[heap] Use live memory as heuristic for spawning compaction tasks

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/base/sys-info.h"
 #include "src/code-stubs.h"
@@ skipping 3293 matching lines @@
   }
   p->ResetLiveBytes();
   return true;
 }
 
 
 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 value depending on the live memory in evacuation candidates
   // - a hard limit
-  const int kPagesPerCompactionTask = 4;
+  //
+  // TODO(mlippautz): Instead of basing the limit on live memory, we could also
+  //   compute the number from the time it takes to evacuate memory and a given
+  //   desired time in which compaction should be finished.
+  const int kLiveMemoryPerCompactionTask = 2 * Page::kPageSize;
   const int kMaxCompactionTasks = 8;
+  int live_bytes = 0;
+  for (Page* page : evacuation_candidates_) {
+    live_bytes += page->LiveBytes();
+  }
   return Min(kMaxCompactionTasks,
-             Min(1 + evacuation_candidates_.length() / kPagesPerCompactionTask,
+             Min(1 + live_bytes / kLiveMemoryPerCompactionTask,
                  Max(1, base::SysInfo::NumberOfProcessors() - 1)));
 }
 
 
 void MarkCompactCollector::EvacuatePagesInParallel() {
   const int num_pages = evacuation_candidates_.length();
   if (num_pages == 0) return;
 
   const int num_tasks = NumberOfParallelCompactionTasks();
 
@@ skipping 1262 matching lines @@
     MarkBit mark_bit = Marking::MarkBitFrom(host);
     if (Marking::IsBlack(mark_bit)) {
       RelocInfo rinfo(pc, RelocInfo::CODE_TARGET, 0, host);
       RecordRelocSlot(&rinfo, target);
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8