[Performance] Raise default HTTP/2 initial window size to 16 MiB

client/src/main/java/org/asynchttpclient/AsyncHttpClientConfig.java

@@ -287,10 +287,10 @@ default boolean isHttp2Enabled() {
     }
 
     /**
-     * @return the HTTP/2 initial window size in bytes, defaults to 65535
+     * @return the HTTP/2 initial window size in bytes, defaults to 16777216 (16 MiB)
      */
     default int getHttp2InitialWindowSize() {
-        return 65_535;
+        return 16_777_216;
     }
 
     /**

client/src/main/resources/org/asynchttpclient/config/ahc-default.properties

@@ -55,7 +55,7 @@ org.asynchttpclient.ioThreadsCount=-1
 org.asynchttpclient.hashedWheelTimerTickDuration=100
 org.asynchttpclient.hashedWheelTimerSize=512
 org.asynchttpclient.expiredCookieEvictionDelay=30000
-org.asynchttpclient.http2InitialWindowSize=65535
+org.asynchttpclient.http2InitialWindowSize=16777216
 org.asynchttpclient.http2MaxFrameSize=16384
 org.asynchttpclient.http2HeaderTableSize=4096
 org.asynchttpclient.http2MaxHeaderListSize=8192

client/src/test/java/org/asynchttpclient/DefaultAsyncHttpClientConfigTest.java

@@ -32,6 +32,22 @@ void testHttp2MaxDecompressedResponseSize_ZeroDisables() {
         assertEquals(0L, config.getHttp2MaxDecompressedResponseSize(), "0 must disable the limit");
     }
 
+    @Test
+    void testHttp2InitialWindowSize_DefaultIs16MiB() {
+        DefaultAsyncHttpClientConfig config = new DefaultAsyncHttpClientConfig.Builder().build();
+        assertEquals(16 * 1024 * 1024, config.getHttp2InitialWindowSize(),
+                "Default HTTP/2 initial window size should be 16 MiB (matches the JDK HttpClient and OkHttp defaults)");
+    }
+
+    @Test
+    void testHttp2InitialWindowSize_SetCustom() {
+        DefaultAsyncHttpClientConfig config = new DefaultAsyncHttpClientConfig.Builder()
+                .setHttp2InitialWindowSize(65_535)
+                .build();
+        assertEquals(65_535, config.getHttp2InitialWindowSize(),
+                "Builder must plumb the configured value through");
+    }
+
     @Test
     void testStripAuthorizationOnRedirect_DefaultIsFalse() {
         DefaultAsyncHttpClientConfig config = new DefaultAsyncHttpClientConfig.Builder().build();