diff --git a/runtime/vm/heap/heap.cc b/runtime/vm/heap/heap.cc
index 9475b4f28ed..0c943fadebe 100644
--- a/runtime/vm/heap/heap.cc
+++ b/runtime/vm/heap/heap.cc
@@ -565,7 +565,12 @@ void Heap::CheckConcurrentMarking(Thread* thread, GCReason reason) {
 
   switch (phase) {
     case PageSpace::kMarking:
-      // TODO(rmacnak): Have this thread help with marking.
+      // TODO(rmacnak): Make the allocator of a large page mark size equals to
+      // the large page?
+      COMPILE_ASSERT(kOldPageSize == 512 * KB);
+      COMPILE_ASSERT(kNewPageSize == 512 * KB);
+      old_space_.IncrementalMarkWithSizeBudget(512 * KB);
+      return;
     case PageSpace::kSweepingLarge:
     case PageSpace::kSweepingRegular:
       return;  // Busy.
diff --git a/runtime/vm/heap/pages.cc b/runtime/vm/heap/pages.cc
index e7f1933ffee..dbd960ac5f1 100644
--- a/runtime/vm/heap/pages.cc
+++ b/runtime/vm/heap/pages.cc
@@ -1674,19 +1674,17 @@ void PageSpaceController::RecordUpdate(SpaceUsage before,
       after.CombinedUsedInWords() + (kOldPageSizeInWords * growth_in_pages);
 
 #if defined(TARGET_ARCH_IA32)
-  // No concurrent marking.
-  soft_gc_threshold_in_words_ = threshold;
-  hard_gc_threshold_in_words_ = threshold;
+  bool concurrent_mark = false;
 #else
-  // Start concurrent marking when old-space has less than half of new-space
-  // available or less than 5% available.
-  // Note that heap_ can be null in some unit tests.
-  const intptr_t new_space =
-      heap_ == nullptr ? 0 : heap_->new_space()->CapacityInWords();
-  const intptr_t headroom = Utils::Maximum(new_space / 2, threshold / 20);
-  soft_gc_threshold_in_words_ = threshold;
-  hard_gc_threshold_in_words_ = threshold + headroom;
+  bool concurrent_mark = FLAG_concurrent_mark && (FLAG_marker_tasks != 0);
 #endif
+  if (concurrent_mark) {
+    soft_gc_threshold_in_words_ = threshold;
+    hard_gc_threshold_in_words_ = kIntptrMax / kWordSize;
+  } else {
+    soft_gc_threshold_in_words_ = kIntptrMax / kWordSize;
+    hard_gc_threshold_in_words_ = threshold;
+  }
 
   // Set a tight idle threshold.
   idle_gc_threshold_in_words_ =