Allow more scavenges during idle times by pushing down the idle new space limit.

BUG=

Review URL: https://codereview.chromium.org/1313083002

Cr-Commit-Position: refs/heads/master@{#30356}

src/heap/gc-idle-time-handler.cc

@@ -119,13 +119,17 @@ bool GCIdleTimeHandler::ShouldDoScavenge(
     // It is better to do full GC for the background tab.
     return false;
   }
-  size_t new_space_allocation_limit =
-      kMaxScheduledIdleTime * scavenge_speed_in_bytes_per_ms;
+
+  // Calculates how much memory are we able to scavenge in
+  // kMaxFrameRenderingIdleTime ms. If scavenge_speed_in_bytes_per_ms is 0 we
+  // will take care of this later.
+  size_t idle_new_space_allocation_limit =
+      kMaxFrameRenderingIdleTime * scavenge_speed_in_bytes_per_ms;
 
   // If the limit is larger than the new space size, then scavenging used to be
   // really fast. We can take advantage of the whole new space.
-  if (new_space_allocation_limit > new_space_size) {
-    new_space_allocation_limit = new_space_size;
+  if (idle_new_space_allocation_limit > new_space_size) {
+    idle_new_space_allocation_limit = new_space_size;
   }
 
   // We do not know the allocation throughput before the first scavenge.
@@ -134,31 +138,43 @@ bool GCIdleTimeHandler::ShouldDoScavenge(
     // We have to trigger scavenge before we reach the end of new space.
     size_t adjust_limit = new_space_allocation_throughput_in_bytes_per_ms *
                           kTimeUntilNextIdleEvent;
-    if (adjust_limit > new_space_allocation_limit) {
-      new_space_allocation_limit = 0;
+    if (adjust_limit > idle_new_space_allocation_limit) {
+      idle_new_space_allocation_limit = 0;
     } else {
-      new_space_allocation_limit -= adjust_limit;
+      idle_new_space_allocation_limit -= adjust_limit;
     }
   }
 
-  if (new_space_allocation_throughput_in_bytes_per_ms <
-      kLowAllocationThroughput) {
-    new_space_allocation_limit =
-        Min(new_space_allocation_limit,
-            static_cast<size_t>(new_space_size * kConservativeTimeRatio));
-  }
-
   // The allocated new space limit to trigger a scavange has to be at least
   // kMinimumNewSpaceSizeToPerformScavenge.
-  if (new_space_allocation_limit < kMinimumNewSpaceSizeToPerformScavenge) {
-    new_space_allocation_limit = kMinimumNewSpaceSizeToPerformScavenge;
+  if (idle_new_space_allocation_limit < kMinimumNewSpaceSizeToPerformScavenge) {
+    idle_new_space_allocation_limit = kMinimumNewSpaceSizeToPerformScavenge;
   }
 
+  // Set an initial scavenge speed if it is unknown.
   if (scavenge_speed_in_bytes_per_ms == 0) {
     scavenge_speed_in_bytes_per_ms = kInitialConservativeScavengeSpeed;
   }
 
-  if (new_space_allocation_limit <= used_new_space_size) {
+  // We apply a max factor to the new space size to make sure that a slowly
+  // allocating application still leaves enough of wiggle room to schedule a
+  // scavenge.
+  size_t max_limit;
+  const double kMaxNewSpaceSizeFactorLongIdleTimes = 0.5;
+  const double kMaxNewSpaceSizeFactorShortIdleTimes = 0.8;
+  if (idle_time_in_ms > kMaxFrameRenderingIdleTime) {
+    max_limit = static_cast<size_t>(new_space_size *
+                                    kMaxNewSpaceSizeFactorLongIdleTimes);
+  } else {
+    max_limit = static_cast<size_t>(new_space_size *
+                                    kMaxNewSpaceSizeFactorShortIdleTimes);
+  }
+  idle_new_space_allocation_limit =
+      Min(idle_new_space_allocation_limit, max_limit);
+
+  // We perform a scavenge if we are over the idle new space limit and
+  // a scavenge fits into the given idle time bucket.
+  if (idle_new_space_allocation_limit <= used_new_space_size) {
     if (used_new_space_size / scavenge_speed_in_bytes_per_ms <=
         idle_time_in_ms) {
       return true;