dgram: use uv_udp_try_send()

This improves dgram performance by avoiding unnecessary async
operations.

One issue with this commit is that it seems hard to actually create
conditions under which the fallback path to the async case is
actually taken, for all supported OS, so an internal CLI option
is used for testing that path.

Another caveat is that the lack of an async operation means
that there are slight timing differences (essentially `nextTick()`
rather than `setImmediate()` for the send callback).

PR-URL: #29832
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: addaleax

benchmark/dgram/array-vs-concat.js

@@ -29,17 +29,25 @@ function main({ dur, len, num, type, chunks }) {
 
   function onsendConcat() {
     if (sent++ % num === 0) {
-      for (var i = 0; i < num; i++) {
-        socket.send(Buffer.concat(chunk), PORT, '127.0.0.1', onsend);
-      }
+      // The setImmediate() is necessary to have event loop progress on OSes
+      // that only perform synchronous I/O on nonblocking UDP sockets.
+      setImmediate(() => {
+        for (var i = 0; i < num; i++) {
+          socket.send(Buffer.concat(chunk), PORT, '127.0.0.1', onsend);
+        }
+      });
     }
   }
 
   function onsendMulti() {
     if (sent++ % num === 0) {
-      for (var i = 0; i < num; i++) {
-        socket.send(chunk, PORT, '127.0.0.1', onsend);
-      }
+      // The setImmediate() is necessary to have event loop progress on OSes
+      // that only perform synchronous I/O on nonblocking UDP sockets.
+      setImmediate(() => {
+        for (var i = 0; i < num; i++) {
+          socket.send(chunk, PORT, '127.0.0.1', onsend);
+        }
+      });
     }
   }
 

benchmark/dgram/multi-buffer.js

@@ -27,9 +27,13 @@ function main({ dur, len, num, type, chunks }) {
 
   function onsend() {
     if (sent++ % num === 0) {
-      for (var i = 0; i < num; i++) {
-        socket.send(chunk, PORT, '127.0.0.1', onsend);
-      }
+      // The setImmediate() is necessary to have event loop progress on OSes
+      // that only perform synchronous I/O on nonblocking UDP sockets.
+      setImmediate(() => {
+        for (var i = 0; i < num; i++) {
+          socket.send(chunk, PORT, '127.0.0.1', onsend);
+        }
+      });
     }
   }
 

benchmark/dgram/offset-length.js

@@ -23,9 +23,13 @@ function main({ dur, len, num, type }) {
 
   function onsend() {
     if (sent++ % num === 0) {
-      for (var i = 0; i < num; i++) {
-        socket.send(chunk, 0, chunk.length, PORT, '127.0.0.1', onsend);
-      }
+      // The setImmediate() is necessary to have event loop progress on OSes
+      // that only perform synchronous I/O on nonblocking UDP sockets.
+      setImmediate(() => {
+        for (var i = 0; i < num; i++) {
+          socket.send(chunk, 0, chunk.length, PORT, '127.0.0.1', onsend);
+        }
+      });
     }
   }
 

benchmark/dgram/single-buffer.js

@@ -23,9 +23,13 @@ function main({ dur, len, num, type }) {
 
   function onsend() {
     if (sent++ % num === 0) {
-      for (var i = 0; i < num; i++) {
-        socket.send(chunk, PORT, '127.0.0.1', onsend);
-      }
+      // The setImmediate() is necessary to have event loop progress on OSes
+      // that only perform synchronous I/O on nonblocking UDP sockets.
+      setImmediate(() => {
+        for (var i = 0; i < num; i++) {
+          socket.send(chunk, PORT, '127.0.0.1', onsend);
+        }
+      });
     }
   }
 

lib/dgram.js

@@ -666,6 +666,14 @@ function doSend(ex, self, ip, list, address, port, callback) {
   else
     err = state.handle.send(req, list, list.length, !!callback);
 
