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. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/