Relax memory order for faster tickers

Summary:
The default behavior for atomic operations is sequentially consistent ordering
which is not needed for simple counters (see:
http://en.cppreference.com/w/cpp/atomic/memory_order). Change the memory order
to std::memory_order_relaxed for better performance.

Test Plan: make clean all check

Reviewers: rven, anthony, yhchiang, aekmekji, sdong, MarkCallaghan, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46953

util/statistics.cc

@@ -62,7 +62,7 @@ void StatisticsImpl::setTickerCount(uint32_t tickerType, uint64_t count) {
       tickerType < INTERNAL_TICKER_ENUM_MAX :
       tickerType < TICKER_ENUM_MAX);
   if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
-    tickers_[tickerType].value = count;
+    tickers_[tickerType].value.store(count, std::memory_order_relaxed);
   }
   if (stats_ && tickerType < TICKER_ENUM_MAX) {
     stats_->setTickerCount(tickerType, count);
@@ -75,7 +75,7 @@ void StatisticsImpl::recordTick(uint32_t tickerType, uint64_t count) {
       tickerType < INTERNAL_TICKER_ENUM_MAX :
       tickerType < TICKER_ENUM_MAX);
   if (tickerType < TICKER_ENUM_MAX || enable_internal_stats_) {
-    tickers_[tickerType].value += count;
+    tickers_[tickerType].value.fetch_add(count, std::memory_order_relaxed);
   }
   if (stats_ && tickerType < TICKER_ENUM_MAX) {
     stats_->recordTick(tickerType, count);