Re: Why is throughput so bad with lots of virtual stations?

[Christian Lamparter <chunkeey@googlemail.com>]

On Saturday 31 March 2012 05:56:30 Ben Greear wrote:
> I notice that when I get, say, 200 virtual stations on a radio (ath9k),
> the total throughput becomes about 100 packets per second.  It can run
> several thousands of packets per second with 10 or so stations.
is this tcp or udp "throughput"?

Anyway, I don't know much about your AP. But If it's mac80211 based
then you might also run into a problem with ath9k vs mac80211 for
every sta_info_get_bss [used by ieee80211_find_sta et. al] call.
You see, the station hash table uses the last byte of a station's
MAC as the "HASH" [see STA_HASH]. So for 00:11:22:33:44:55, the hash
is "55". Now, ath9k uses a mac mask for it's VIFs as well, only it 
starts from the other direction [i.e.:] xy:11:22:33:44:55. 
[e.g.: vif1: 00:11:22:33:44:55, vif2: 04:11:22:33:44:55,
vif3: 0c:11:22:33:44:55, ...]

So, mac80211's sta hash table really becomes a "long list". Which has
to be traversed by the driver and stack several times. 

> According to the xmit debugfs file, the radio is basically idle.
> It's queues are empty almost all of the time, few retransmits, etc.
> 
> This is on the 3.0.26 kernel.
> 
> If anyone has any ideas of what might need work, please let me know.
> 
> I'll start digging into the code to see what I can see.

Regards,
	Chr