Re: [Qemu-devel] [PATCH] qemu-clock: add an alarm timer based on timerfd

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-09-19 09:26, Paolo Bonzini wrote:
> Il 18/09/2012 22:37, Anthony Liguori ha scritto:
>> Unfortunately, there's a lot of Windows code in qemu-timer.c and main-loop.c
>> right now otherwise the refactoring would be trivial.  I'll leave that for
>> another day.
>>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Jan Kiszka <jan.kiszka@siemens.com>
>> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>> ---
>> Please note, this is lightly tested.  Since this is such a fundamental change,
>> I'd like to do some performance analysis before committing but wanted to share
>> early.
> 
> Looks good.  I think Peter Portante tested something similar, and found no big
> difference between the two.  But it's a good thing and, in my opinion, for
> non-timerfd OSes we should simply adjust the select() timeout and not bother
> with signals.

What would be the advantage of timerfd over select? On Linux, both use
hrtimers (and low slack for RT processes). I'm starting to like the
select/WaitForMultipleObjects pattern as it would allow to consolidate
over basically two versions of timers and simplify the code.

Jan

> 
> I'm not sure if the same can be done for Windows, but I think it's possible as long
> as you keep the timeBeginPeriod/timeEndPeriod calls.  As a start, Stefan, can you
> check if the win32 timer works for you with the calls added?  Like this:
> 
> diff --git a/qemu-timer.c b/qemu-timer.c
> index c7a1551..721c769 100644
> --- a/qemu-timer.c
> +++ b/qemu-timer.c
> @@ -673,6 +673,10 @@ static int win32_start_timer(struct qemu_alarm_timer *t)
>      HANDLE hTimer;
>      BOOLEAN success;
>  
> +    timeGetDevCaps(&mm_tc, sizeof(mm_tc));
> +
> +    timeBeginPeriod(mm_tc.wPeriodMin);
> +
>      /* If you call ChangeTimerQueueTimer on a one-shot timer (its period
>         is zero) that has already expired, the timer is not updated.  Since
>         creating a new timer is relatively expensive, set a bogus one-hour
> @@ -688,6 +692,7 @@ static int win32_start_timer(struct qemu_alarm_timer *t)
>      if (!success) {
>          fprintf(stderr, "Failed to initialize win32 alarm timer: %ld\n",
>                  GetLastError());
> +        timeEndPeriod(mm_tc.wPeriodMin);
>          return -1;
>      }
>  
> @@ -702,6 +707,7 @@ static void win32_stop_timer(struct qemu_alarm_timer *t)
>      if (hTimer) {
>          DeleteTimerQueueTimer(NULL, hTimer, NULL);
>      }
> +    timeEndPeriod(mm_tc.wPeriodMin);
>  }
>  
>  static void win32_rearm_timer(struct qemu_alarm_timer *t,
> 
> Paolo
> 

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