Replaced sync thread nudge queue with a single pending nudge.

This partially fixes a high QPS issue we're seeing with notifications.

BUG=55279
TEST=Manually

Review URL: http://codereview.chromium.org/3410002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@59336 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 30 insertions, 26 deletions

diff --git a/chrome/browser/sync/engine/syncer_thread.cc b/chrome/browser/sync/engine/syncer_thread.cc
index a3f9da8..92322b4 100644
--- a/chrome/browser/sync/engine/syncer_thread.cc
+++ b/chrome/browser/sync/engine/syncer_thread.cc
@@ -294,10 +294,10 @@ void SyncerThread::ThreadMainLoop() {
         WaitInterval::THROTTLED;
     // If we are throttled, we must wait.  Otherwise, wait until either the next
     // nudge (if one exists) or the poll interval.
-    const TimeTicks end_wait = throttled ? next_poll :
-        !vault_.nudge_queue_.empty() &&
-        vault_.nudge_queue_.top().first < next_poll ?
-        vault_.nudge_queue_.top().first : next_poll;
+    TimeTicks end_wait = next_poll;
+    if (!throttled && !vault_.pending_nudge_time_.is_null()) {
+      end_wait = std::min(end_wait, vault_.pending_nudge_time_);
+    }
     LOG(INFO) << "end_wait is " << end_wait.ToInternalValue();
     LOG(INFO) << "next_poll is " << next_poll.ToInternalValue();
 
@@ -527,13 +527,17 @@ bool SyncerThread::UpdateNudgeSource(bool was_throttled,
   }
   // Update the nudge source if a new nudge has come through during the
   // previous sync cycle.
-  while (!vault_.nudge_queue_.empty() &&
-         TimeTicks::Now() >= vault_.nudge_queue_.top().first) {
+  if (!vault_.pending_nudge_time_.is_null()) {
     if (!was_throttled && !nudged) {
-      nudge_source = vault_.nudge_queue_.top().second;
+      nudge_source = vault_.pending_nudge_source_;
       nudged = true;
     }
-    vault_.nudge_queue_.pop();
+    LOG(INFO) << "Clearing pending nudge from "
+              << vault_.pending_nudge_source_
+              << " at tick "
+              << vault_.pending_nudge_time_.ToInternalValue();
+    vault_.pending_nudge_source_ = kUnknown;
+    vault_.pending_nudge_time_ = base::TimeTicks();
   }
   SetUpdatesSource(nudged, nudge_source, initial_sync);
   return nudged;
@@ -700,8 +704,16 @@ void SyncerThread::NudgeSyncImpl(int milliseconds_from_now,
 
   const TimeTicks nudge_time = TimeTicks::Now() +
       TimeDelta::FromMilliseconds(milliseconds_from_now);
-  NudgeObject nudge_object(nudge_time, source);
-  vault_.nudge_queue_.push(nudge_object);
+  if (nudge_time <= vault_.pending_nudge_time_) {
+    LOG(INFO) << "Nudge for source " << source
+              << " dropped due to existing later pending nudge";
+    return;
+  }
+
+  LOG(INFO) << "Replacing pending nudge for source "
+            << source << " at " << nudge_time.ToInternalValue();
+  vault_.pending_nudge_source_ = source;
+  vault_.pending_nudge_time_ = nudge_time;
   vault_field_changed_.Broadcast();
 }
 
diff --git a/chrome/browser/sync/engine/syncer_thread.h b/chrome/browser/sync/engine/syncer_thread.h
index 687f8b7..3bf96c3 100644
--- a/chrome/browser/sync/engine/syncer_thread.h
+++ b/chrome/browser/sync/engine/syncer_thread.h
@@ -10,7 +10,6 @@
 #pragma once
 
 #include <list>
-#include <queue>
 #include <vector>
 
 #include "base/basictypes.h"
@@ -169,17 +168,6 @@ class SyncerThread : public base::RefCountedThreadSafe<SyncerThread>,
   // Handle of the running thread.
   base::Thread thread_;
 
-  typedef std::pair<base::TimeTicks, NudgeSource> NudgeObject;
-
-  struct IsTimeTicksGreater {
-    inline bool operator() (const NudgeObject& lhs, const NudgeObject& rhs) {
-      return lhs.first > rhs.first;
-    }
-  };
-
-  typedef std::priority_queue<NudgeObject, std::vector<NudgeObject>,
-                              IsTimeTicksGreater> NudgeQueue;
-
   // Fields that are modified / accessed by multiple threads go in this struct
   // for clarity and explicitness.
   struct ProtectedFields {
@@ -197,9 +185,12 @@ class SyncerThread : public base::RefCountedThreadSafe<SyncerThread>,
     // State of the server connection.
     bool connected_;
 
-    // A queue of all scheduled nudges.  One insertion for every call to
-    // NudgeQueue().
-    NudgeQueue nudge_queue_;
+    // kUnknown if there is no pending nudge.  (Theoretically, there
+    // could be a pending nudge of type kUnknown, so it's better to
+    // check pending_nudge_time_.)
+    NudgeSource pending_nudge_source_;
+    // null iff there is no pending nudge.
+    base::TimeTicks pending_nudge_time_;
 
     // The wait interval for to the current iteration of our main loop.  This is
     // only written to by the syncer thread, and since the only reader from a
@@ -213,7 +204,8 @@ class SyncerThread : public base::RefCountedThreadSafe<SyncerThread>,
           pause_requested_(false),
           paused_(false),
           syncer_(NULL),
-          connected_(false) {}
+          connected_(false),
+          pending_nudge_source_(kUnknown) {}
   } vault_;
 
   // Gets signaled whenever a thread outside of the syncer thread changes a