feat(udp): use recvmmsg

Read up to `BATCH_SIZE = 32` with single `recvmmsg` syscall.

Previously `neqo_bin::udp::Socket::recv` would use `recvmmsg`, but provide a
single buffer to write into only, effectively using `recvmsg` instead of
`recvmmsg`.

With this commit `Socket::recv` provides `BATCH_SIZE` number of buffers on each
`recvmmsg` syscall, thus reading more than one datagram at a time if available.

neqo-bin/src/udp.rs

@@ -9,6 +9,7 @@
 
 use std::{
     io::{self, IoSliceMut},
+    mem::MaybeUninit,
     net::{SocketAddr, ToSocketAddrs},
     slice,
 };
@@ -17,6 +18,13 @@ use neqo_common::{Datagram, IpTos};
 use quinn_udp::{EcnCodepoint, RecvMeta, Transmit, UdpSocketState};
 use tokio::io::Interest;
 
+#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+// Chosen somewhat arbitrarily; might benefit from additional tuning.
+pub(crate) const BATCH_SIZE: usize = 32;
+
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+pub(crate) const BATCH_SIZE: usize = 1;
+
 /// Socket receive buffer size.
 ///
 /// Allows reading multiple datagrams in a single [`Socket::recv`] call.
@@ -25,7 +33,8 @@ const RECV_BUF_SIZE: usize = u16::MAX as usize;
 pub struct Socket {
     socket: tokio::net::UdpSocket,
     state: UdpSocketState,
-    recv_buf: Vec<u8>,
+    // TODO: Rename
+    recv_buf: [Vec<u8>; BATCH_SIZE],
 }
 
 impl Socket {
@@ -36,7 +45,11 @@ impl Socket {
         Ok(Self {
             state: quinn_udp::UdpSocketState::new((&socket).into())?,
             socket: tokio::net::UdpSocket::from_std(socket)?,
-            recv_buf: vec![0; RECV_BUF_SIZE],
+            recv_buf: (0..BATCH_SIZE)
+                .map(|_| vec![0; RECV_BUF_SIZE])
+                .collect::<Vec<_>>()
+                .try_into()
+                .expect("successful array instantiation"),
         })
     }
 
@@ -77,18 +90,25 @@ impl Socket {
 
     /// Receive a UDP datagram on the specified socket.
     pub fn recv(&mut self, local_address: &SocketAddr) -> Result<Vec<Datagram>, io::Error> {
-        let mut meta = RecvMeta::default();
-
-        match self.socket.try_io(Interest::READABLE, || {
-            self.state.recv(
-                (&self.socket).into(),
-                &mut [IoSliceMut::new(&mut self.recv_buf)],
-                slice::from_mut(&mut meta),
-            )
+        let mut metas = [RecvMeta::default(); BATCH_SIZE];
+
+        // TODO: Safe?
+        let mut iovs = MaybeUninit::<[IoSliceMut<'_>; BATCH_SIZE]>::uninit();
+        for (i, buf) in self.recv_buf.iter_mut().enumerate() {
+            unsafe {
+                iovs.as_mut_ptr()
+                    .cast::<IoSliceMut>()
+                    .add(i)
+                    .write(IoSliceMut::new(buf));
+            };
+        }
+        let mut iovs = unsafe { iovs.assume_init() };
+
+        let msgs = match self.socket.try_io(Interest::READABLE, || {
+            self.state
+                .recv((&self.socket).into(), &mut iovs, &mut metas)
         }) {
-            Ok(n) => {
-                assert_eq!(n, 1, "only passed one slice");
-            }
+            Ok(n) => n,
             Err(ref err)
                 if err.kind() == io::ErrorKind::WouldBlock
                     || err.kind() == io::ErrorKind::Interrupted =>
@@ -100,27 +120,34 @@ impl Socket {
             }
         };
 
-        if meta.len == 0 {
-            eprintln!("zero length datagram received?");
-            return Ok(vec![]);
-        }
-        if meta.len == self.recv_buf.len() {
-            eprintln!(
-                "Might have received more than {} bytes",
-                self.recv_buf.len()
-            );
-        }
-
-        Ok(self.recv_buf[0..meta.len]
-            .chunks(meta.stride.min(self.recv_buf.len()))
-            .map(|d| {
-                Datagram::new(
-                    meta.addr,
-                    *local_address,
-                    meta.ecn.map(|n| IpTos::from(n as u8)).unwrap_or_default(),
-                    None, // TODO: get the real TTL https://github.com/quinn-rs/quinn/issues/1749
-                    d,
-                )
+        // TODO
+        // if meta.len == 0 {
+        //     eprintln!("zero length datagram received?");
+        //     return Ok(vec![]);
+        // }
+        // if meta.len == self.recv_buf.len() {
+        //     eprintln!(
+        //         "Might have received more than {} bytes",
+        //         self.recv_buf.len()
+        //     );
+        // }
+
+        Ok(metas
+            .iter()
+            .zip(iovs.iter())
+            .take(msgs)
+            .flat_map(|(meta, buf)| {
+                buf[0..meta.len]
+                    .chunks(meta.stride.min(buf.len()))
+                    .map(|d| {
+                        Datagram::new(
+                            meta.addr,
+                            *local_address,
+                            meta.ecn.map(|n| IpTos::from(n as u8)).unwrap_or_default(),
+                            None, // TODO: get the real TTL https://github.com/quinn-rs/quinn/issues/1749
+                            d,
+                        )
+                    })
             })
             .collect())
     }