[heap] Flag for parallel ephemeron marking

New flag for disabling of parallel marking after visiting all
ephemerons.

Bug: chromium:844008
Change-Id: I3dfc4e6a05ffc8e065313ef4955279617246802c
Reviewed-on: https://chromium-review.googlesource.com/1100468Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Cr-Commit-Position: refs/heads/master@{#53728}

src/flag-definitions.h

@@ -689,6 +689,9 @@ DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING_BOOL,
             "use concurrent marking")
 DEFINE_BOOL(parallel_marking, true, "use parallel marking in atomic pause")
 DEFINE_IMPLICATION(parallel_marking, concurrent_marking)
+DEFINE_BOOL(
+    parallel_ephemeron_marking, true,
+    "use parallel marking of objects after visiting ephemerons in atomic pause")
 DEFINE_BOOL(trace_concurrent_marking, false, "trace concurrent marking")
 DEFINE_BOOL(black_allocation, true, "use black allocation")
 DEFINE_BOOL(concurrent_store_buffer, true,

src/heap/mark-compact.cc

@@ -1588,7 +1588,7 @@ void MarkCompactCollector::MarkLiveObjects() {
 
   {
     TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_MAIN);
-    if (FLAG_parallel_marking) {
+    if (FLAG_parallel_marking && FLAG_parallel_ephemeron_marking) {
       DCHECK(FLAG_concurrent_marking);
       heap_->concurrent_marking()->RescheduleTasksIfNeeded();
     }