size arena to match batch size

interface.go

@@ -37,6 +37,7 @@ type InterfaceConfig struct {
 	DropLocalBroadcast bool
 	DropMulticast      bool
 	routines           int
+	batchSize          int
 	MessageMetrics     *MessageMetrics
 	version            string
 	relayManager       *relayManager
@@ -70,6 +71,7 @@ type Interface struct {
 	dropLocalBroadcast    bool
 	dropMulticast         bool
 	routines              int
+	batchSize             int
 	disconnectInvalid     atomic.Bool
 	closed                atomic.Bool
 	relayManager          *relayManager
@@ -190,6 +192,7 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 		dropLocalBroadcast:    c.DropLocalBroadcast,
 		dropMulticast:         c.DropMulticast,
 		routines:              c.routines,
+		batchSize:             c.batchSize,
 		version:               c.version,
 		writers:               make([]udp.Conn, c.routines),
 		readers:               make([]tio.Queue, c.routines),
@@ -260,8 +263,8 @@ func (f *Interface) activate() error {
 	}
 	f.readers = f.inside.Readers()
 	for i := range f.readers {
-		arena := batch.NewArena(batch.DefaultPassthroughArenaCap)
+		arena := batch.NewArena(max(f.batchSize, 1) * udp.MTU)
-		f.batchers[i] = batch.NewPassthrough(f.readers[i], arena)
+		f.batchers[i] = batch.NewPassthrough(f.readers[i], f.batchSize, arena)
 	}
 
 	f.wg.Add(1) // for us to wait on Close() to return

main.go

@@ -215,6 +215,7 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		DropLocalBroadcast:    c.GetBool("tun.drop_local_broadcast", false),
 		DropMulticast:         c.GetBool("tun.drop_multicast", false),
 		routines:              routines,
+		batchSize:             c.GetInt("listen.batch", 64),
 		MessageMetrics:        messageMetrics,
 		version:               buildVersion,
 		relayManager:          NewRelayManager(ctx, l, hostMap, c),

overlay/batch/passthrough.go

@@ -20,10 +20,10 @@ const passthroughBaseNumSlots = 128
 // standalone Passthrough batcher: 128 slots × udp.MTU ≈ 1.1 MiB.
 const DefaultPassthroughArenaCap = passthroughBaseNumSlots * udp.MTU
 
-func NewPassthrough(w io.Writer, arena *Arena) *Passthrough {
+func NewPassthrough(w io.Writer, slots int, arena *Arena) *Passthrough {
 	return &Passthrough{
 		out:   w,
-		slots: make([][]byte, 0, passthroughBaseNumSlots),
+		slots: make([][]byte, 0, slots),
 		arena: arena,
 	}
 }