Device polling, kern.polling.burst_max and gig-e
steve at expertcity.com
Fri Jan 30 10:33:09 PST 2004
We have a 4.9-RELEASE-p1 box dedicated to some traffic analysis. It
monitors on two em interfaces: about 200,000 pps on one interface, and
180,000 pps on the other.
It's been dealing with that OK, but our traffic levels are increasing -
we reached over 240,000 pps on one interface last week. This made CPU
reach 100%, and some packets not get processed.
So, last night we enabled polling on the nics.
Initially, great result - CPU dropped from 82% load (45% system load due
to interupts) yesterday to 55% load today (12% in system), for same pps
load (about 300,000 pps total) at the time.
However, input errors went from 0 to about 1200 (oddly, it was 1200
every other second, and 0 for the seconds in-between.)
A bit of digging around led me to increase kern.polling.burst_max.
According to http://info.iet.unipi.it/~luigi/polling/, "The default
value is enough for a 100Mbit ethernet". I increased it gradually to
900, whcih has almost (but not entirely) eliminated the errors. Now the
errors are zero for most intervals, but every 10 or so intervals there
are between 10 and 100 input errors.
- does it make sense to leave the default at 150, in this day of gigabit
- is there a danger in increasing the burt_max? (My burst size goes
straight to the max of 900.)
- can it be increased more ?
- are there other variables that make sense to increase for gigabit?
Since I increased the burst max, I now have slowly incrementing
kern.polling.lost_polls - about 1 every 2 seconds. Anything to worry about?
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