feat(1gbps): batched send via sendmmsg (Transport::send_batch)

First increment of the 1 Gbps send-path rework (the measured bottleneck): the
native data plane did one send() syscall per packet — at ~125k pkt/s (1 Gbps
wire) that burns a core on syscalls. Port the proven GameStream sendmmsg path
into the core Transport seam.

- Transport gains `send_batch(&[&[u8]]) -> usize` (count handed to the kernel;
  caller counts the rest as send-buffer drops). Default = the scalar send loop
  (loopback transport + non-Linux).
- UdpTransport overrides it on Linux with `sendmmsg` (64 datagrams/syscall);
  the connected socket needs no per-message address. Non-blocking-aware: a full
  send buffer yields a short count / EAGAIN, and we stop + report what went out
  rather than block or retry (same lossy, FEC-protected contract as send()).
- Session::submit_frame seals every shard then hands the whole frame to
  send_batch in ONE call instead of looping send() — ~64x fewer syscalls per
  frame on the native + GameStream-over-core paths; send_dropped accounting
  preserved (total - sent).

~125k → ~2k syscalls/sec at 1 Gbps line rate. Verified: new loopback-UDP test
send_batch_delivers_over_loopback (100 batched packets arrive intact, datagram
boundaries preserved); full core suite + clippy + fmt green.

Next increments: a paced send thread (microburst shaping so a real NIC doesn't
drop line-rate bursts) and recvmmsg on the client.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-core/src/transport/mod.rs

@@ -15,5 +15,23 @@ pub trait Transport: Send + Sync {
     /// recovers, surfaced so the caller can count it (`packets_send_dropped`) instead of it being
     /// invisible. `Err` = a real send failure.
     fn send(&self, packet: &[u8]) -> std::io::Result<bool>;
+
+    /// Send a whole frame's packets in as few syscalls as possible, returning how many were
+    /// handed to the kernel (the caller counts `packets.len() - sent` as send-buffer drops). This
+    /// is the 1 Gbps+ lever: the [`UdpTransport`](super::UdpTransport) override uses `sendmmsg`
+    /// (~64 packets/syscall) instead of one `send` each — at ~125k pkt/s that is the difference
+    /// between ~2k and ~125k syscalls/sec. The default is the scalar `send` loop (correct for the
+    /// loopback transport and non-Linux builds). On a full send buffer it stops early and reports
+    /// the partial count rather than blocking — same lossy, FEC-protected contract as `send`.
+    fn send_batch(&self, packets: &[&[u8]]) -> std::io::Result<usize> {
+        let mut sent = 0;
+        for p in packets {
+            if self.send(p)? {
+                sent += 1;
+            }
+        }
+        Ok(sent)
+    }
+
     fn recv(&self) -> std::io::Result<Option<Vec<u8>>>;
 }

crates/punktfunk-core/src/transport/udp.rs

@@ -1,8 +1,9 @@
 //! Real UDP datagram transport — native sockets, no async runtime.
 //!
-//! M1 uses one `recv` syscall per packet; the latency budget (§7) calls for
-//! `sendmmsg`/UDP-GSO batching to cut syscalls, which is a P2 optimization layered on
-//! this same [`Transport`] seam.
+//! Send is batched via `sendmmsg` ([`Transport::send_batch`], up to 64 datagrams/syscall) — the
+//! 1 Gbps+ syscall lever (~125k → ~2k syscalls/sec at line rate). Recv is still one syscall per
+//! packet; a `recvmmsg` batch on the client + a paced send thread on the host are the remaining
+//! steps of the 1 Gbps data-plane work, layered on this same [`Transport`] seam.
 
 use super::Transport;
 use crate::packet::MAX_DATAGRAM_BYTES;
@@ -81,6 +82,54 @@ impl Transport for UdpTransport {
         }
     }
 
