[PATCH 3/7] ntdll: Use CLOCK_REALTIME_COARSE for NtQuerySystemTime() if it has sufficient resolution.

[Huw Davies]

This will only affect users running with HZ=1000.  On an i7-8700 CPU @
3.20GHz it cuts the call cost from ~30ns to ~12ns.

Signed-off-by: Huw Davies 
---
 dlls/ntdll/time.c | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/dlls/ntdll/time.c b/dlls/ntdll/time.c
index d0a7954377..37401ce2e1 100644
--- a/dlls/ntdll/time.c
+++ b/dlls/ntdll/time.c
@@ -458,19 +458,43 @@ void WINAPI RtlTimeToElapsedTimeFields( const LARGE_INTEGER *Time, PTIME_FIELDS
  * Get the current system time.
  *
  * PARAMS
- *   Time [O] Destination for the current system time.
+ *   time [O] Destination for the current system time.
  *
  * RETURNS
  *   Success: STATUS_SUCCESS.
  *   Failure: An NTSTATUS error code indicating the problem.
  */
-NTSTATUS WINAPI NtQuerySystemTime( PLARGE_INTEGER Time )
+NTSTATUS WINAPI NtQuerySystemTime( LARGE_INTEGER *time )
 {
-    struct timeval now;
+#if defined(HAVE_CLOCK_GETTIME)
+    struct timespec ts;
+    static clockid_t clock_id = -1;

-    gettimeofday( &now, 0 );
-    Time->QuadPart = now.tv_sec * (ULONGLONG)TICKSPERSEC + TICKS_1601_TO_1970;
-    Time->QuadPart += now.tv_usec * 10;
+    if (clock_id == -1)
+    {
+        struct timespec res;
+
+        /* Use CLOCK_REALTIME_COARSE if it has 1 ms or better resolution */
+        if (!clock_getres( CLOCK_REALTIME_COARSE, &res ) && res.tv_sec == 0 && res.tv_nsec <= 1000000)
+            clock_id = CLOCK_REALTIME_COARSE;
+        else
+            clock_id = CLOCK_REALTIME;
+    }
+
+    if (!clock_gettime( clock_id, &ts ))
+    {
+        time->QuadPart = ts.tv_sec * (ULONGLONG)TICKSPERSEC + TICKS_1601_TO_1970;
+        time->QuadPart += (ts.tv_nsec + 50) / 100;
+    }
+    else
+#endif
+    {
+        struct timeval now;
+
+        gettimeofday( &now, 0 );
+        time->QuadPart = now.tv_sec * (ULONGLONG)TICKSPERSEC + TICKS_1601_TO_1970;
+        time->QuadPart += now.tv_usec * 10;
+    }
     return STATUS_SUCCESS;
 }