Reland of "[heap] Add more tasks for parallel compaction"

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 87e1b34f5c021acd57af14d222a5f107620a5e47..d090de066932274a5c344eaee91cf03c0d73fc6f 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -6,6 +6,7 @@
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
+#include "src/base/sys-info.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/cpu-profiler.h"
@@ -572,7 +573,6 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
   heap()->paged_space(OLD_SPACE)->ResetUnsweptFreeBytes();
   heap()->paged_space(CODE_SPACE)->ResetUnsweptFreeBytes();
   heap()->paged_space(MAP_SPACE)->ResetUnsweptFreeBytes();
-
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap && !evacuation()) {
     VerifyEvacuation(heap_);
@@ -593,7 +593,6 @@ bool MarkCompactCollector::IsSweepingCompleted() {
 
 void MarkCompactCollector::RefillFreeList(PagedSpace* space) {
   FreeList* free_list;
-
   if (space == heap()->old_space()) {
     free_list = free_list_old_space_.get();
   } else if (space == heap()->code_space()) {
@@ -3370,52 +3369,57 @@ bool MarkCompactCollector::EvacuateLiveObjectsFromPage(
 }
 
 
+int MarkCompactCollector::NumberOfParallelCompactionTasks() {
+  if (!FLAG_parallel_compaction) return 1;
+  // We cap the number of parallel compaction tasks by
+  // - (#cores - 1)
+  // - a value depending on the list of evacuation candidates
+  // - a hard limit
+  const int kPagesPerCompactionTask = 4;
+  const int kMaxCompactionTasks = 8;
+  return Min(kMaxCompactionTasks,
+             Min(1 + evacuation_candidates_.length() / kPagesPerCompactionTask,
+                 Max(1, base::SysInfo::NumberOfProcessors() - 1)));
+}
+
+
 void MarkCompactCollector::EvacuatePagesInParallel() {
   if (evacuation_candidates_.length() == 0) return;
 
-  int num_tasks = 1;
-  if (FLAG_parallel_compaction) {
-    num_tasks = NumberOfParallelCompactionTasks();
-  }
+  const int num_tasks = NumberOfParallelCompactionTasks();
 
   // Set up compaction spaces.
-  CompactionSpaceCollection** compaction_spaces_for_tasks =
+  CompactionSpaceCollection** spaces_for_tasks =
       new CompactionSpaceCollection*[num_tasks];
   for (int i = 0; i < num_tasks; i++) {
-    compaction_spaces_for_tasks[i] = new CompactionSpaceCollection(heap());
+    spaces_for_tasks[i] = new CompactionSpaceCollection(heap());
   }
-
-  compaction_spaces_for_tasks[0]->Get(OLD_SPACE)->MoveOverFreeMemory(
-      heap()->old_space());
-  compaction_spaces_for_tasks[0]
-      ->Get(CODE_SPACE)
-      ->MoveOverFreeMemory(heap()->code_space());
+  heap()->old_space()->DivideMemory(spaces_for_tasks, num_tasks, 1 * MB);
+  heap()->code_space()->DivideMemory(spaces_for_tasks, num_tasks, 1 * MB);
 
   compaction_in_progress_ = true;
   // Kick off parallel tasks.
   for (int i = 1; i < num_tasks; i++) {
     concurrent_compaction_tasks_active_++;
     V8::GetCurrentPlatform()->CallOnBackgroundThread(
-        new CompactionTask(heap(), compaction_spaces_for_tasks[i]),
+        new CompactionTask(heap(), spaces_for_tasks[i]),
         v8::Platform::kShortRunningTask);
   }
 
-  // Contribute in main thread. Counter and signal are in principal not needed.
-  concurrent_compaction_tasks_active_++;
-  EvacuatePages(compaction_spaces_for_tasks[0], &migration_slots_buffer_);
-  pending_compaction_tasks_semaphore_.Signal();
+  // Perform compaction on the main thread.
+  EvacuatePages(spaces_for_tasks[0], &migration_slots_buffer_);
 
   WaitUntilCompactionCompleted();
 
   // Merge back memory (compacted and unused) from compaction spaces.
   for (int i = 0; i < num_tasks; i++) {
     heap()->old_space()->MergeCompactionSpace(
-        compaction_spaces_for_tasks[i]->Get(OLD_SPACE));
+        spaces_for_tasks[i]->Get(OLD_SPACE));
     heap()->code_space()->MergeCompactionSpace(
-        compaction_spaces_for_tasks[i]->Get(CODE_SPACE));
-    delete compaction_spaces_for_tasks[i];
+        spaces_for_tasks[i]->Get(CODE_SPACE));
+    delete spaces_for_tasks[i];
   }
-  delete[] compaction_spaces_for_tasks;
+  delete[] spaces_for_tasks;
 
   // Finalize sequentially.
   const int num_pages = evacuation_candidates_.length();