+    /// Batched send via `sendmmsg` (up to 64 datagrams per syscall) — the connected socket needs
+    /// no per-message address. The socket is non-blocking, so a full send buffer surfaces as a
+    /// short count (or `EAGAIN` with nothing sent); we stop and report what went out rather than
+    /// block or retry — the data plane is lossy + FEC-protected, and blocking would queue stale
+    /// frames + add latency. Ports the proven GameStream `sendmmsg_all`. Non-Linux falls back to
+    /// the trait's scalar `send` loop (no `sendmmsg`).
+    #[cfg(target_os = "linux")]
+    fn send_batch(&self, packets: &[&[u8]]) -> std::io::Result<usize> {
+        use std::os::fd::AsRawFd;
+        const CHUNK: usize = 64;
+        let fd = self.socket.as_raw_fd();
+        let mut total_sent = 0usize;
+        for chunk in packets.chunks(CHUNK) {
+            // `hdrs` borrow `iovs` by raw pointer; both stay alive through the `sendmmsg` call.
+            let mut iovs: Vec<libc::iovec> = chunk
+                .iter()
+                .map(|p| libc::iovec {
+                    iov_base: p.as_ptr() as *mut libc::c_void,
+                    iov_len: p.len(),
+                })
+                .collect();
+            let mut hdrs: Vec<libc::mmsghdr> = iovs
+                .iter_mut()
+                .map(|iov| {
+                    let mut h: libc::mmsghdr = unsafe { std::mem::zeroed() };
+                    h.msg_hdr.msg_iov = iov;
+                    h.msg_hdr.msg_iovlen = 1;
+                    h
+                })
+                .collect();
+            let n = unsafe { libc::sendmmsg(fd, hdrs.as_mut_ptr(), hdrs.len() as libc::c_uint, 0) };
+            if n < 0 {
+                let err = std::io::Error::last_os_error();
+                // Nothing fit in the send buffer — drop this + the remaining chunks (counted by
+                // the caller). A real error (not WouldBlock) still tears the session down.
+                if err.kind() == std::io::ErrorKind::WouldBlock {
+                    break;
+                }
+                return Err(err);
+            }
+            total_sent += n as usize;
+            if (n as usize) < chunk.len() {
+                break; // buffer filled mid-chunk — drop the remainder
+            }
+        }
+        Ok(total_sent)
+    }
+
     fn recv(&self) -> std::io::Result<Option<Vec<u8>>> {
         let mut buf = vec![0u8; RECV_BUF];
         match self.socket.recv(&mut buf) {
@@ -96,3 +145,56 @@ impl Transport for UdpTransport {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::transport::Transport;
+
+    /// `send_batch` delivers a whole frame's worth of packets over real loopback UDP — exercising
+    /// the `sendmmsg` path on Linux (the scalar-loop default elsewhere). 100 × 200 B = 20 KB fits
+    /// the socket buffer, so loopback is lossless and every packet must arrive intact + in order.
+    #[test]
+    fn send_batch_delivers_over_loopback() {
+        let rx = std::net::UdpSocket::bind("127.0.0.1:0").unwrap();
+        rx.set_read_timeout(Some(std::time::Duration::from_millis(500)))
+            .unwrap();
+        let rx_addr = rx.local_addr().unwrap().to_string();
+        let tx = UdpTransport::connect("127.0.0.1:0", &rx_addr).unwrap();
+
+        const N: u32 = 100;
+        let payloads: Vec<Vec<u8>> = (0..N)
+            .map(|i| {
+                let mut v = vec![0u8; 200];
+                v[0..4].copy_from_slice(&i.to_le_bytes());
+                v
+            })
+            .collect();
+        let refs: Vec<&[u8]> = payloads.iter().map(|p| p.as_slice()).collect();
+        let sent = tx.send_batch(&refs).unwrap();
+        assert_eq!(
+            sent, N as usize,
+            "send_batch should hand all packets to the kernel"
+        );
+
+        let mut seen = std::collections::HashSet::new();
+        let mut buf = [0u8; 2048];
+        while seen.len() < N as usize {
+            match rx.recv(&mut buf) {
+                Ok(n) => {
+                    assert_eq!(
+                        n, 200,
+                        "datagram boundaries preserved (one packet per recv)"
+                    );
+                    seen.insert(u32::from_le_bytes(buf[0..4].try_into().unwrap()));
+                }
+                Err(_) => break, // read timeout — stop and let the assert report the shortfall
+            }
+        }
+        assert_eq!(
+            seen.len(),
+            N as usize,
+            "every batched packet should arrive over loopback"
+        );
+    }
+}