[gc] Delay start of memory reducer.

Creates a feature (flag): transition from Done -> Wait
schedules a timer after 30s instead of 8s.
In local benchmark, this reduces by 50% cpu time spent doing
incremental marking and sweeping.

Bug: chromium:1330940
Change-Id: Iff9121243b88d0ed87d0b921e285ece52a83eaa9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3696168
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81283}

src/flags/flag-definitions.h

@@ -1416,6 +1416,9 @@ DEFINE_BOOL(
 DEFINE_BOOL(gc_experiment_less_compaction, false,
             "less compaction in non-memory reducing mode")
 
+DEFINE_INT(gc_memory_reducer_start_delay_ms, 8000,
+           "Delay before memory reducer start")
+
 DEFINE_BOOL(disable_abortjs, false, "disables AbortJS runtime function")
 
 DEFINE_BOOL(randomize_all_allocations, false,

src/heap/memory-reducer.cc

@@ -156,17 +156,14 @@ MemoryReducer::State MemoryReducer::Step(const State& state,
                 state.committed_memory_at_last_run + kCommittedMemoryDelta)) {
           return state;
         } else {
-          return State(kWait, 0, event.time_ms + kLongDelayMs,
-                       event.type == kMarkCompact ? event.time_ms
-                                                  : state.last_gc_time_ms,
+          return State(kWait, 0, event.time_ms + kLongDelayMs, event.time_ms,
                        0);
         }
       } else {
         DCHECK_EQ(kPossibleGarbage, event.type);
-        return State(
-            kWait, 0, event.time_ms + kLongDelayMs,
-            event.type == kMarkCompact ? event.time_ms : state.last_gc_time_ms,
-            0);
+        return State(kWait, 0,
+                     event.time_ms + FLAG_gc_memory_reducer_start_delay_ms,
+                     state.last_gc_time_ms, 0);
       }
     case kWait:
       switch (event.type) {

src/heap/memory-reducer.h

@@ -166,6 +166,7 @@ class V8_EXPORT_PRIVATE MemoryReducer {
   State state_;
   unsigned int js_calls_counter_;
   double js_calls_sample_time_ms_;
+  int start_delay_ms_ = false;
 
   // Used in cctest.
   friend class heap::HeapTester;

test/unittests/heap/memory-reducer-unittest.cc

@@ -123,7 +123,7 @@ TEST(MemoryReducer, FromDoneToWait) {
       state0,
       MarkCompactEventGarbageLeft(2, MemoryReducer::kCommittedMemoryDelta));
   EXPECT_EQ(MemoryReducer::kWait, state1.action);
-  EXPECT_EQ(MemoryReducer::kLongDelayMs + 2, state1.next_gc_start_ms);
+  EXPECT_EQ(FLAG_gc_memory_reducer_start_delay_ms + 2, state1.next_gc_start_ms);
   EXPECT_EQ(0, state1.started_gcs);
   EXPECT_EQ(2, state1.last_gc_time_ms);
 
@@ -131,13 +131,13 @@ TEST(MemoryReducer, FromDoneToWait) {
       state0,
       MarkCompactEventNoGarbageLeft(2, MemoryReducer::kCommittedMemoryDelta));
   EXPECT_EQ(MemoryReducer::kWait, state1.action);
-  EXPECT_EQ(MemoryReducer::kLongDelayMs + 2, state1.next_gc_start_ms);
+  EXPECT_EQ(FLAG_gc_memory_reducer_start_delay_ms + 2, state1.next_gc_start_ms);
   EXPECT_EQ(0, state1.started_gcs);
   EXPECT_EQ(2, state1.last_gc_time_ms);
 
   state1 = MemoryReducer::Step(state0, PossibleGarbageEvent(0));
   EXPECT_EQ(MemoryReducer::kWait, state1.action);
-  EXPECT_EQ(MemoryReducer::kLongDelayMs, state1.next_gc_start_ms);
+  EXPECT_EQ(FLAG_gc_memory_reducer_start_delay_ms, state1.next_gc_start_ms);
   EXPECT_EQ(0, state1.started_gcs);
   EXPECT_EQ(state0.last_gc_time_ms, state1.last_gc_time_ms);
 
@@ -147,7 +147,7 @@ TEST(MemoryReducer, FromDoneToWait) {
                   2, static_cast<size_t>(
                          1000 * MB * MemoryReducer::kCommittedMemoryFactor)));
   EXPECT_EQ(MemoryReducer::kWait, state1.action);
-  EXPECT_EQ(MemoryReducer::kLongDelayMs + 2, state1.next_gc_start_ms);
+  EXPECT_EQ(FLAG_gc_memory_reducer_start_delay_ms + 2, state1.next_gc_start_ms);
   EXPECT_EQ(0, state1.started_gcs);
   EXPECT_EQ(2, state1.last_gc_time_ms);
 }