Change priority increasing scheme

lib/nghttp2_outbound_item.h

@@ -56,12 +56,13 @@ typedef struct {
      inipri. The item is chosen from the queue based on pri and
      seq. For control frames, they consist of just 1 frame and pri
      does not change. For DATA frame, they could split up to several
-     frames. After sending a frame, the pri is increased by 1. If it
-     becomes more than lowest priority, then it returns back to inipri
-     and do the same sequence again and again. By doing this, the
-     higher priority long DATA frames don't starve the lower
-     prioritized streams. */
+     frames. After sending a frame, the pri becomes |inipri| +
+     |pridecay| and |pridecay| is multiplied by 2. If it becomes more
+     than lowest priority, then it returns back to |inipri| and do the
+     same sequence again and again. By doing this, the higher priority
+     long DATA frames don't starve the lower prioritized streams. */
   int pri;
+  uint32_t pridecay;
   int64_t seq;
 } nghttp2_outbound_item;
 

lib/nghttp2_session.c

@@ -388,6 +388,7 @@ int nghttp2_session_add_frame(nghttp2_session *session,
     return r;
   }
   item->inipri = item->pri;
+  item->pridecay = 1;
   return 0;
 }
 
@@ -979,12 +980,19 @@ nghttp2_outbound_item* nghttp2_session_pop_next_ob_item
 static void nghttp2_outbound_item_adjust_pri(nghttp2_session *session,
                                              nghttp2_outbound_item *item)
 {
+  assert(item->pri > 0);
   if(item->pri == NGHTTP2_PRI_LOWEST) {
-    item->pri = item->inipri;
-  } else if(item->pri > (int32_t)NGHTTP2_PRI_LOWEST/2) {
+    nghttp2_stream *stream;
+    stream = nghttp2_session_get_stream
+      (session, nghttp2_outbound_item_get_data_frame(item)->hd.stream_id);
+    assert(stream);
+    item->pri = item->inipri = stream->pri;
+    item->pridecay = 1;
+  } else if(item->pri + item->pridecay > NGHTTP2_PRI_LOWEST) {
     item->pri = NGHTTP2_PRI_LOWEST;
   } else {
-    item->pri *= 2;
+    item->pri = item->inipri + item->pridecay;
+    item->pridecay *= 2;
   }
 }