Fix Resource Priorities and Scheduling (Chrome Side)

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 e222143f31c3be16a3fd5d376f4f8e3faf1991e0..e1098ebf4aa1cddb69069d50dfe31537a234f0dd 100644
--- a/content/browser/loader/resource_scheduler.cc
+++ b/content/browser/loader/resource_scheduler.cc
@@ -2,10 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <set>
-
 #include "content/browser/loader/resource_scheduler.h"
 
+#include <stdint.h>
+#include <set>
+
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/stl_util.h"
@@ -36,6 +37,16 @@ const char kThrottleCoalesceFieldTrialCoalesce[] = "Coalesce";
 const char kRequestLimitFieldTrial[] = "OutstandingRequestLimiting";
 const char kRequestLimitFieldTrialGroupPrefix[] = "Limit";
 
+const char kResourcePrioritiesFieldTrial[] = "ResourcePriorities";
+
+// Flags identifying various attributes of the request that are used
+// when making scheduling decisions.
+using RequestAttributes = uint8_t;
+const RequestAttributes kAttributeNone = 0x00;
+const RequestAttributes kAttributeInFlight = 0x01;
+const RequestAttributes kAttributeDelayable = 0x02;
+const RequestAttributes kAttributeLayoutBlocking = 0x04;
+
 // Post ResourceScheduler histograms of the following forms:
 // If |histogram_suffix| is NULL or the empty string:
 //   ResourceScheduler.base_name.histogram_name
@@ -73,9 +84,12 @@ const char* GetNumClientsString(size_t num_clients) {
 }  // namespace
 
 static const size_t kCoalescedTimerPeriod = 5000;
-static const size_t kMaxNumDelayableRequestsPerClient = 10;
+static const size_t kDefaultMaxNumDelayableRequestsPerClient = 10;
 static const size_t kMaxNumDelayableRequestsPerHost = 6;
 static const size_t kMaxNumThrottledRequestsPerClient = 1;
+static const size_t kDefaultMaxNumDelayableWhileLayoutBlocking = 1;
+static const net::RequestPriority
+    kDefaultLayoutBlockingPriorityThreshold = net::LOW;
 
 struct ResourceScheduler::RequestPriorityParams {
   RequestPriorityParams()
@@ -175,7 +189,7 @@ class ResourceScheduler::ScheduledResourceRequest : public ResourceThrottle {
         ready_(false),
         deferred_(false),
         is_async_(is_async),
-        classification_(NORMAL_REQUEST),
+        attributes_(kAttributeNone),
         scheduler_(scheduler),
         priority_(priority),
         fifo_ordering_(0) {
@@ -239,11 +253,11 @@ class ResourceScheduler::ScheduledResourceRequest : public ResourceThrottle {
   void set_fifo_ordering(uint32 fifo_ordering) {
     fifo_ordering_ = fifo_ordering;
   }
-  RequestClassification classification() const {
-    return classification_;
+  RequestAttributes attributes() const {
+    return attributes_;
   }
-  void set_classification(RequestClassification classification) {
-    classification_ = classification;
+  void set_attributes(RequestAttributes attributes) {
+    attributes_ = attributes;
   }
 
  private:
@@ -276,7 +290,7 @@ class ResourceScheduler::ScheduledResourceRequest : public ResourceThrottle {
   bool ready_;
   bool deferred_;
   bool is_async_;
-  RequestClassification classification_;
+  RequestAttributes attributes_;
   ResourceScheduler* scheduler_;
   RequestPriorityParams priority_;
   uint32 fifo_ordering_;
@@ -321,7 +335,7 @@ class ResourceScheduler::Client {
         is_visible_(is_visible),
         is_loaded_(false),
         is_paused_(false),
-        has_body_(false),
+        has_html_body_(false),
         using_spdy_proxy_(false),
         load_started_time_(base::TimeTicks::Now()),
         scheduler_(scheduler),
@@ -340,11 +354,11 @@ class ResourceScheduler::Client {
 
   void ScheduleRequest(net::URLRequest* url_request,
                        ScheduledResourceRequest* request) {
+    SetRequestAttributes(request, DetermineRequestAttributes(request));
     if (ShouldStartRequest(request) == START_REQUEST)
       StartRequest(request);
     else
       pending_requests_.Insert(request);
-    SetRequestClassification(request, ClassifyRequest(request));
   }
 
