[heap] Add --fast-forward-schedule and disable it for experimenting

Add a flag for disabling FastForwardSchedule() during incremental
marking.

This CL disables "schedule fast forwarding" by default to experiment
with this setup for a few days and to see whether this causes
performance regressions.

Bug: v8:12775
Change-Id: Id2c0a62f4b67b3a2379816581800cfb62361b40c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3865003
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82806}

src/flags/flag-definitions.h

@@ -1259,6 +1259,7 @@ DEFINE_BOOL(incremental_marking_task, true, "use tasks for incremental marking")
 DEFINE_INT(incremental_marking_soft_trigger, 0,
            "threshold for starting incremental marking via a task in percent "
            "of available space: limit - size")
+DEFINE_BOOL(fast_forward_schedule, false, "Fast forwards marking schedule")
 DEFINE_INT(incremental_marking_hard_trigger, 0,
            "threshold for starting incremental marking immediately in percent "
            "of available space: limit - size")

src/heap/incremental-marking.cc

@@ -677,6 +677,8 @@ bool IncrementalMarking::TryInitializeTaskTimeout() {
 }
 
 void IncrementalMarking::FastForwardSchedule() {
+  DCHECK(FLAG_fast_forward_schedule);
+
   if (scheduled_bytes_to_mark_ < bytes_marked_) {
     scheduled_bytes_to_mark_ = bytes_marked_;
     if (FLAG_trace_incremental_marking) {
@@ -717,7 +719,9 @@ void IncrementalMarking::ScheduleBytesToMarkBasedOnTime(double time_ms) {
 
 void IncrementalMarking::AdvanceAndFinalizeIfComplete() {
   ScheduleBytesToMarkBasedOnTime(heap()->MonotonicallyIncreasingTimeInMs());
+  if (FLAG_fast_forward_schedule) {
     FastForwardScheduleIfCloseToFinalization();
+  }
   Step(kStepSizeInMs, StepOrigin::kTask);
   heap()->FinalizeIncrementalMarkingIfComplete(
       GarbageCollectionReason::kFinalizeMarkingViaTask);