[heap] Make the scavenge task trigger configurable via a runtime flags.

Change-Id: If9252354404efcac89ab8fbbd41250a6311fdfd6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135731Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66985}

src/flags/flag-definitions.h

@@ -892,6 +892,8 @@ DEFINE_BOOL(incremental_marking_wrappers, true,
 DEFINE_BOOL(trace_unmapper, false, "Trace the unmapping")
 DEFINE_BOOL(parallel_scavenge, true, "parallel scavenge")
 DEFINE_BOOL(scavenge_task, true, "schedule scavenge tasks")
+DEFINE_INT(scavenge_task_trigger, 80,
+           "scavenge task trigger in percent of the current heap limit")
 DEFINE_BOOL(trace_parallel_scavenge, false, "trace parallel scavenge")
 DEFINE_BOOL(write_protect_code_memory, true, "write protect code memory")
 #ifdef V8_CONCURRENT_MARKING

src/heap/scavenge-job.cc

@@ -30,8 +30,7 @@ class ScavengeJob::Task : public CancelableTask {
 };
 
 size_t ScavengeJob::YoungGenerationTaskTriggerSize(Heap* heap) {
-  static constexpr double kTaskTriggerFactor = 0.8;
-  return heap->new_space()->Capacity() * kTaskTriggerFactor;
+  return heap->new_space()->Capacity() * FLAG_scavenge_task_trigger / 100;
 }
 
 bool ScavengeJob::YoungGenerationSizeTaskTriggerReached(Heap* heap) {