transport: optimize header field allocation

internal/transport/http2_server.go

@@ -945,6 +945,19 @@ func appendHeaderFieldsFromMD(headerFields []hpack.HeaderField, md metadata.MD)
 	return headerFields
 }
 
+// headerFieldsCountFromMD returns the total number of header fields that
+// appendHeaderFieldsFromMD will produce from md. Reserved headers are dropped
+// by appendHeaderFieldsFromMD, so this may slightly over-count, which is
+// preferable to growing the slice.
+func headerFieldsCountFromMD(md metadata.MD) int {
+	n := 0
+	for _, vv := range md {
+		n += len(vv)
+	}
+
+	return n
+}
+
 func (t *http2Server) checkForHeaderListSize(hf []hpack.HeaderField) bool {
 	if t.maxSendHeaderListSize == nil {
 		return true
@@ -1041,9 +1054,14 @@ func (t *http2Server) writeHeader(s *ServerStream, md metadata.MD) error {
 }
 
 func (t *http2Server) writeHeaderLocked(s *ServerStream) error {
-	// TODO(mmukhi): Benchmark if the performance gets better if count the metadata and other header fields
-	// first and create a slice of that exact size.
-	headerFields := make([]hpack.HeaderField, 0, 2) // at least :status, content-type will be there if none else.
+	// Count the metadata header fields as well so the slice is not reallocated
+	// while they are appended below.
+	headerFieldCount := 2 // :status, content-type
+	headerFieldCount += headerFieldsCountFromMD(s.header)
+	if s.sendCompress != "" {
+		headerFieldCount++
+	}
+	headerFields := make([]hpack.HeaderField, 0, headerFieldCount)
 	headerFields = append(headerFields, hpack.HeaderField{Name: ":status", Value: "200"})
 	headerFields = append(headerFields, hpack.HeaderField{Name: "content-type", Value: grpcutil.ContentType(s.contentSubtype)})
 	if s.sendCompress != "" {
@@ -1087,10 +1105,18 @@ func (t *http2Server) writeStatus(s *ServerStream, st *status.Status) error {
 		return nil
 	}
 
-	// TODO(mmukhi): Benchmark if the performance gets better if count the metadata and other header fields
-	// first and create a slice of that exact size.
-	headerFields := make([]hpack.HeaderField, 0, 2) // grpc-status and grpc-message will be there if none else.
-	if !s.updateHeaderSent() {                      // No headers have been sent.
+	// Count the metadata header fields as well so the slice is not reallocated
+	// while they are appended below.
+	headersAlreadySent := s.updateHeaderSent()
+	headerFieldCount := 2 // grpc-status, grpc-message
+	headerFieldCount += headerFieldsCountFromMD(s.trailer)
+	if !headersAlreadySent && len(s.header) == 0 {
+		// Trailer only response gets :status and content-type as well.
+		headerFieldCount += 2
+	}
+	// +1 for optional grpc-status-details-bin
+	headerFields := make([]hpack.HeaderField, 0, headerFieldCount+1)
+	if !headersAlreadySent { // No headers have been sent.
 		if len(s.header) > 0 { // Send a separate header frame.
 			if err := t.writeHeaderLocked(s); err != nil {
 				return err

internal/transport/http2_server_header_bench_test.go

@@ -0,0 +1,63 @@
+/*
+ *
+ * Copyright 2026 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package transport
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/net/http2/hpack"
+
+	"google.golang.org/grpc/metadata"
+)
+
+// BenchmarkAppendHeaderFieldsFromMD measures the slice re-allocation overhead
+// of appendHeaderFieldsFromMD when the initial slice capacity is too small
+// (old behavior) versus pre-sized to the exact metadata count (new behavior).
+func BenchmarkAppendHeaderFieldsFromMD(b *testing.B) {
+	for _, mdCount := range []int{0, 4, 12, 48} {
+		b.Run(fmt.Sprintf("mdCount=%d", mdCount), func(b *testing.B) {
+			md := make(metadata.MD, mdCount)
+			for i := 0; i < mdCount; i++ {
+				md[fmt.Sprintf("header-%d", i)] = []string{
+					fmt.Sprintf("value-%d-a", i),
+					fmt.Sprintf("value-%d-b", i),
+				}
+			}
+
+			b.Run("unsized", func(b *testing.B) {
+				b.ReportAllocs()
+				for b.Loop() {
+					headerFields := make([]hpack.HeaderField, 0, 2)
+					headerFields = appendHeaderFieldsFromMD(headerFields, md)
+					_ = headerFields
+				}
+			})
+
+			b.Run("sized", func(b *testing.B) {
+				b.ReportAllocs()
+				for b.Loop() {
+					headerFields := make([]hpack.HeaderField, 0, headerFieldsCountFromMD(md))
+					headerFields = appendHeaderFieldsFromMD(headerFields, md)
+					_ = headerFields
+				}
+			})
+		})
+	}
+}