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

[Jan Kiszka]

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On 2012-09-19 16:15, Peter Portante wrote:
> On Wed, Sep 19, 2012 at 3:44 AM, Jan Kiszka 
> <address@hidden<mailto:address@hidden>> wrote:
> 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 <address@hidden<mailto:address@hidden>>
>>> Cc: Jan Kiszka <address@hidden<mailto:address@hidden>>
>>> Signed-off-by: Anthony Liguori <address@hidden<mailto:address@hidden>>
>>> ---
>>> 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 am not sure the comparison is timerfd v. select, but timerfd v signal based 
> timer (setitimer). The timerfd path allows you to integrate with 
> select/poll/epoll loops, where as signal based timers make that more 
> difficult. One can do the same thing with signalfd, but only for one signal, 
> where as you can setup multiple timers at the expense of file descriptors.
> 
> Additionally, FWIW, select() has a resolution capped by its use of struct 
> timeval, which is microseconds, where timerfd_settime allows for nanosecond 
> resolution.

< 1µs resolution is pointless, even on RT-hardened kernels with fast
hardware underneath and when running natively.

> 
> 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.
> 
> With timerfd, signalfd and eventfd, Linux seems to have provided all the 
> coverage needed to make that happen.

The advantage is that timers based on select/poll timeouts will allow to
unify a lot of code for _all_ host platforms, i.e. even Windows. We
still need to evaluate the precise impact and look for potentially
missed limitations (aka: someone has to write patches and test them).
But if there are no relevant ones, it should be the better architecture.

That said, a timerfd based solution for Linux may be an intermediate
step of the select-based work takes longer.

Jan

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