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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