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

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

Please turn of HTML in you mailer. It's very hard to parse your reply.

On 2012-09-19 16:15, Peter Portante wrote:
> On Wed, Sep 19, 2012 at 3:44 AM, Jan Kiszka <jan.kiszka@siemens.com<mailto:jan.kiszka@siemens.com>> 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 <pbonzini@redhat.com<mailto:pbonzini@redhat.com>>
>>> Cc: Jan Kiszka <jan.kiszka@siemens.com<mailto:jan.kiszka@siemens.com>>
>>> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com<mailto: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 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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