device: add MaxBatchSizeOverride global for default batch-size control

device/device.go

@@ -307,6 +307,10 @@ func NewDevice(tunDevice tun.Device, bind conn.Bind, logger *Logger) *Device {
 	device.rate.limiter.Init()
 	device.indexTable.Init()
 
+	if MaxBatchSizeOverride > 0 {
+		device.batchSizeOverride.Store(int32(MaxBatchSizeOverride))
+	}
+
 	device.PopulatePools()
 
 	// create queues

device/pool_size.go

@@ -5,6 +5,12 @@
 
 package device
 
+// MaxBatchSizeOverride, when nonzero, replaces the per-Device batch size used
+// to size eager buffer allocations in RoutineReceiveIncoming and
+// RoutineReadFromTUN. Changes affect Devices created after this assignment.
+// Use SetMaxBatchSizeOverride to change it safely at runtime.
+var MaxBatchSizeOverride uint32 = 0
+
 // SetPreallocatedBuffersPerPool sets the cap on the number of buffers held by
 // each per-Device pool. Zero disables the cap (upstream default on
 // non-mobile platforms). Changes affect Devices created after this call.
@@ -13,6 +19,13 @@ func SetPreallocatedBuffersPerPool(n uint32) {
 	PreallocatedBuffersPerPool = n
 }
 
+// SetMaxBatchSizeOverride sets the global batch size override applied to
+// Devices created after this call. Zero disables the override. Existing
+// Devices are unaffected; use Device.SetMaxBatchSize for per-instance.
+func SetMaxBatchSizeOverride(n uint32) {
+	MaxBatchSizeOverride = n
+}
+
 // SetPreallocatedBuffersPerPool updates the cap on this Device's pools in
 // place. Takes effect immediately; goroutines blocked in Get are unblocked if
 // the cap was raised. Has no effect if the Device was created with