26µs    -3.28%

internal/transport/transport.go

@@ -285,6 +285,10 @@ type Stream struct {
 	contentSubtype string
 }
 
+func (s *Stream) ID() uint32 {
+	return s.id
+}
+
 // isHeaderSent is only valid on the server-side.
 func (s *Stream) isHeaderSent() bool {
 	return atomic.LoadUint32(&s.headerSent) == 1

server.go

@@ -712,15 +712,53 @@ 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
+}
+
+// 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
-	st.HandleStreams(func(stream *transport.Stream) {
-		wg.Add(1)
+
+	streamChannels := make([]chan *transport.Stream, numWorkers)
+	for i := range streamChannels {
+		ch := make(chan *transport.Stream)
 		go func() {
-			defer wg.Done()
-			s.handleStream(st, stream, s.traceInfo(st, stream))
+			for stream := range ch {
+				s.handleStream(st, stream, s.traceInfo(st, stream))
+				wg.Done()
+			}
 		}()
+		streamChannels[i] = ch
+	}
+
+	st.HandleStreams(func(stream *transport.Stream) {
+		wg.Add(1)
+		select {
+		case streamChannels[stream.ID() & workerMask] <- stream:
+		default:
+			go func() {
+				s.handleStream(st, stream, s.traceInfo(st, stream))
+				wg.Done()
+			}()
+		}
 	}, func(ctx context.Context, method string) context.Context {
 		if !EnableTracing {
 			return ctx
@@ -729,6 +767,10 @@ func (s *Server) serveStreams(st transport.ServerTransport) {
 		return trace.NewContext(ctx, tr)
 	})
 	wg.Wait()
+
+	for _, ch := range streamChannels {
+		close(ch)
+	}
 }
 
 var _ http.Handler = (*Server)(nil)