Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let thetimekeeper sleep

[Andy Lutomirski]

On Wed, Dec 18, 2013 at 1:57 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Dec 18, 2013 at 01:53:18PM -0800, Andy Lutomirski wrote:
>> On Wed, Dec 18, 2013 at 1:49 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Wed, Dec 18, 2013 at 01:29:53PM -0800, Andy Lutomirski wrote:
>> >> On 12/18/2013 09:43 AM, Frederic Weisbecker wrote:
>> >> > On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote:
>> >> >> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote:
>> >> >>> So this is what this series brings, more details following:
>> >> >>>
>> >> >>> * Some code, naming and whitespace cleanups
>> >> >>>
>> >> >>> * Allow all CPUs outside the nohz_full range to handle the timekeeping
>> >> >>>   duty, not just CPU 0. Balancing the timekeeping duty should improve
>> >> >>>   powersavings.
>> >> >>
>> >> >> If the system just has one nohz_full cpu running, it will need another
>> >> >> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living.
>> >> >> From powersaving POV, that is not good compare to normal nohz idle.
>> >> >
>> >> > Sure, but everything has a tradeoff :)
>> >> >
>> >> > We could theoretically run with the timekeeper purely idle if the other
>> >> > CPU in full dynticks mode runs in userspace for a long while and seldom
>> >> > do syscalls and faults. Timekeeping could be updated on kernel/user
>> >> > boundaries in this case without much impact on performances.
>> >> >
>> >> > But then there is one strict condition for that: it can't read the timeofday
>> >> > through the vdso but only through a syscall.
>> >>
>> >> Where's your ambition? :)
>> >>
>> >> If the vdso timing functions could see that it's been too long since a
>> >> real timekeeping update, they could fall back to a syscall.  Otherwise,
>> >> they could using rdtsc or whatever is in use.
>> >
>> > One objection to that approach in the past has been that it injects
>> > avoidable latency into the worker CPUs.  I suppose that you could argue
>> > that the cache misses due to a timekeeping-CPU update are not free, but
>> > then again, the syscall is likely to also incur a few cache misses as
>> > well.
>> >
>> > I bet that the timekeeping-CPU approach wins, but it would be cool to
>> > see you prove me wrong.
>>
>> There's already some (very vague) discussion about having a scheduled
>> time at which the clock frequency and/or offset will change, and this
>> wouldn't be a huge departure from that.  The goal there is to avoid
>> waiting for timekeeping if vclock_gettime runs concurrently with an
>> update, but the same approach could apply here (albeit with one extra
>> branch).
>>
>> Anyway, syscalls aren't *that* expensive.
>
> Like I said, it would be cool to see you prove me wrong, but that will
> need to be with patches and performance results rather than rhetoric.

Fair enough.  Don't hold your breath.  (And certainly don't take this
as an objection to these patches.)

--Andy
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