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

[Ken Thomases]

On May 2, 2019, at 3:45 AM, Huw Davies <huw at codeweavers.com> wrote:
> 
> 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 <huw at codeweavers.com>
> ---
> 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;

Having clock_gettime() doesn't necessarily imply having CLOCK_REALTIME_COARSE.  It seems that baseline POSIX only requires CLOCK_REALTIME.

-Ken