ath(4) irq and taskq cpu usage

[Sam Leffler]

Eric van Gyzen wrote:
> The irq and taskq for my ath(4) card often use excessive amounts of CPU
> time, even when my network is idle.  They are often above 10% and 15%,
> respectively; occasionally, they are as high as 27% and 44%.
> 
> The system is an AMD Athlon64 2800+ running FreeBSD 6.2-RELEASE i386
> with a custom kernel including the wlan_* stuff, ath, ath_hal, and
> ath_rate_sample.  It is a station using WPA2-PSK with AES-CCMP.  The
> access point is also a FreeBSD machine with an ath(4) card.
> 
> During periods of high CPU usage, the
> 
>     rx failed 'cuz of PHY err
>         OFDM timing
> 
> fields of the athstats output are increasing rather quickly.  For
> example, while CPU usage was 25% and 46%, the OFDM timing field was
> increasing by 43,000 per second.
> 
> Can anyone explain this?  Is it a sign of failing hardware?

It means you're seeing lots of noise in the environment.  The numbers
you cite are way too high (43K/sec is crazy) and the %cpu times see too
high for your processor but that's hard to evaluate.  You don't indicate
what your h/w is (mac+phy) revs but presumably it's old enough that PHY
errors are not counted in h/w but instead sent to the host as little
packets that must be processed.  If you actually use the radio you'll
see the error counts go down because the radio will be busy doing useful
work.

High phy error rates can also be caused by things like faulty antenna
connections and/or radio overload (i.e. sta and ap being too close
and/or using high power radios).

I can add a knob to the driver to turn off this stuff but then you will
likely see degraded performance as the PHY errors are used to tune the
baseband when there is noise and/or when the case temperature changes
(this can significantly affect radio operation).

	Sam