Add option to prioritize timers

Summary: libev allows you to set prioritiy for watchers. This sets the priority of timers to the max, for both libev timer watchers and the timer fds. It's not clear if this will make a big difference

Reviewed By: kvtsoy

Differential Revision: D65384729

fbshipit-source-id: 95e7882d63254311caa8e99eff29a4d6c92901c3

quic/common/events/LibevQuicEventBase.cpp

@@ -126,6 +126,9 @@ void LibevQuicEventBase::scheduleLibevTimeoutImpl(
         libEvTimeoutCallback,
         seconds /* after */,
         0. /* repeat */);
+    if (prioritizeTimers_) {
+      ev_set_priority(&wrapper->ev_timer_, EV_MAXPRI);
+    }
     setImplHandle(timerCallback, wrapper);
   } else {
     // We already have a wrapper. Just re-arm it.
@@ -144,7 +147,8 @@ void LibevQuicEventBase::scheduleTimerFDTimeoutImpl(
   if (wrapper == nullptr) {
     // This is the first time this timer callback is getting scheduled. Create
     // a wrapper for it.
-    wrapper = new TimerCallbackWrapperTimerFD(timerCallback, ev_loop_);
+    wrapper = new TimerCallbackWrapperTimerFD(
+        timerCallback, ev_loop_, prioritizeTimers_);
     wrapper->ev_io_watcher_.data = wrapper;
     setImplHandle(timerCallback, wrapper);
   }

quic/common/events/LibevQuicEventBase.h

@@ -140,6 +140,10 @@ class LibevQuicEventBase
 #endif
   }
 
+  void prioritizeTimers() {
+    prioritizeTimers_ = true;
+  }
+
   struct ev_loop* getLibevLoop() {
     return ev_loop_;
   }
@@ -191,9 +195,10 @@ class LibevQuicEventBase
   class TimerCallbackWrapperTimerFD
       : public QuicTimerCallback::TimerCallbackImpl {
    public:
-    explicit TimerCallbackWrapperTimerFD(
+    TimerCallbackWrapperTimerFD(
         QuicTimerCallback* callback,
-        struct ev_loop* ev_loop)
+        struct ev_loop* ev_loop,
+        bool prioritizeTimers)
         : callback_(callback), ev_loop_(ev_loop) {
 #if defined(HAS_TIMERFD)
       ev_io_watcher_.fd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK);
@@ -211,6 +216,9 @@ class LibevQuicEventBase
           },
           ev_io_watcher_.fd,
           EV_READ);
+      if (prioritizeTimers) {
+        ev_set_priority(&ev_io_watcher_, EV_MAXPRI);
+      }
 #else
       LOG(FATAL) << "TimerFD not supported on this platform";
 #endif
@@ -339,5 +347,6 @@ class LibevQuicEventBase
   ev_timer ev_timer_internal_;
   bool internalTimerInitialized_{false};
   bool useTimerFd_{false};
+  bool prioritizeTimers_{false};
 };
 } // namespace quic