Date: Thu, 05 Feb 2015 11:03:03 -0800 From: Sean Bruno <sbruno@ignoranthack.me> To: FreeBSD Net <freebsd-net@freebsd.org> Subject: Silly experiments with netisr Message-ID: <54D3BE67.8060502@ignoranthack.me>
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQF8BAEBCgBmBQJU075kXxSAAAAAAC4AKGlzc3Vlci1mcHJAbm90YXRpb25zLm9w ZW5wZ3AuZmlmdGhob3JzZW1hbi5uZXRCQUFENDYzMkU3MTIxREU4RDIwOTk3REQx MjAxRUZDQTFFNzI3RTY0AAoJEBIB78oecn5kc4kH/02ttXzvapAG2PSML9Ml0Kwf XblpOHnrhUU8jsTauGhh8q4C94rb9hFDCzL4cEAI87QMXoBQHi9CWE0v/XdeR+8M ajpHlNyd78XbmIKOVksesYWzLbVFjC0A3emnkH4dUX1XD6tJoihVaUQVcrAbNm+I p6Z4yXrOXUP9UxBgkCSe5m3Y/K3vcmIPvFSnO/nN/2tckEh6+uuj1n3QyFXkUJJg 9erFanvDXr3nOyR6IWXIKxuy1yta32SpOPxywIl81qSBh1n/IOor41WqpzOnlNdM d0np+ZD/d+Z9OQJZnuJunCrV6Cv2EFKJe5qBzCdOjLj0KvpNDnFXyndWpeXyvgI=3D =3D+FSJ -----END PGP SIGNATURE-----
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