linux-kernel - Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer

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Date:	Sat, 25 Apr 2015 11:37:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	viresh kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Miao <realmz6@...il.com>, shashim@...eaurora.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, cl@...nel.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate
 running timer

On Thu, 2015-04-23 at 14:45 +0200, Thomas Gleixner wrote:

> You definitely have a point from the high throughput networking
> perspective.
> 
> Though in a power optimizing scenario with minimal network traffic
> this might be the wrong decision. We have to gather data from the
> power maniacs whether this matters or not. The FULL_NO_HZ camp might
> be pretty unhappy about the above.

Sure, I understand.


To make this clear, here the profile on a moderately loaded TCP server,
pushing ~20Gbits of data. Most of TCP output is ACK clock driven (thus
from softirq context).

(using regular sendmsg() system calls, that why the
get_nohz_timer_target() is 'only' second in the profile, but add the
find_next_bit() to it and this is very close being at first position)



   PerfTop:    4712 irqs/sec  kernel:96.7%  exact:  0.0% [4000Hz cycles],  (all, 72 CPUs)
-----------------------------------------------------------------------------------------
    10.16%  [kernel]          [k] copy_user_enhanced_fast_string            
     5.66%  [kernel]          [k] get_nohz_timer_target                     
     5.59%  [kernel]          [k] _raw_spin_lock                            
     2.53%  [kernel]          [k] __netif_receive_skb_core                  
     2.27%  [kernel]          [k] find_next_bit                             
     1.90%  [kernel]          [k] tcp_ack                                   

Maybe a reasonable heuristic would be to
change /proc/sys/kernel/timer_migration default to 0 on hosts with more
than 32 cpus.

profile with timer_migration = 0

   PerfTop:    3656 irqs/sec  kernel:94.3%  exact:  0.0% [4000Hz cycles],  (all, 72 CPUs)
-----------------------------------------------------------------------------------------
    13.95%  [kernel]          [k] copy_user_enhanced_fast_string            
     4.65%  [kernel]          [k] _raw_spin_lock                            
     2.57%  [kernel]          [k] __netif_receive_skb_core                  
     2.33%  [kernel]          [k] tcp_ack               

Thanks.


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