[rust-server] Reland: fix TCP-layer TTFT stalls (#33026) (#33269)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

rust/sglang-server/Cargo.toml

@@ -24,6 +24,7 @@ futures = { workspace = true }
 pyo3 = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+socket2 = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }

rust/sglang-server/src/api_server.rs

@@ -88,6 +88,14 @@ pub async fn serve(
     // runtime drops → detached handlers cancel → their `AbortGuard`s fire, release
     // `Senders` clones → tok/detok channels close → workers exit. Full drain is
     // deferred (see `request_shutdown`).
+    // Match Python (asyncio sets TCP_NODELAY); avoids a ~13 ms
+    // Nagle/delayed-ACK penalty on keep-alive connections.
+    use axum::serve::ListenerExt;
+    let listener = listener.tap_io(|io| {
+        if let Err(e) = io.set_nodelay(true) {
+            tracing::debug!(error = %e, "set_nodelay failed");
+        }
+    });
     // `with_connect_info` exposes the peer address to the access-log middleware.
     let serve = axum::serve(
         listener,

rust/sglang-server/src/runtime.rs

@@ -93,9 +93,28 @@ impl Drop for Runtime {
 /// startup misconfiguration (e.g. no tokenizer for a non-skip server).
 pub fn start(cfg: RuntimeConfig) -> Result<Runtime, String> {
     // Bind the API server port before spawning any thread, so an unavailable
-    // port (EADDRINUSE) is a hard startup error.
-    let listener = std::net::TcpListener::bind(cfg.rust_server_args.http_addr)
-        .map_err(|e| format!("bind {} failed: {e}", cfg.rust_server_args.http_addr))?;
+    // port (EADDRINUSE) is a hard startup error. socket2 rather than
+    // `std::net::TcpListener` so SO_RCVBUF can be set before `listen`.
+    let addr = cfg.rust_server_args.http_addr;
+    let socket = socket2::Socket::new(
+        socket2::Domain::for_address(addr),
+        socket2::Type::STREAM,
+        Some(socket2::Protocol::TCP),
+    )
+    .map_err(|e| format!("socket for {addr} failed: {e}"))?;
+    socket
+        .set_reuse_address(true)
+        .map_err(|e| format!("set_reuseaddr failed: {e}"))?;
+    if let Err(e) = socket.set_recv_buffer_size(16 * 1024 * 1024) {
+        eprintln!("warning: set_recv_buffer_size on listener failed: {e}");
+    }
+    socket
+        .bind(&addr.into())
+        .map_err(|e| format!("bind {addr} failed: {e}"))?;
+    socket
+        .listen(1024)
+        .map_err(|e| format!("listen on {addr} failed: {e}"))?;
+    let listener: std::net::TcpListener = socket.into();
     listener
         .set_nonblocking(true)
         .map_err(|e| format!("listener set_nonblocking failed: {e}"))?;