Re: [PATCH] gettime: minimize integer division

[Jens Axboe <axboe@kernel.dk>]

On 2012-12-20 01:52, Sam Bradshaw wrote:
> 
> This patch generally converts a division to a subtraction in fio_gettime().
> 
> Shows ~1% better iops with synthetic benchmarking at roughly the same cpu 
> time spent in fio_gettime().
> 
> Signed-off-by: Sam Bradshaw <sbradshaw@micron.com>
> 
> diff --git a/gettime.c b/gettime.c
> index 248f146..e2a6241 100644
> --- a/gettime.c
> +++ b/gettime.c
> @@ -163,17 +163,23 @@ void fio_gettime(struct timeval *tp, void fio_unused *caller)
>  		}
>  #ifdef ARCH_HAVE_CPU_CLOCK
>  	case CS_CPUCLOCK: {
> -		unsigned long long usecs, t;
> +		unsigned long long usecs, t, delta = 0;
> 
>  		t = get_cpu_clock();
>  		if (tv && t < tv->last_cycles) {
>  			dprint(FD_TIME, "CPU clock going back in time\n");
>  			t = tv->last_cycles;
> -		} else if (tv)
> +		} else if (tv) {
> +			if (tv->last_tv_valid)
> +				delta = t - tv->last_cycles;
>  			tv->last_cycles = t;
> +		}
> 
>  		usecs = t / cycles_per_usec;
> -		tp->tv_sec = usecs / 1000000;
> +		if (delta > 1000000)
> +			tp->tv_sec = tv->last_tv.tv_sec;
> +		else
> +			tp->tv_sec = usecs / 1000000;

Shouldn't that be delta < 1000000? What am I missing? If the diff is
more than 1M usecs, then do the division. If not, we can reuse the
seconds from the last one.

-- 
Jens Axboe