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, 2 insertions, 6 deletions

diff --git a/src/network/access/qhttp2protocolhandler_p.h b/src/network/access/qhttp2protocolhandler_p.h
index 82eea21818..b52f8ae10c 100644
--- a/src/network/access/qhttp2protocolhandler_p.h
+++ b/src/network/access/qhttp2protocolhandler_p.h
@@ -176,13 +176,9 @@ private:
     quint32 maxConcurrentStreams = Http2::maxConcurrentStreams;
 
     // Control flow:
-    static const qint32 sessionMaxRecvWindowSize = Http2::defaultSessionWindowSize * 10;
-    // Signed integer, it can become negative (it's still a valid window size):
-    qint32 sessionRecvWindowSize = sessionMaxRecvWindowSize;
 
-    // We do not negotiate this window size
-    // We have to send WINDOW_UPDATE frames to our peer also.
-    static const qint32 streamInitialRecvWindowSize = Http2::defaultSessionWindowSize;
+    // Signed integer, it can become negative (it's still a valid window size):
+    qint32 sessionRecvWindowSize = Http2::initialSessionReceiveWindowSize;
 
     // Updated by SETTINGS and WINDOW_UPDATE.
     qint32 sessionSendWindowSize = Http2::defaultSessionWindowSize;