Re: dhclient sucks cpu usage...

From: Alexander V. Chernikov <melifaro_at_FreeBSD.org>
Date: Tue, 10 Jun 2014 13:17:53 +0400
On 10.06.2014 07:03, Bryan Venteicher wrote:
> Hi,
>
> ----- Original Message -----
>> So, after finding out that nc has a stupidly small buffer size (2k
>> even though there is space for 16k), I was still not getting as good
>> as performance using nc between machines, so I decided to generate some
>> flame graphs to try to identify issues...  (Thanks to who included a
>> full set of modules, including dtraceall on memstick!)
>>
>> So, the first one is:
>> https://www.funkthat.com/~jmg/em.stack.svg
>>
>> As I was browsing around, the em_handle_que was consuming quite a bit
>> of cpu usage for only doing ~50MB/sec over gige..  Running top -SH shows
>> me that the taskqueue for em was consuming about 50% cpu...  Also pretty
>> high for only 50MB/sec...  Looking closer, you'll see that bpf_mtap is
>> consuming ~3.18% (under ether_nh_input)..  I know I'm not running tcpdump
>> or anything, but I think dhclient uses bpf to be able to inject packets
>> and listen in on them, so I kill off dhclient, and instantly, the taskqueue
>> thread for em drops down to 40% CPU... (transfer rate only marginally
>> improves, if it does)
>>
>> I decide to run another flame graph w/o dhclient running:
>> https://www.funkthat.com/~jmg/em.stack.nodhclient.svg
>>
>> and now _rxeof drops from 17.22% to 11.94%, pretty significant...
>>
>> So, if you care about performance, don't run dhclient...
>>
> Yes, I've noticed the same issue. It can absolutely kill performance
> in a VM guest. It is much more pronounced on only some of my systems,
> and I hadn't tracked it down yet. I wonder if this is fallout from
> the callout work, or if there was some bpf change.
>
> I've been using the kludgey workaround patch below.
Hm, pretty interesting.
dhclient should setup proper filter (and it looks like it does so:
13:10 [0] m_at_ptichko s netstat -B
   Pid  Netif   Flags      Recv      Drop     Match Sblen Hblen Command
  1224    em0 -ifs--l  41225922         0        11     0     0 dhclient
)
see "match" count.
And BPF itself adds the cost of read rwlock (+ bgp_filter() calls for 
each consumer on interface).
It should not introduce significant performance penalties.

>
> diff --git a/sys/net/bpf.c b/sys/net/bpf.c
> index cb3ed27..9751986 100644
> --- a/sys/net/bpf.c
> +++ b/sys/net/bpf.c
> _at__at_ -2013,9 +2013,11 _at__at_ bpf_gettime(struct bintime *bt, int tstype, struct mbuf *m)
>   			return (BPF_TSTAMP_EXTERN);
>   		}
>   	}
> +#if 0
>   	if (quality == BPF_TSTAMP_NORMAL)
>   		binuptime(bt);
>   	else
> +#endif
bpf_getttime() is called IFF packet filter matches some traffic.
Can you show your "netstat -B" output ?
>   		getbinuptime(bt);
>   
>   	return (quality);
>
>
>> --
>>    John-Mark Gurney				Voice: +1 415 225 5579
>>
>>       "All that I will do, has been done, All that I have, has not."
>> _______________________________________________
>> freebsd-current_at_freebsd.org mailing list
>> http://lists.freebsd.org/mailman/listinfo/freebsd-current
>> To unsubscribe, send any mail to "freebsd-current-unsubscribe_at_freebsd.org"
>>
> _______________________________________________
> freebsd-net_at_freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-net
> To unsubscribe, send any mail to "freebsd-net-unsubscribe_at_freebsd.org"
>
Received on Tue Jun 10 2014 - 07:19:52 UTC

This archive was generated by hypermail 2.4.0 : Wed May 19 2021 - 11:40:49 UTC