Re: Prevent auto scan in wireless devices

[Marc Herbert <Marc Herbert gmail com>]

Dan Williams <dcbw redhat com> writes:

> 1) Passive scanning on any recent wifi chipset does not consume
> significantly more power than normal TX operation

OK, so I made some more power comparisons between ifupdown and
NetworkManager. Theories are good but nothing replaces actual
experience.

With ifupdown, my laptop draws about 9.3W +- 0.1W, as reported by
powertop querying ACPI. Switching the wireless interface to connected,
disconnected, rf_kill-ed, rmmod-ed, has almost no measurable effect on
this.

With NetworkManager, my laptop draws about 10W when I am connected,
and about 10.2W (!) when I disable wireless. I assume this is all
because of background scanning, about which I could not care less.

I am afraid I will have to give up NetworkManager for the
moment. Maybe I will give it another try the next time I will upgrade
my distribution. And I will come prepared this time :-)


> 2) If the user is not actively using the wifi connection, there is no
> reason to keep the wifi chip powered on  That's it. 

Well, that does not solve my almost +1W power problem when I _am_
actively using it. And I unfortunately do not see this rf_kill
happening yet when I am not connected.


> Keep clear, delineated lines between your
> components and your system will be simpler and more reliable.
> [...]
> Because NetworkManager is not a power saving daemon.  As explained
> above, power saving needs to be system wide and NetworkManager should
> not be talking to a bunch of other components to get power state.  If we
> did, we'd have to write new code each time somebody wrote a new power
> daemon.  Instead, NetworkManager provides a rich D-Bus interface that
> things like power daemons can talk to to get network state and figure
> out what exactly they need to do to implement their power saving policy.

This sound like great architectural design, but:
- as said above, this does not seem to happen yet
- background scanning is a feature totally useless to me in any
  situation. The only time I need a scan is before connecting, period.
- With my specific hardware, background scanning is a killer.  Once I
  get rid of background scanning I do not even need this super-duper
  power manager of the future to control my wireless card: my power
  issues are already solved.


Is background scanning really rooted deep down in the design of
NetworkManager? If yes, I am afraid I will have to give it up and
never come back to it. Too bad I have to go to the command line again.



>> > But at the end of the day, if you want to save power, you'll want to
>> > turn off the wifi chipset when the user isn't going to use it.

Well, not necessarily. With my hardware, turning off background scanning
seems enough to save power.


>> >> I prefer to loose 2 or 3 seconds to get the first list of available
>> >> APs than loose battery life during the X minutes (or hours) scanning
>> >> and keeping the wireless card on. 

Definitely yes.