HTTP/2 protocol handler: tweak receive window sizes

We were using the default ones, provided by RFC7540. It appears they are way
too restrictive and conservative: when downloading something relatively big,
a stream keeps spending the whole session/its own 'recv' windows and thus
we have to constantly send WINDOW_UPDATE frames. This significantly slows
down our HTTP/2 implementation, making it orders of magnitude slower than
HTTP/1.1. To fix this:

- We send SETTINGS_INITIAL_WINDOW_SIZE in the first SETTINGS frame
  to inform our peer that per-stream WINDOW is bigger than 64Kb
- We increase the session's receive window size.

Task-number: QTBUG-63722
Change-Id: I31312fcfd5f0fc0aee6aaa5d3562cc7d1b931adc
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/network/access/http2/http2protocol_p.h b/src/network/access/http2/http2protocol_p.h
index b26ff0e9f4..c64e960002 100644
--- a/src/network/access/http2/http2protocol_p.h
+++ b/src/network/access/http2/http2protocol_p.h
@@ -129,10 +129,20 @@ enum Http2PredefinedParameters
     maxConcurrentStreams = 100 // HTTP/2, 6.5.2
 };
 
-// It's int, it has internal linkage, it's ok to have it in headers -
-// no ODR violation is possible.
+// These are ints, const, they have internal linkage, it's ok to have them in
+// headers - no ODR violation.
 const quint32 lastValidStreamID((quint32(1) << 31) - 1); // HTTP/2, 5.1.1
 
+// The default size of 64K is too small and limiting: if we use it, we end up
+// sending WINDOW_UPDATE frames on a stream/session all the time, for each
+// 2 DATE frames of size 16K (also default) we'll send a WINDOW_UPDATE frame
+// for a given stream and have a download speed order of magnitude lower than
+// our own HTTP/1.1 protocol handler. We choose a bigger window size (normally,
+// HTTP/2 servers are not afraid to immediately set it to possible max anyway)
+// and split this window size between our concurrent streams.
+const qint32 initialSessionReceiveWindowSize = defaultSessionWindowSize * 10000;
+const qint32 initialStreamReceiveWindowSize = initialSessionReceiveWindowSize / maxConcurrentStreams;
+
 extern const Q_AUTOTEST_EXPORT char Http2clientPreface[clientPrefaceLength];
 void prepare_for_protocol_upgrade(QHttpNetworkRequest &request);