On Thu, May 14, 2009 at 11:54 AM, Dan Williams <dcbw@xxxxxxxxxx> wrote: > On Thu, 2009-05-14 at 10:52 -0700, Luis R. Rodriguez wrote: >> I'm told Network Manager scans every 60 seconds. When TX'ing or RX'ing >> a lot of data you will see a big dip in throughput and sometimes it >> may cause issues with some connections. Jouni pointed out a possible >> nice option here: split the scans per channel through time. Now with >> nl80211 this is possible but right now Network Manager uses wext >> through wpa_supplicant in many distributions and this won't change for >> a bit (maybe by the next major distribution releases?). Since we're >> stuck with wext for the current distribution releases I'd like to hear >> feedback on a possible nice solution. Should we simply cancel scan? > > Libertas libertas_tf (the mac80211 driver) ? Or the fullmac one? > splits scans up into 3 parts with a short return to the > operating channel between each part. There's nothing that requires > cfg80211 for that to work... We can do tricks in drivers but I'd like to see this handled in mac80211. > Something I've tossed around for a while is counting traffic on the > device and if its over a certain bitrate for a period of time, postpone > the scan for a while. But after a certain amount of time, there's going > to be a scan no matter what. Sure, makes sense. I take it the effort to make this more intelligent is part of the the roaming intelligence we need to enhance. > The problem here is that at any time an application (say, wifi location > app) could ask for the list of access points. If you don't scan > periodically, all APs other than your associated AP (and others on the > same channel) will gradually drop off because their beacons are > received. Hard to wifi position or get area statistics if there's only > one AP in the list. Makes sense. > Secondarily, scanning is a tradeoff between better roaming latency and > continuous high throughput. If you don't scan, you have no idea what's > around, and when you move and the current AP becomes marginal, you > *have* to take the hit no matter what, so you can scan and find a new AP > to associate with. True however I'm inclined to believe that generally when you are sending or receiving a lot of data you are stationary so roaming might not be that urgent. For 802.11p (vehicle stuff) I suppose this may be a bit different but I have yet to see this take off. > I would have though that the periodic scanning would be more of an > annoyance when doing VOIP or SSH other latency sensitive tasks, but when > just downloading a file, a few second drop in transfer rate gets lost in > the bucket in the grand scheme of things. Good point, how about we use pm_qos for disabling scan when we are sending a lot of data (whatever we define this to mean)? Then applications can just write to this pm_qos stuff and you won't see this. I forget when pm_qos was introduced though. Luis -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
