Silly experiments with netisr

[Sean Bruno]

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Some questions came up around the office and we ended up doing some
quite silly things with lo0 and netcat.

If one runs a continuous netcat on localhost to another netcat listener
on localhost that writes the output to /dev/null, netisr gets super busy
doing stuff/things.

E.g.
  -- listener running "nc -k -l 10000 > /dev/null"
  -  sender running in a while loop "nc -N localhost 10000 <
/var/tmp/testfile"

Interesting things start happening on the machine.  top -SH shows netisr
eating up about 1/2 of a cpu core.  If you drop the MTU on lo0 to 1500
(so that it looks like something in the real world), netisr will peg out
a cpu core.  This seems logical, in that smaller MTU means busier
netisr.  Its interesting though.

Looking at some pmcstat things, shows that the system is busilly
chugging along in tcp_do_segment().  I wonder if this is meaningful in
anyway or just "interesting".

PMC: [FR_RETIRED_X86_INSTRUCTIONS] Samples: 267614 (100.0%) , 12350
unresolved

%SAMP IMAGE      FUNCTION             CALLERS
  5.5 kernel     in_cksumdata         in_cksum_skip
  5.0 kernel     tcp_output           tcp_do_segment:4.2 tcp_usr_rcvd:0.5
  4.6 kernel     __rw_wlock_hard      tcp_usr_send:3.7 tcp_usr_rcvd:0.8
  3.8 pf.ko      pf_test              pf_check_in:2.0 pf_check_out:1.8
  3.6 kernel     sched_idletd         fork_exit
  3.2 pf.ko      pf_test_state_tcp    pf_test
  3.1 kernel     bzero                pf_test:0.8 pf_test_state_tcp:0.7
  3.1 kernel     bcopy                m_copydata:1.3 tcp_addoptions:0.7
tcp_dooptions:0.5
  2.7 kernel     tcp_do_segment       tcp_input


sean
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