   void RemoveRequest(ScheduledResourceRequest* request) {
@@ -362,10 +376,10 @@ class ResourceScheduler::Client {
   RequestSet StartAndRemoveAllRequests() {
     // First start any pending requests so that they will be moved into
     // in_flight_requests_. This may exceed the limits
-    // kMaxNumDelayableRequestsPerClient, kMaxNumDelayableRequestsPerHost and
-    // kMaxNumThrottledRequestsPerClient, so this method must not do anything
-    // that depends on those limits before calling ClearInFlightRequests()
-    // below.
+    // kDefaultMaxNumDelayableRequestsPerClient, kMaxNumDelayableRequestsPerHost
+    // and kMaxNumThrottledRequestsPerClient, so this method must not do
+    // anything that depends on those limits before calling
+    // ClearInFlightRequests() below.
     while (!pending_requests_.IsEmpty()) {
       ScheduledResourceRequest* request =
           *pending_requests_.GetNextHighestIterator();
@@ -377,7 +391,7 @@ class ResourceScheduler::Client {
     for (RequestSet::iterator it = in_flight_requests_.begin();
          it != in_flight_requests_.end(); ++it) {
       unowned_requests.insert(*it);
-      (*it)->set_classification(NORMAL_REQUEST);
+      (*it)->set_attributes(kAttributeNone);
     }
     ClearInFlightRequests();
     return unowned_requests;
@@ -482,12 +496,12 @@ class ResourceScheduler::Client {
   }
 
   void OnNavigate() {
-    has_body_ = false;
+    has_html_body_ = false;
     is_loaded_ = false;
   }
 
   void OnWillInsertBody() {
-    has_body_ = true;
+    has_html_body_ = true;
     LoadAnyStartablePendingRequests();
   }
 
@@ -503,11 +517,9 @@ class ResourceScheduler::Client {
                            RequestPriorityParams new_priority_params) {
     request->url_request()->SetPriority(new_priority_params.priority);
     request->set_request_priority_params(new_priority_params);
+    SetRequestAttributes(request, DetermineRequestAttributes(request));
     if (!pending_requests_.IsQueued(request)) {
       DCHECK(ContainsKey(in_flight_requests_, request));
-      // The priority of the request and priority support of the server may
-      // have changed, so update the delayable count.
-      SetRequestClassification(request, ClassifyRequest(request));
       // Request has already started.
       return;
     }
@@ -574,14 +586,14 @@ class ResourceScheduler::Client {
 
   void InsertInFlightRequest(ScheduledResourceRequest* request) {
     in_flight_requests_.insert(request);
-    SetRequestClassification(request, ClassifyRequest(request));
+    SetRequestAttributes(request, DetermineRequestAttributes(request));
   }
 
   void EraseInFlightRequest(ScheduledResourceRequest* request) {
     size_t erased = in_flight_requests_.erase(request);
     DCHECK_EQ(1u, erased);
     // Clear any special state that we were tracking for this request.
-    SetRequestClassification(request, NORMAL_REQUEST);
+    SetRequestAttributes(request, kAttributeNone);
   }
 
   void ClearInFlightRequests() {
@@ -590,71 +602,101 @@ class ResourceScheduler::Client {
     total_layout_blocking_count_ = 0;
   }
 
-  size_t CountRequestsWithClassification(
-      const RequestClassification classification, const bool include_pending) {
-    size_t classification_request_count = 0;
+  size_t CountRequestsWithAttributes(
+      const RequestAttributes attributes,
+      ScheduledResourceRequest* current_request) {
+    size_t matching_request_count = 0;
     for (RequestSet::const_iterator it = in_flight_requests_.begin();
          it != in_flight_requests_.end(); ++it) {
-      if ((*it)->classification() == classification)
-        classification_request_count++;
+      if (RequestAttributesAreSet((*it)->attributes(), attributes))
+        matching_request_count++;
     }
-    if (include_pending) {
+    if (!RequestAttributesAreSet(attributes, kAttributeInFlight)) {
+      bool current_request_is_pending = false;
       for (RequestQueue::NetQueue::const_iterator
            it = pending_requests_.GetNextHighestIterator();
            it != pending_requests_.End(); ++it) {
-        if ((*it)->classification() == classification)
-          classification_request_count++;
+        if (RequestAttributesAreSet((*it)->attributes(), attributes))
+          matching_request_count++;
+        if (*it == current_request)
+          current_request_is_pending = true;
+      }
+      // Account for the current request if it is not in one of the lists yet.
+      if (current_request &&
+          !ContainsKey(in_flight_requests_, current_request) &&
+          !current_request_is_pending) {
+        if (RequestAttributesAreSet(current_request->attributes(), attributes))
+          matching_request_count++;
       }
     }
-    return classification_request_count;
+    return matching_request_count;
   }
 
