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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