+  if (err >= 1) {
+    // Synchronous finish. The return code is msg_length + 1 so that we can
+    // distinguish between synchronous success and asynchronous success.
+    if (callback)
+      process.nextTick(callback, null, err - 1);
+    return;
+  }
+
   if (err && callback) {
     // Don't emit as error, dgram_legacy.js compatibility
     const ex = exceptionWithHostPort(err, 'send', address, port);

src/node_options.cc

@@ -453,6 +453,8 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             "write warnings to file instead of stderr",
             &EnvironmentOptions::redirect_warnings,
             kAllowedInEnvironment);
+  AddOption("--test-udp-no-try-send", "",  // For testing only.
+            &EnvironmentOptions::test_udp_no_try_send);
   AddOption("--throw-deprecation",
             "throw an exception on deprecations",
             &EnvironmentOptions::throw_deprecation,

src/node_options.h

@@ -135,6 +135,7 @@ class EnvironmentOptions : public Options {
   bool heap_prof = false;
 #endif  // HAVE_INSPECTOR
   std::string redirect_warnings;
+  bool test_udp_no_try_send = false;
   bool throw_deprecation = false;
   bool trace_deprecation = false;
   bool trace_sync_io = false;

src/udp_wrap.cc

@@ -429,11 +429,6 @@ void UDPWrap::DoSend(const FunctionCallbackInfo<Value>& args, int family) {
   size_t count = args[2].As<Uint32>()->Value();
   const bool have_callback = sendto ? args[5]->IsTrue() : args[3]->IsTrue();
 
-  SendWrap* req_wrap;
-  {
-    AsyncHooks::DefaultTriggerAsyncIdScope trigger_scope(wrap);
-    req_wrap = new SendWrap(env, req_wrap_obj, have_callback);
-  }
   size_t msg_size = 0;
 
   MaybeStackBuffer<uv_buf_t, 16> bufs(count);
@@ -448,8 +443,6 @@ void UDPWrap::DoSend(const FunctionCallbackInfo<Value>& args, int family) {
     msg_size += length;
   }
 
-  req_wrap->msg_size = msg_size;
-
   int err = 0;
   struct sockaddr_storage addr_storage;
   sockaddr* addr = nullptr;
@@ -462,18 +455,47 @@ void UDPWrap::DoSend(const FunctionCallbackInfo<Value>& args, int family) {
     }
   }
 
+  uv_buf_t* bufs_ptr = *bufs;
+  if (err == 0 && !UNLIKELY(env->options()->test_udp_no_try_send)) {
+    err = uv_udp_try_send(&wrap->handle_, bufs_ptr, count, addr);
+    if (err == UV_ENOSYS || err == UV_EAGAIN) {
+      err = 0;
+    } else if (err >= 0) {
+      size_t sent = err;
+      while (count > 0 && bufs_ptr->len <= sent) {
+        sent -= bufs_ptr->len;
+        bufs_ptr++;
+        count--;
+      }
+      if (count > 0) {
+        CHECK_LT(sent, bufs_ptr->len);
+        bufs_ptr->base += sent;
+        bufs_ptr->len -= sent;
+      } else {
+        CHECK_EQ(static_cast<size_t>(err), msg_size);
+        // + 1 so that the JS side can distinguish 0-length async sends from
+        // 0-length sync sends.
+        args.GetReturnValue().Set(static_cast<uint32_t>(msg_size) + 1);
+        return;
+      }
+    }
+  }
+
   if (err == 0) {
+    AsyncHooks::DefaultTriggerAsyncIdScope trigger_scope(wrap);
+    SendWrap* req_wrap = new SendWrap(env, req_wrap_obj, have_callback);
+    req_wrap->msg_size = msg_size;
+
     err = req_wrap->Dispatch(uv_udp_send,
                              &wrap->handle_,
-                             *bufs,
+                             bufs_ptr,
                              count,
                              addr,
                              OnSend);
+    if (err)
+      delete req_wrap;
   }
 
-  if (err)
-    delete req_wrap;
-
   args.GetReturnValue().Set(err);
 }
 

test/async-hooks/test-udpsendwrap.js

@@ -1,3 +1,4 @@
+// Flags: --test-udp-no-try-send
 'use strict';
 
 const common = require('../common');

test/parallel/test-dgram-send-callback-recursive.js

@@ -22,7 +22,7 @@ function onsend() {
 client.on('listening', function() {
   port = this.address().port;
 
-  setImmediate(function() {
+  process.nextTick(() => {
     async = true;
   });