-  void SetRequestClassification(ScheduledResourceRequest* request,
-                                RequestClassification classification) {
-    RequestClassification old_classification = request->classification();
-    if (old_classification == classification)
+  bool RequestAttributesAreSet(RequestAttributes request_attributes,
+                               RequestAttributes matching_attributes) const {
+    return (request_attributes & matching_attributes) == matching_attributes;
+  }
+
+  void SetRequestAttributes(ScheduledResourceRequest* request,
+                            RequestAttributes attributes) {
+    RequestAttributes old_attributes = request->attributes();
+    if (old_attributes == attributes)
       return;
 
-    if (old_classification == IN_FLIGHT_DELAYABLE_REQUEST)
+    if (RequestAttributesAreSet(old_attributes,
+                                kAttributeInFlight | kAttributeDelayable)) {
       in_flight_delayable_count_--;
-    if (old_classification == LAYOUT_BLOCKING_REQUEST)
+    }
+    if (RequestAttributesAreSet(old_attributes, kAttributeLayoutBlocking))
       total_layout_blocking_count_--;
 
-    if (classification == IN_FLIGHT_DELAYABLE_REQUEST)
+    if (RequestAttributesAreSet(attributes,
+                                kAttributeInFlight | kAttributeDelayable)) {
       in_flight_delayable_count_++;
-    if (classification == LAYOUT_BLOCKING_REQUEST)
+    }
+    if (RequestAttributesAreSet(attributes, kAttributeLayoutBlocking))
       total_layout_blocking_count_++;
 
-    request->set_classification(classification);
-    DCHECK_EQ(
-        CountRequestsWithClassification(IN_FLIGHT_DELAYABLE_REQUEST, false),
+    request->set_attributes(attributes);
+    DCHECK_EQ(CountRequestsWithAttributes(
+        kAttributeInFlight | kAttributeDelayable, request),
         in_flight_delayable_count_);
-    DCHECK_EQ(CountRequestsWithClassification(LAYOUT_BLOCKING_REQUEST, true),
+    DCHECK_EQ(CountRequestsWithAttributes(kAttributeLayoutBlocking, request),
               total_layout_blocking_count_);
   }
 
-  RequestClassification ClassifyRequest(ScheduledResourceRequest* request) {
-    // If a request is already marked as layout-blocking make sure to keep the
-    // classification across redirects unless the priority was lowered.
-    if (request->classification() == LAYOUT_BLOCKING_REQUEST &&
-        request->url_request()->priority() > net::LOW) {
-      return LAYOUT_BLOCKING_REQUEST;
-    }
-
-    if (!has_body_ && request->url_request()->priority() > net::LOW)
-      return LAYOUT_BLOCKING_REQUEST;
-
-    if (request->url_request()->priority() < net::LOW) {
+  RequestAttributes DetermineRequestAttributes(
+      ScheduledResourceRequest* request) {
+    RequestAttributes attributes = kAttributeNone;
+
+    if (ContainsKey(in_flight_requests_, request))
+      attributes |= kAttributeInFlight;
+
+    if (RequestAttributesAreSet(request->attributes(),
+                                kAttributeLayoutBlocking)) {
+      // If a request is already marked as layout-blocking make sure to keep the
+      // attribute across redirects.
+      attributes |= kAttributeLayoutBlocking;
+    } else if (!has_html_body_ &&
+               request->url_request()->priority() >
+               scheduler_->non_delayable_threshold()) {
+      // Requests that are above the non_delayable threshold before the HTML
+      // body has been parsed are inferred to be layout-blocking.
+      attributes |= kAttributeLayoutBlocking;
+    } else if (request->url_request()->priority() <
+               scheduler_->non_delayable_threshold()) {
+      // Resources below the non_delayable_threshold that are being requested
+      // from a server that does not support native prioritization are
+      // considered delayable.
       net::HostPortPair host_port_pair =
           net::HostPortPair::FromURL(request->url_request()->url());
       net::HttpServerProperties& http_server_properties =
           *request->url_request()->context()->http_server_properties();
-      if (!http_server_properties.SupportsRequestPriority(host_port_pair) &&
-          ContainsKey(in_flight_requests_, request)) {
-        return IN_FLIGHT_DELAYABLE_REQUEST;
-      }
+      if (!http_server_properties.SupportsRequestPriority(host_port_pair))
+        attributes |= kAttributeDelayable;
     }
-    return NORMAL_REQUEST;
+
+    return attributes;
   }
 
