dhclient sucks cpu usage...

[Bryan Venteicher]

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.

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
@@ -2013,9 +2013,11 @@ 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
 		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."
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