dhclient sucks cpu usage...

[John-Mark Gurney]

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...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."