   bool ShouldKeepSearching(
@@ -735,25 +777,21 @@ class ResourceScheduler::Client {
     const net::URLRequest& url_request = *request->url_request();
     // Syncronous requests could block the entire render, which could impact
     // user-observable Clients.
-    if (!request->is_async()) {
+    if (!request->is_async())
       return START_REQUEST;
-    }
 
     // TODO(simonjam): This may end up causing disk contention. We should
     // experiment with throttling if that happens.
     // TODO(aiolos): We probably want to Coalesce these as well to avoid
     // waking the disk.
-    if (!url_request.url().SchemeIsHTTPOrHTTPS()) {
+    if (!url_request.url().SchemeIsHTTPOrHTTPS())
       return START_REQUEST;
-    }
 
-    if (throttle_state_ == COALESCED) {
+    if (throttle_state_ == COALESCED)
       return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
-    }
 
-    if (using_spdy_proxy_ && url_request.url().SchemeIs(url::kHttpScheme)) {
+    if (using_spdy_proxy_ && url_request.url().SchemeIs(url::kHttpScheme))
       return START_REQUEST;
-    }
 
     // Implementation of the kRequestLimitFieldTrial.
     if (scheduler_->limit_outstanding_requests() &&
@@ -769,9 +807,8 @@ class ResourceScheduler::Client {
     // 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(host_port_pair)) {
+    if (http_server_properties.SupportsRequestPriority(host_port_pair))
       return START_REQUEST;
-    }
 
     if (throttle_state_ == THROTTLED &&
         in_flight_requests_.size() >= kMaxNumThrottledRequestsPerClient) {
@@ -780,30 +817,55 @@ class ResourceScheduler::Client {
       return DO_NOT_START_REQUEST_AND_KEEP_SEARCHING;
     }
 
-    // High-priority and layout-blocking requests.
-    if (url_request.priority() >= net::LOW) {
+    // Non-delayable requests.
+    if (!RequestAttributesAreSet(request->attributes(), kAttributeDelayable))
       return START_REQUEST;
-    }
 
-    if (in_flight_delayable_count_ >= kMaxNumDelayableRequestsPerClient) {
+    if (in_flight_delayable_count_ >=
+        scheduler_->max_num_delayable_requests()) {
       return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
     }
 
     if (ShouldKeepSearching(host_port_pair)) {
-      // There may be other requests for other hosts we'd allow,
+      // There may be other requests for other hosts that may be allowed,
       // so keep checking.
       return DO_NOT_START_REQUEST_AND_KEEP_SEARCHING;
     }
 
-    bool have_immediate_requests_in_flight =
-        in_flight_requests_.size() > in_flight_delayable_count_;
-    if (have_immediate_requests_in_flight &&
-        (!has_body_ || total_layout_blocking_count_ != 0) &&
-        // Do not allow a low priority request through in parallel if
-        // we are in a limit field trial.
-        (scheduler_->limit_outstanding_requests() ||
-         in_flight_delayable_count_ != 0)) {
-      return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
+    // The in-flight requests consist of layout-blocking requests,
+    // normal requests and delayable requests.  Everything except for
+    // delayable requests is handled above here so this is deciding what to
+    // do with a delayable request while we are in the layout-blocking phase
+    // of loading.
+    if (!has_html_body_ || total_layout_blocking_count_ != 0) {
+      size_t non_delayable_requests_in_flight_count =
+          in_flight_requests_.size() - in_flight_delayable_count_;
+      if (scheduler_->enable_in_flight_non_delayable_threshold()) {
+        if (non_delayable_requests_in_flight_count >
+            scheduler_->in_flight_non_delayable_threshold()) {
+          // Too many higher priority in-flight requests to allow lower priority
+          // requests through.
+          return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
+        }
+        if (in_flight_requests_.size() > 0 &&
+            (scheduler_->limit_outstanding_requests() ||
+             in_flight_delayable_count_ >=
+             scheduler_->max_num_delayable_while_layout_blocking())) {
+          // Block the request if at least one request is in flight and the
+          // number of in-flight delayable requests has hit the configured
+          // limit.
+          return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
+        }
+      } else if (non_delayable_requests_in_flight_count > 0 &&
+          (scheduler_->limit_outstanding_requests() ||
+           in_flight_delayable_count_ >=
+           scheduler_->max_num_delayable_while_layout_blocking())) {
+        // If there are no high-priority requests in flight the floodgates open.
+        // If there are high-priority requests in-flight then limit the number
+        // of lower-priority requests (or zero if a limit field trial is
+        // active).
+        return DO_NOT_START_REQUEST_AND_STOP_SEARCHING;
+      }
     }
 
