Silly experiments with netisr

[Sean Bruno]

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On 02/05/15 11:13, Adrian Chadd wrote:
> On 5 February 2015 at 11:03, Sean Bruno <sbruno at ignoranthack.me>
> wrote:
>> 
> 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
> 
>> UHm, on a recent intel, use CPU_CLK_UNHALTED instead, so you get
>> an idea of which instructions are spending the most time doing
>> "stuff". Some instructions are costlier than others (eg things
>> that cause memory bus stalls.)

Oh right.  Hrm, running this on an AMD system.  I should have thought
about that one.

> 
> %SAMP IMAGE      FUNCTION             CALLERS 5.5 kernel
> in_cksumdata         in_cksum_skip
> 
>> .. we're checksumming localhost tcp? :)
> 

Yeah, and the ipv6 checksumming cannot be turned off apparently.

% ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 1500
	options=600000<RXCSUM_IPV6,TXCSUM_IPV6>
	inet6 ::1 prefixlen 128
	inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
	inet 127.0.0.1 netmask 0xff000000
	nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
	groups: lo
% ifconfig lo0 -rxcsum6
ifconfig: -rxcsum6: Operation not supported


> 
> 
>> -adrian
> 
> 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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> 

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