Silly experiments with netisr

[hiren panchasara]

On 02/05/15 at 11:13P, Adrian Chadd wrote:
> On 5 February 2015 at 11:03, Sean Bruno <sbruno at ignoranthack.me> wrote:
> >
> > -----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
> 
> 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.)
> 
> > %SAMP IMAGE      FUNCTION             CALLERS
> >   5.5 kernel     in_cksumdata         in_cksum_skip
> 
> .. we're checksumming localhost tcp? :)

We can get rid of it with RXCSUM/TXCSUM flags on lo0 right? 

From man 4 lo

     If the transmit checksum offload capability flag is enabled on a loopback
     interface, checksums will not be generated by IP, UDP, or TCP for packets
     sent on the interface.

Or am I missing something?

cheers,
Hiren
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 618 bytes
Desc: not available
URL: <http://lists.freebsd.org/pipermail/freebsd-net/attachments/20150205/172f0af5/attachment.sig>