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: https://github.com/nodejs/node/pull/29832
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

1 files changed, 14 insertions, 6 deletions

diff --git a/benchmark/dgram/array-vs-concat.js b/benchmark/dgram/array-vs-concat.js
index 669cf47df4..d260a48063 100644
--- a/benchmark/dgram/array-vs-concat.js
+++ b/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);
+        }
+      });
     }
   }