Adjust batch size in fifo qdisc to match the fdbased batch size.

These two are frequently used together, so this change slightly optimizes
the number of write syscalls.

PiperOrigin-RevId: 463161851

pkg/tcpip/link/fdbased/endpoint.go

@@ -66,6 +66,11 @@ type linkDispatcher interface {
 type PacketDispatchMode int
 
 const (
+	// BatchSize is the number of packets to write in each syscall. It is 47
+	// because when GvisorGSO is in use then a single 65KB TCP segment can get
+	// split into 46 segments of 1420 bytes and a single 216 byte segment.
+	BatchSize = 47
+
 	// Readv is the default dispatch mode and is the least performant of the
 	// dispatch options but the one that is supported by all underlying FD
 	// types.
@@ -670,11 +675,7 @@ func (e *endpoint) sendBatch(batchFDInfo fdInfo, pkts []*stack.PacketBuffer) (in
 //   - pkt.NetworkProtocolNumber
 func (e *endpoint) WritePackets(pkts stack.PacketBufferList) (int, tcpip.Error) {
 	// Preallocate to avoid repeated reallocation as we append to batch.
-	// batchSz is 47 because when GvisorGSO is in use then a single 65KB TCP
-	// segment can get split into 46 segments of 1420 bytes and a single 216
-	// byte segment.
-	const batchSz = 47
-	batch := make([]*stack.PacketBuffer, 0, batchSz)
+	batch := make([]*stack.PacketBuffer, 0, BatchSize)
 	batchFDInfo := fdInfo{fd: -1, isSocket: false}
 	sentPackets := 0
 	for _, pkt := range pkts.AsSlice() {

pkg/tcpip/link/qdisc/fifo/fifo.go

@@ -28,9 +28,11 @@ import (
 var _ stack.QueueingDiscipline = (*discipline)(nil)
 
 const (
-	// BatchSize represents the number of packets written to the
-	// lower link endpoint during calls to WritePackets.
-	BatchSize   = 32
+	// BatchSize is the number of packets to write in each syscall. It is 47
+	// because when GvisorGSO is in use then a single 65KB TCP segment can get
+	// split into 46 segments of 1420 bytes and a single 216 byte segment.
+	BatchSize = 47
+
 	qDiscClosed = 1
 )