windows: improve timer precision

Improve timing precision by using QueryPerformanceCounter.

This is part of the fix for Node.js' test-timers-first-fire.js.

include/uv-win.h

@@ -316,8 +316,6 @@ RB_HEAD(uv_timer_tree_s, uv_timer_s);
   HANDLE iocp;                                                                \
   /* The current time according to the event loop. in msecs. */               \
   uint64_t time;                                                              \
-  /* GetTickCount() result when the event loop time was last updated. */      \
-  DWORD last_tick_count;                                                      \
   /* Tail of a single-linked circular queue of pending reqs. If the queue */  \
   /* is empty, tail_ is NULL. If there is only one item, */                   \
   /* tail_->next_req == tail_ */                                              \

src/win/core.c

@@ -137,7 +137,6 @@ int uv_loop_init(uv_loop_t* loop) {
    * to zero before calling uv_update_time for the first time.
    */
   loop->time = 0;
-  loop->last_tick_count = 0;
   uv_update_time(loop);
 
   QUEUE_INIT(&loop->wq);
@@ -314,13 +313,17 @@ static void uv_poll(uv_loop_t* loop, DWORD timeout) {
     /* Package was dequeued */
     req = uv_overlapped_to_req(overlapped);
     uv_insert_pending_req(loop, req);
+
+    /* Some time might have passed waiting for I/O,
+     * so update the loop time here.
+     */
+    uv_update_time(loop);
   } else if (GetLastError() != WAIT_TIMEOUT) {
     /* Serious error */
     uv_fatal_error(GetLastError(), "GetQueuedCompletionStatus");
-  } else {
-    /* We're sure that at least `timeout` milliseconds have expired, but
-     * this may not be reflected yet in the GetTickCount() return value.
-     * Therefore we ensure it's taken into account here.
+  } else if (timeout > 0) {
+    /* GetQueuedCompletionStatus can occasionally return a little early.
+     * Make sure that the desired timeout is reflected in the loop time.
      */
     uv__time_forward(loop, timeout);
   }
@@ -347,13 +350,17 @@ static void uv_poll_ex(uv_loop_t* loop, DWORD timeout) {
       req = uv_overlapped_to_req(overlappeds[i].lpOverlapped);
       uv_insert_pending_req(loop, req);
     }
+
+    /* Some time might have passed waiting for I/O,
+     * so update the loop time here.
+     */
+    uv_update_time(loop);
   } else if (GetLastError() != WAIT_TIMEOUT) {
     /* Serious error */
     uv_fatal_error(GetLastError(), "GetQueuedCompletionStatusEx");
   } else if (timeout > 0) {
-    /* We're sure that at least `timeout` milliseconds have expired, but
-     * this may not be reflected yet in the GetTickCount() return value.
-     * Therefore we ensure it's taken into account here.
+    /* GetQueuedCompletionStatus can occasionally return a little early.
+     * Make sure that the desired timeout is reflected in the loop time.
      */
     uv__time_forward(loop, timeout);
   }
@@ -412,7 +419,6 @@ int uv_run(uv_loop_t *loop, uv_run_mode mode) {
        * UV_RUN_NOWAIT makes no guarantees about progress so it's omitted from
        * the check.
        */
-      uv_update_time(loop);
       uv_process_timers(loop);
     }
 

src/win/internal.h

@@ -322,6 +322,7 @@ void uv__fs_poll_endgame(uv_loop_t* loop, uv_fs_poll_t* handle);
  */
 void uv__util_init();
 
+uint64_t uv__hrtime(double scale);
 int uv_parent_pid();
 __declspec(noreturn) void uv_fatal_error(const int errorno, const char* syscall);
 

src/win/timer.c

@@ -28,39 +28,17 @@
 #include "handle-inl.h"
 
 
-void uv_update_time(uv_loop_t* loop) {
-  DWORD ticks;
-  ULARGE_INTEGER time;
-
-  ticks = GetTickCount();
+/* The number of milliseconds in one second. */
+#define UV__MILLISEC 1000
 
-  time.QuadPart = loop->time;
 
