Add a new stats level to exclude tickers (facebook#7329)

Summary:
Currently, application may pass a statistics object to db but later
wants to reduce stats tracking overhead by setting stats level to
kExceptHistogramOrTimers (the current lowest level). Tickers will still
be incremented, causing up to 1% CPU. We can add a new lowest stats
level `kExceptTickers` to disable ticker incrementing as well, thus
reducing CPU cycles spent on tickers.

Test Plan (devserver):
```
make check
make clean
DEBUG_LEVEL=0 make db_bench
./db_bench -perf_level=1 -stats_level=0 -statistics -benchmarks=fillseq,readrandom -duration=120
```

Measure CPU util (%) before and after change:
CPU util by rocksdb::RecordTick: 1.1 vs (<0.1)

Pull Request resolved: facebook#7329

Reviewed By: pdillinger

Differential Revision: D23434014

Pulled By: riversand963

fbshipit-source-id: 72ff0f02a192ac476d4b0044b9f37fd4a22ff0d4

db/db_statistics_test.cc

@@ -142,6 +142,19 @@ TEST_F(DBStatisticsTest, ResetStats) {
   }
 }
 
+TEST_F(DBStatisticsTest, ExcludeTickers) {
+  Options options = CurrentOptions();
+  options.statistics = ROCKSDB_NAMESPACE::CreateDBStatistics();
+  DestroyAndReopen(options);
+  options.statistics->set_stats_level(StatsLevel::kExceptTickers);
+  ASSERT_OK(Put("foo", "value"));
+  ASSERT_EQ(0, options.statistics->getTickerCount(BYTES_WRITTEN));
+  options.statistics->set_stats_level(StatsLevel::kExceptHistogramOrTimers);
+  Reopen(options);
+  ASSERT_EQ("value", Get("foo"));
+  ASSERT_GT(options.statistics->getTickerCount(BYTES_READ), 0);
+}
+
 }  // namespace ROCKSDB_NAMESPACE
 
 int main(int argc, char** argv) {

include/rocksdb/statistics.h

@@ -488,6 +488,10 @@ struct HistogramData {
 // Usage:
 //   options.statistics->set_stats_level(StatsLevel::kExceptTimeForMutex);
 enum StatsLevel : uint8_t {
+  // Disable all metrics
+  kDisableAll,
+  // Disable tickers
+  kExceptTickers = kDisableAll,
   // Disable timer stats, and skip histogram stats
   kExceptHistogramOrTimers,
   // Skip timer stats

monitoring/statistics.cc

@@ -329,11 +329,17 @@ uint64_t StatisticsImpl::getAndResetTickerCount(uint32_t tickerType) {
 }
 
 void StatisticsImpl::recordTick(uint32_t tickerType, uint64_t count) {
-  assert(tickerType < TICKER_ENUM_MAX);
-  per_core_stats_.Access()->tickers_[tickerType].fetch_add(
-      count, std::memory_order_relaxed);
-  if (stats_ && tickerType < TICKER_ENUM_MAX) {
-    stats_->recordTick(tickerType, count);
+  if (get_stats_level() <= StatsLevel::kExceptTickers) {
+    return;
+  }
+  if (tickerType < TICKER_ENUM_MAX) {
+    per_core_stats_.Access()->tickers_[tickerType].fetch_add(
+        count, std::memory_order_relaxed);
+    if (stats_) {
+      stats_->recordTick(tickerType, count);
+    }
+  } else {
+    assert(false);
   }
 }