[heap] Add flag for experimenting with the number of marking tasks.

This adds --gc-experiment-reduce-concurrent-marking-tasks to be used
in a Finch experiment.

Bug: v8:10442
Change-Id: Ie2adf4faa20c99d1793907dfc6857497743f8d5c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2280093Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68673}

src/flags/flag-definitions.h

@@ -1067,6 +1067,8 @@ DEFINE_BOOL(gc_experiment_background_schedule, false,
             "new background GC schedule heuristics")
 DEFINE_BOOL(gc_experiment_less_compaction, false,
             "less compaction in non-memory reducing mode")
+DEFINE_BOOL(gc_experiment_reduce_concurrent_marking_tasks, false,
+            "reduce the number of concurrent marking tasks")
 
 DEFINE_BOOL(disable_abortjs, false, "disables AbortJS runtime function")
 

src/heap/concurrent-marking.cc

@@ -527,6 +527,10 @@ void ConcurrentMarking::ScheduleTasks() {
     // thread.
     total_task_count_ = Max(1, Min(kMaxTasks, num_cores - 2));
 #endif  // defined(OS_MACOSX)
+    if (FLAG_gc_experiment_reduce_concurrent_marking_tasks) {
+      // Use at most half of the cores in the experiment.
+      total_task_count_ = Max(1, Min(kMaxTasks, (num_cores / 2) - 1));
+    }
     DCHECK_LE(total_task_count_, kMaxTasks);
     // One task is for the main thread.
     STATIC_ASSERT(kMaxTasks + 1 <= MarkingWorklist::kMaxNumTasks);