http2: tune control flow defaults

The current defaults are unnecessarily conservative which makes http2 control flow over
high latency connections (such as public internet) unbearably slow.

Author: ronag

lib/internal/http2/core.js

@@ -3288,12 +3288,16 @@ function connectionListener(socket) {
 
   socket[kServer] = this;
 
+  // NGHTTP2 default is 64KB, but we want to allow more data to be
+  // buffered without applying backpressure.
+  session.setLocalWindowSize(512 * 1024);
+
   this.emit('session', session);
 }
 
 function initializeOptions(options) {
   assertIsObject(options, 'options');
-  options = { ...options };
+  options = { ...getDefaultSettings(), ...options };
   assertIsObject(options.settings, 'options.settings');
   options.settings = { ...options.settings };
 

lib/internal/http2/util.js

@@ -364,6 +364,10 @@ function getDefaultSettings() {
       settingsBuffer[IDX_SETTINGS_ENABLE_CONNECT_PROTOCOL] === 1;
   }
 
+  // NGHTTP2 default is 64KB, but we want to allow more data to be
+  // buffered without applying backpressure.
+  holder.initialWindowSize = 256 * 1024;
+
   if (settingsBuffer[IDX_SETTINGS_FLAGS + 1]) holder.customSettings = addCustomSettingsToObj();
 
   return holder;

test/parallel/test-http2-binding.js

@@ -17,7 +17,7 @@ const settings = http2.getDefaultSettings();
 assert.strictEqual(settings.headerTableSize, 4096);
 assert.strictEqual(settings.enablePush, true);
 assert.strictEqual(settings.maxConcurrentStreams, 4294967295);
-assert.strictEqual(settings.initialWindowSize, 65535);
+assert.strictEqual(settings.initialWindowSize, 256 * 1024);
 assert.strictEqual(settings.maxFrameSize, 16384);
 
 assert.strictEqual(binding.nghttp2ErrorString(-517),