[ResourceScheduler] Throttle H2/QUIC requests just like we do for 1.1

content/browser/loader/resource_scheduler.cc

Index: content/browser/loader/resource_scheduler.cc
diff --git a/content/browser/loader/resource_scheduler.cc b/content/browser/loader/resource_scheduler.cc
index 2253b57d4a9a33ceeffee9f8657cd8cdfb100030..f1604d440531544ce4be9860c77d9d5ed45a80af 100644
--- a/content/browser/loader/resource_scheduler.cc
+++ b/content/browser/loader/resource_scheduler.cc
@@ -10,6 +10,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/feature_list.h"
 #include "base/macros.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
@@ -31,6 +32,13 @@ namespace content {
 
 namespace {
 
+// When kPrioritySupportedRequestsDelayable is enabled, requests for
+// H2/QUIC/SPDY resources can be delayed by the ResourceScheduler just as
+// HTTP/1.1 resources are. Disabling this appears to have negative performance
+// impact, see https://crbug.com/655585.
+const base::Feature kPrioritySupportedRequestsDelayable{
+    "PrioritySupportedRequestsDelayable", base::FEATURE_ENABLED_BY_DEFAULT};
+
 enum StartMode {
   START_SYNC,
   START_ASYNC
@@ -513,14 +521,20 @@ class ResourceScheduler::Client {
       attributes |= kAttributeLayoutBlocking;
     } else if (request->url_request()->priority() <
                kDelayablePriorityThreshold) {
-      // Resources below the delayable priority threshold that are being
-      // requested from a server that does not support native prioritization are
-      // considered delayable.
-      url::SchemeHostPort scheme_host_port(request->url_request()->url());
-      net::HttpServerProperties& http_server_properties =
-          *request->url_request()->context()->http_server_properties();
-      if (!http_server_properties.SupportsRequestPriority(scheme_host_port))
+      if (base::FeatureList::IsEnabled(kPrioritySupportedRequestsDelayable)) {

[Randy Smith (Not in Mondays), 2016/10/19 16:41:43]

I note that this lookup seems O(log n) (map) in the set of feature; given that
this code is run per-request, it might be worth a separate boolean.   This would
also make the change I raise below more readable.   Up to you.

[jkarlin, 2016/10/19 17:01:41]

Done.

+        // Resources below the delayable priority threshold that are considered
+        // delayable.
         attributes |= kAttributeDelayable;
+      } else {
+        // Resources below the delayable priority threshold that are being
+        // requested from a server that does not support native prioritization
+        // are considered delayable.
+        url::SchemeHostPort scheme_host_port(request->url_request()->url());
+        net::HttpServerProperties& http_server_properties =
+            *request->url_request()->context()->http_server_properties();
+        if (!http_server_properties.SupportsRequestPriority(scheme_host_port))

[Randy Smith (Not in Mondays), 2016/10/19 16:41:43]

I'll call out that another option (which probably would have been my preferred)
would have been to just combine the conditionals here rather than have the
larger if statement.  But I can see arguments both ways, and I presume you made
the choice you did consciously, so just a thought.

[jkarlin, 2016/10/19 17:01:41]

Acknowledged.

+          attributes |= kAttributeDelayable;
+      }
     }
 
     return attributes;
@@ -595,20 +609,24 @@ class ResourceScheduler::Client {
     if (!url_request.url().SchemeIsHTTPOrHTTPS())
       return START_REQUEST;
 
-    if (using_spdy_proxy_ && url_request.url().SchemeIs(url::kHttpScheme))
-      return START_REQUEST;
-
     net::HostPortPair host_port_pair =
         net::HostPortPair::FromURL(url_request.url());
-    url::SchemeHostPort scheme_host_port(url_request.url());
-    net::HttpServerProperties& http_server_properties =
-        *url_request.context()->http_server_properties();
-
-    // TODO(willchan): We should really improve this algorithm as described in
-    // crbug.com/164101. Also, theoretically we should not count a
-    // request-priority capable request against the delayable requests limit.
-    if (http_server_properties.SupportsRequestPriority(scheme_host_port))
-      return START_REQUEST;
+
+    if (!base::FeatureList::IsEnabled(kPrioritySupportedRequestsDelayable)) {
+      if (using_spdy_proxy_ && url_request.url().SchemeIs(url::kHttpScheme))
+        return START_REQUEST;
+
+      url::SchemeHostPort scheme_host_port(url_request.url());
+
+      net::HttpServerProperties& http_server_properties =
+          *url_request.context()->http_server_properties();
+
+      // TODO(willchan): We should really improve this algorithm as described in
+      // crbug.com/164101. Also, theoretically we should not count a
+      // request-priority capable request against the delayable requests limit.
+      if (http_server_properties.SupportsRequestPriority(scheme_host_port))
+        return START_REQUEST;
+    }
 
     // Non-delayable requests.
     if (!RequestAttributesAreSet(request->attributes(), kAttributeDelayable))