Do not set startWork_ in EventBase if time measurement is disabled

Summary: startWork_ is used only if enableTimeMeasurement is set. It gives ~0.5% performance win for mcrouter.

Reviewed By: yfeldblum

Differential Revision: D2590176

fb-gh-sync-id: 07f2189ebdec751cd0d91d191d8f595780d2808a

folly/io/async/EventBase.cpp

@@ -459,12 +459,14 @@ bool EventBase::bumpHandlingTime() {
     " (loop) latest " << latestLoopCnt_ << " next " << nextLoopCnt_;
   if(nothingHandledYet()) {
     latestLoopCnt_ = nextLoopCnt_;
+    if (enableTimeMeasurement_) {
       // set the time
       startWork_ = std::chrono::duration_cast<std::chrono::microseconds>(
         std::chrono::steady_clock::now().time_since_epoch()).count();
 
       VLOG(11) << "EventBase " << this << " " << __PRETTY_FUNCTION__ <<
         " (loop) startWork_ " << startWork_;
+    }
     return true;
   }
   return false;