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Date: Tue, 5 May 2015 15:00:41 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Eric Dumazet <eric.dumazet@...il.com>
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 Sat, 25 Apr 2015, Eric Dumazet wrote:
> 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
Is that with the static key patch applied?
Thanks,
tglx
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