mfi(4) IO performance regression, post 8.1

[Adrian Chadd]

Hm! A timer related bug?

I'll CC mav@ on this, as it was his commit (and work in his general area.)

I wonder what's going on - is it something to do with the two ACPI
calls inserted there, or is it something to do with the change in
event timer values?

mav? Any ideas?


Adrian

On 17 July 2012 13:39, Steve McCoy <smccoy at greatbaysoftware.com> wrote:

> Alright, I've finally narrowed it down to r209897, which only affects
> acpi_cpu_idle():
>
> --- stable/8/sys/dev/acpica/acpi_cpu.c  2010/06/23 17:04:42     209471
> +++ stable/8/sys/dev/acpica/acpi_cpu.c  2010/07/11 11:58:46     209897
> @@ -930,12 +930,16 @@
>
>      /*
>       * Execute HLT (or equivalent) and wait for an interrupt.  We can't
> -     * calculate the time spent in C1 since the place we wake up is an
> -     * ISR.  Assume we slept half of quantum and return.
> +     * precisely calculate the time spent in C1 since the place we wake up
> +     * is an ISR.  Assume we slept no more then half of quantum.
>       */
>      if (cx_next->type == ACPI_STATE_C1) {
> -       sc->cpu_prev_sleep = (sc->cpu_prev_sleep * 3 + 500000 / hz) / 4;
> +       AcpiHwRead(&start_time, &AcpiGbl_FADT.XPmTimerBlock);
>         acpi_cpu_c1();
> +       AcpiHwRead(&end_time, &AcpiGbl_FADT.XPmTimerBlock);
> +        end_time = acpi_TimerDelta(end_time, start_time);
> +       sc->cpu_prev_sleep = (sc->cpu_prev_sleep * 3 +
> +           min(PM_USEC(end_time), 500000 / hz)) / 4;
>         return;
>      }
>
> My current guess is that AcpiHwRead() is a problem on our hardware. It's an
> isolated change and, to my desperate eyes, the commit message implies that
> it isn't critical — Do you think we could buy ourselves some time by pulling
> it out of our version of the kernel? Or is this essential for correctness?
> Any thoughts are appreciated, thanks!
>
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