-  /* GetTickCount() can conceivably wrap around, so when the current tick
-   * count is lower than the last tick count, we'll assume it has wrapped.
-   * uv_poll must make sure that the timer can never overflow more than
-   * once between two subsequent uv_update_time calls.
-   */
-  time.LowPart = ticks;
-  if (ticks < loop->last_tick_count)
-    time.HighPart++;
-
-  /* Remember the last tick count. */
-  loop->last_tick_count = ticks;
-
-  /* The GetTickCount() resolution isn't too good. Sometimes it'll happen
-   * that GetQueuedCompletionStatus() or GetQueuedCompletionStatusEx() has
-   * waited for a couple of ms but this is not reflected in the GetTickCount
-   * result yet. Therefore whenever GetQueuedCompletionStatus times out
-   * we'll add the number of ms that it has waited to the current loop time.
-   * When that happened the loop time might be a little ms farther than what
-   * we've just computed, and we shouldn't update the loop time.
-   */
-  if (loop->time < time.QuadPart)
-    loop->time = time.QuadPart;
+void uv_update_time(uv_loop_t* loop) {
+  uint64_t new_time = uv__hrtime(UV__MILLISEC);
+  if (new_time > loop->time) {
+    loop->time = new_time;
+  }
 }
 
-
 void uv__time_forward(uv_loop_t* loop, uint64_t msecs) {
   loop->time += msecs;
 }
@@ -191,16 +169,9 @@ DWORD uv__next_timeout(const uv_loop_t* loop) {
   timer = RB_MIN(uv_timer_tree_s, &((uv_loop_t*)loop)->timers);
   if (timer) {
     delta = timer->due - loop->time;
-    if (delta >= UINT_MAX >> 1) {
-      /* A timeout value of UINT_MAX means infinite, so that's no good. But
-       * more importantly, there's always the risk that GetTickCount wraps.
-       * uv_update_time can detect this, but we must make sure that the
-       * tick counter never overflows twice between two subsequent
-       * uv_update_time calls. We do this by never sleeping more than half
-       * the time it takes to wrap  the counter - which is huge overkill,
-       * but hey, it's not so bad to wake up every 25 days.
-       */
-      return UINT_MAX >> 1;
+    if (delta >= UINT_MAX - 1) {
+      /* A timeout value of UINT_MAX means infinite, so that's no good. */
+      return UINT_MAX - 1;
     } else if (delta < 0) {
       /* Negative timeout values are not allowed */
       return 0;

src/win/util.c

@@ -52,16 +52,15 @@
 #define MAX_TITLE_LENGTH 8192
 
 /* The number of nanoseconds in one second. */
-#undef NANOSEC
-#define NANOSEC 1000000000
+#define UV__NANOSEC 1000000000
 
 
 /* Cached copy of the process title, plus a mutex guarding it. */
 static char *process_title;
 static CRITICAL_SECTION process_title_lock;
 
-/* Frequency (ticks per nanosecond) of the high-resolution clock. */
-static double hrtime_frequency_ = 0;
+/* Interval (in seconds) of the high-resolution clock. */
+static double hrtime_interval_ = 0;
 
 
 /*
@@ -73,11 +72,14 @@ void uv__util_init() {
   /* Initialize process title access mutex. */
   InitializeCriticalSection(&process_title_lock);
 
-  /* Retrieve high-resolution timer frequency. */
-  if (QueryPerformanceFrequency(&perf_frequency))
-    hrtime_frequency_ = (double) perf_frequency.QuadPart / (double) NANOSEC;
-  else
-    hrtime_frequency_= 0;
+  /* Retrieve high-resolution timer frequency
+   * and precompute its reciprocal. 
+   */
+  if (QueryPerformanceFrequency(&perf_frequency)) {
+    hrtime_interval_ = 1.0 / perf_frequency.QuadPart;
+  } else {
+    hrtime_interval_= 0;
+  }
 }
 
 
@@ -463,26 +465,27 @@ int uv_get_process_title(char* buffer, size_t size) {
 
 
 uint64_t uv_hrtime(void) {
-  LARGE_INTEGER counter;
-
   uv__once_init();
+  return uv__hrtime(UV__NANOSEC);
+}
+
+uint64_t uv__hrtime(double scale) {
+  LARGE_INTEGER counter;
 
-  /* If the performance frequency is zero, there's no support. */
-  if (hrtime_frequency_ == 0) {
-    /* uv__set_sys_error(loop, ERROR_NOT_SUPPORTED); */
+  /* If the performance interval is zero, there's no support. */
+  if (hrtime_interval_ == 0) {
     return 0;
   }
 
   if (!QueryPerformanceCounter(&counter)) {
-    /* uv__set_sys_error(loop, GetLastError()); */
     return 0;
   }
 
   /* Because we have no guarantee about the order of magnitude of the
-   * performance counter frequency, integer math could cause this computation
+   * performance counter interval, integer math could cause this computation
    * to overflow. Therefore we resort to floating point math.
    */
-  return (uint64_t) ((double) counter.QuadPart / hrtime_frequency_);
+  return (uint64_t) ((double) counter.QuadPart * hrtime_interval_ * scale);
 }