ACPI-fast default timecounter, but HPET 83% faster

[Garrett Cooper]

On Sun, Apr 26, 2009 at 4:50 PM, Pieter de Goeje <pieter at degoeje.nl> wrote:
> Dear hackers,
>
> While fiddling with the sysctl kern.timecounter.hardware, I found out that on
> my system HPET is significantly faster than ACPI-fast. Using the program
> below I measured the number of clock_gettime() calls the system can execute
> per second. I ran the program 10 times for each configuration and here are
> the results:
>
> x ACPI-fast
> + HPET
> +-------------------------------------------------------------------------+
> |x                                                                       +|
> |x                                                                       +|
> |x                                                                      ++|
> |x                                                                      ++|
> |x                                                                      ++|
> |x                                                                      ++|
> |A                                                                      |A|
> +-------------------------------------------------------------------------+
>    N           Min           Max        Median           Avg        Stddev
> x  10        822032        823752        823551      823397.8     509.43254
> +  10       1498348       1506862       1502830     1503267.4     2842.9779
> Difference at 95.0% confidence
>        679870 +/- 1918.94
>        82.5688% +/- 0.233052%
>        (Student's t, pooled s = 2042.31)
>
> System details: Intel(R) Core(TM)2 Duo CPU E6750  @ 2.66GHz (3200.02-MHz
> 686-class CPU), Gigabyte P35-DS3R motherboard running i386 -CURRENT updated
> today.
>
> Unfortunately I only have one system with a HPET timecounter, so I cannot
> verify these results on another system. If similar results are obtained on
> other machines, I think the HPET timecounter quality needs to be increased
> beyond that of ACPI-fast.
>
> Regards,
>
> Pieter de Goeje
>
> ----- 8< ----- clock_gettime.c ----- 8< ------
> #include <sys/time.h>
> #include <stdio.h>
> #include <time.h>
>
> #define COUNT 1000000
>
> int main() {
>        struct timespec ts_start, ts_stop, ts_read;
>        double time;
>        int i;
>
>        clock_gettime(CLOCK_MONOTONIC, &ts_start);
>        for(i = 0; i < COUNT; i++) {
>                clock_gettime(CLOCK_MONOTONIC, &ts_read);
>        }
>        clock_gettime(CLOCK_MONOTONIC, &ts_stop);
>
>        time = (ts_stop.tv_sec - ts_start.tv_sec) + (ts_stop.tv_nsec -
> ts_start.tv_nsec) * 1E-9;
>        printf("%.0f\n", COUNT / time);
> }

I'm seeing similar results.

[root at orangebox /usr/home/gcooper]# dmesg | grep 'Timecounter "'
Timecounter "i8254" frequency 1193182 Hz quality 0
Timecounter "ACPI-fast" frequency 3579545 Hz quality 1000
Timecounter "HPET" frequency 14318180 Hz quality 900
[root at orangebox /usr/home/gcooper]# ./cgt
1369355
[root at orangebox /usr/home/gcooper]# sysctl
kern.timecounter.hardware="ACPI-fast"
kern.timecounter.hardware: HPET -> ACPI-fast
[root at orangebox /usr/home/gcooper]# ./cgt
772289

Why's the default ACPI-fast? For power-saving functionality or because
of the `quality' factor? What is the criteria that determines the
`quality' of a clock as what's being reported above (I know what
determines the quality of a clock visually from a oscilloscope =])?

Thanks,
-Garrett