Passthrough.Flush hands its whole lane to a new tio.BatchWriter in one
WriteBatch call when the queue implements it, and NewSingleQueue
exposes WriteBatch when its source does. On darwin, tun.WriteBatch
writes up to 64 packets per sendmsg_x on the utun control socket,
which is a connected datagram socket, so xnu takes the batched
sendit_x path rather than one sendit per message. utun registers no
send_list handler, so the kernel still passes each packet to utun on
its own: this saves syscalls and fd locking, not per-packet kernel work.
Error handling follows what sendmsg_x reports:
- A short count with no error (a packet past the send buffer
mid-call, or a later send failing with ENOBUFS): the remainder goes
out in the next sendmsg_x.
- EMSGSIZE: only returned when the first packet is too large, before
the kernel takes any, so that packet alone goes through Write, which
reports it, and the rest are batched again.
- ENOSYS, EPERM, EOPNOTSUPP: sendmsg_x is private API, so WriteBatch
logs once and falls back to writev per packet for good.
- Anything else: sendmsg_x returns the error without the count it
took, so the call is reported rather than retried, which could
deliver packets twice.
One loss mode has no userspace fix. sendit_x counts each packet as
sent once it is copied into an mbuf; if a later allocation fails, it
frees the copied prefix unsent, returns its length, and sendmsg_x
drops the ENOBUFS because the count is nonzero. That looks exactly
like a real short count, so under mbuf exhaustion WriteBatch silently
drops packets that writev would have delivered.
The iOS tun path (tun_ios.go) is unchanged.
Inside a macOS Network Extension, download goes from 640 to 832 Mbps
against the recvmsg_x change alone in three paired rounds, at 15% less
CPU per Mbps; the likely cause is that its utun charges more per write
than a raw one. Raw dnclient already runs download near line rate,
and there the CPU difference is within noise.
TestTunWriteBatch* round-trip through a connected AF_UNIX datagram
socketpair, which takes sendmsg_x the same way, over both the batched
and the fallback path: order, AF prefixes, skipped invalid packets,
and an oversize packet mid-call and first-in-call. Dropping Namelen
from msghdrX fails them. Mutation testing with gremlins leaves only
equivalent survivors in WriteBatch, plus which error is reported first
when several fail.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
JackDoan