[heap] Add more tasks for parallel compaction

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 6558eb2ddebba2b6349c64eb2fbf4f2c9ea40247..f5d09e0b2678e4a781a917e47c5b696027aa3ce2 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -6,6 +6,7 @@
 #define V8_HEAP_MARK_COMPACT_H_
 
 #include "src/base/bits.h"
+#include "src/base/sys-info.h"
 #include "src/heap/spaces.h"
 
 namespace v8 {
@@ -709,10 +710,15 @@ class MarkCompactCollector {
 
   void EvacuatePagesInParallel();
 
+  // The number of parallel compaction tasks, including the main thread.
   int NumberOfParallelCompactionTasks() {

[Hannes Payer (out of office), 2015/09/23 07:26:03]

Implement this in the cc file. That makes it faster to recompile when playing
with this metric. The include can then also move to the cc file.

[Michael Lippautz, 2015/09/23 07:40:50]

Done.

-    // TODO(hpayer, mlippautz): Figure out some logic to determine the number
-    // of compaction tasks.
-    return 1;
+    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

[Hannes Payer (out of office), 2015/09/23 07:26:03]

Make the magic limits named constants.

[Michael Lippautz, 2015/09/23 07:40:50]

Done.

+    return Min(8, Min(1 + evacuation_candidates_.length() / 4,
+                      Max(1, base::SysInfo::NumberOfProcessors() - 1)));
   }
 
   void WaitUntilCompactionCompleted();