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Re: [patch] clockevents: Reinstate the per cpu tick skew

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On 12/28/2011 6:17 AM, Mike Galbraith wrote:
> On Tue, 2011-12-27 at 10:20 +0100, Mike Galbraith wrote:
>> Quoting removal commit af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867 
>> Historically, Linux has tried to make the regular timer tick on
>> the various CPUs not happen at the same time, to avoid contention
>> on xtime_lock.
>> 
>> Nowadays, with the tickless kernel, this contention no longer
>> happens since time keeping and updating are done differently. In
>> addition, this skew is actually hurting power consumption in a
>> measurable way on many-core systems. End quote
> 
> Hm, nohz enabled, hogs burning up 60 of 64 cores.
> 
> 56.11%  [kernel]      [k] ktime_get 5.54%  [kernel]      [k]
> scheduler_tick 4.02%  [kernel]      [k] cpuacct_charge 3.78%
> [kernel]      [k] __rcu_pending 3.76%  [kernel]      [k]
> tick_sched_timer 3.42%  [kernel]      [k] native_write_msr_safe 
> 1.58%  [kernel]      [k] run_timer_softirq 1.28%  [kernel]      [k]
> __schedule 1.21%  [kernel]      [k] apic_timer_interrupt 1.07%
> [kernel]      [k] _raw_spin_lock 0.81%  [kernel]      [k]
> __switch_to 0.67%  [kernel]      [k] thread_return
> 
> Maybe skew-me wants to become a boot option?

this is 56% of kernel time.. of how much total time?

(and are you using a system where tsc/lapic can be used, or are you
using one of those boatanchors that need hpet?)

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