ath / 802.11n performance issues and timer code

[Adrian Chadd]

On 24 September 2011 05:38, Alexander Motin <mav at freebsd.org> wrote:

>> When I set kern.eventtimer.periodic=1, the 11n TX/RX performance
>> suddenly jumps to where it should be.
>>
>> Would you mind helping me figure out what the problem is?
>
> I would be glad to help, but at this moment I am not sure how network
> traffic related to timer. May be wireless has some specifics, but for
> wired adapters traffic processing happens on network interrupts.

Well, here the interrupt handler just sets up some deferred tasks to
run via taskqueue_schedule().
This isn't the only driver which does this - I think the gige/10gige
NICs also do this.

>> I didn't think kern.eventtimer.periodic was needed?
>
> It should not be needed.
>
> Have you tried to set kern.eventtimer.idletick=1 instead?

I just tested it - it has the same effect.

> kern.eventtimer.periodic=1 on UP system effectively includes
> kern.eventtimer.idletick=1. kern.eventtimer.idletick=0 may somewhat
> increase interrupts overhead due to need to reprogram timer before
> context switch, but under high interrupt rate (about few KHz) kernel
> should dynamically switch to "quick" mode skipping it.

The clock rate interrupt difference is quite startling - at 150mbit
11n bridging (from wlan0 -> arge0 wired) the clock interrupt rate is
around 300/sec for idletick=0, and about 1150/sec for idletick=1.

The other thing to keep in mind is that the wlreless NIC isn't
interrupting per RX or TX completed packet. So although I'm doing ~
19,000 pps, it's only interrupting me ~ 390 times a second.



Adrian