     return START_REQUEST;
@@ -850,7 +912,9 @@ class ResourceScheduler::Client {
   bool is_visible_;
   bool is_loaded_;
   bool is_paused_;
-  bool has_body_;
+  // Tracks if the main HTML parser has reached the body which marks the end of
+  // layout-blocking resources.
+  bool has_html_body_;
   bool using_spdy_proxy_;
   RequestQueue pending_requests_;
   RequestSet in_flight_requests_;
@@ -872,6 +936,13 @@ ResourceScheduler::ResourceScheduler()
       coalesced_clients_(0),
       limit_outstanding_requests_(false),
       outstanding_request_limit_(0),
+      non_delayable_threshold_(
+          kDefaultLayoutBlockingPriorityThreshold),
+      enable_in_flight_non_delayable_threshold_(false),
+      in_flight_non_delayable_threshold_(0),
+      max_num_delayable_while_layout_blocking_(
+          kDefaultMaxNumDelayableWhileLayoutBlocking),
+      max_num_delayable_requests_(kDefaultMaxNumDelayableRequestsPerClient),
       coalescing_timer_(new base::Timer(true /* retain_user_task */,
                                         true /* is_repeating */)) {
   std::string throttling_trial_group =
@@ -896,6 +967,42 @@ ResourceScheduler::ResourceScheduler()
     limit_outstanding_requests_ = true;
     outstanding_request_limit_ = outstanding_limit;
   }
+
+  // Set up the ResourceScheduling field trial options.
+  // The field trial parameters are also encoded into the group name since
+  // the variations component is not available from here and plumbing the
+  // options through the code is overkill for a short experiment.
+  //
+  // The group name encoding looks like this:
+  //  <descriptiveName>_ABCDE_E2_F_G
+  //    A - fetchDeferLateScripts (1 for true, 0 for false)
+  //    B - fetchIncreaseFontPriority (1 for true, 0 for false)
+  //    C - fetchIncreaseAsyncScriptPriority (1 for true, 0 for false)
+  //    D - fetchIncreasePriorities (1 for true, 0 for false)
+  //    E - fetchEnableLayoutBlockingThreshold (1 for true, 0 for false)
+  //    E2 - fetchLayoutBlockingThreshold (Numeric)
+  //    F - fetchMaxNumDelayableWhileLayoutBlocking (Numeric)
+  //    G - fetchMaxNumDelayableRequests (Numeric)
+  std::string resource_priorities_trial_group =
+      base::FieldTrialList::FindFullName(kResourcePrioritiesFieldTrial);
+  std::vector<std::string> resource_priorities_split_group(
+      base::SplitString(resource_priorities_trial_group, "_",
+                        base::KEEP_WHITESPACE, base::SPLIT_WANT_ALL));
+  if (resource_priorities_split_group.size() == 5 &&
+      resource_priorities_split_group[1].length() == 5) {
+    // fetchIncreasePriorities
+    if (resource_priorities_split_group[1].at(3) == '1')
+      non_delayable_threshold_ = net::MEDIUM;
+    enable_in_flight_non_delayable_threshold_ =
+        resource_priorities_split_group[1].at(4) == '1';
+    size_t numeric_value = 0;
+    if (base::StringToSizeT(resource_priorities_split_group[2], &numeric_value))
+      in_flight_non_delayable_threshold_ = numeric_value;
+    if (base::StringToSizeT(resource_priorities_split_group[3], &numeric_value))
+      max_num_delayable_while_layout_blocking_ = numeric_value;
+    if (base::StringToSizeT(resource_priorities_split_group[4], &numeric_value))
+      max_num_delayable_requests_ = numeric_value;
+  }
 }
 
 ResourceScheduler::~ResourceScheduler() {