hwpmc granularity and 6.4 network performance

Vadim Goncharov vadim_nuclight at mail.ru
Mon Nov 24 03:29:23 PST 2008


I've recently perfromed upgrade of busy production router from 6.2 to 6.4-PRE.
I have added two lines to my kernel config and did usual make buildkernel:

device          hwpmc                   # Driver (also a loadable module)
options         HWPMC_HOOKS             # Other necessary kernel hooks

After rebooting with new world and kernel, I've noticed that CPU load has
slightly increased (not measured, it is different every second anyway, as
users do not genereate steady traffic), and in top -S 'swi1: net' became
often in state *Giant, but it not used to do so on 6.2, while kernel config
did not changed much, and device polling is still used. What could happen
to this?

Another question, I've read "Sixty second HWPMC howto" and tried to find out
what exactly eats my CPU. BTW, that instruction did not apply exactly on my
machine, this is what I did:

# cd /tmp
# pmcstat -S instructions -O /tmp/sample.out
# pmcstat -R /tmp/sample.out -k /boot/kernel/kernel -g
# gprof /boot/kernel/kernel p4-instr-retired/kernel.gmon > kernel.gmon.result

Now in file kernel.gmon.result I see the following:

granularity: each sample hit covers 4 byte(s) for 0.00% of 692213.00 seconds

                                  called/total       parents 
index  %time    self descendents  called+self    name           index
                                  called/total       children

[1]     31.7 219129.00        0.00                 ipfw_chk [1]



Why does it show 0.00 in this column ?

On next listing, flat profile, I see more readable listing, but columns are
empty again:

granularity: each sample hit covers 4 byte(s) for 0.00% of 692213.00 seconds

  %   cumulative   self              self     total           
 time   seconds   seconds    calls  ms/call  ms/call  name    
 31.7  219129.00 219129.00                             ipfw_chk [1]
 10.4  291179.00 72050.00                             bcmp [2]
  6.1  333726.00 42547.00                             rn_match [3]
  2.7  352177.00 18451.00                             generic_bzero [4]
  2.4  368960.00 16783.00                             strncmp [5]

OK, I can conclude from this that I should optimize my ipfw ruleset, but
that's all. I know from sources that ipfw_chk() is a big function with a
bunch of 'case's in a large 'switch'. I want to know which parts of that
switch are executed more often. It says in listing that granularity is
4 bytes, I assume that it has a sample for each of 4-byte chunks of binary
code, so that it must have such information. My kernel is compiled with:

makeoptions     DEBUG=-g

so kgdb does know where are instructions for each line of source code.
How can I obtain this info from profiling? It also would be useful to know
which places do calls to that bcmp() and rn_match().

WBR, Vadim Goncharov. ICQ#166852181       mailto:vadim_nuclight at mail.ru
[Moderator of RU.ANTI-ECOLOGY][FreeBSD][http://antigreen.org][LJ:/nuclight]

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