Subject: Re: [RFC PATCHv3 1/2] mac80211: Determine dynamic PS timeout based
 on ps-qos network latency
From: Johannes Berg <johannes@sipsolutions.net>
To: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Cc: "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>
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Date: Mon, 26 Apr 2010 14:11:51 +0200
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On Mon, 2010-04-26 at 15:04 +0300, Juuso Oikarinen wrote:

> > Well I would argue that if the user requires a certain network latency,
> > that should take precedence. The user is unlikely to be thinking in
> > terms of "I want my battery to last that long"; rather they want to last
> > it as long as possible given their quality of service demand
> > constraints?
> 
> Well, this is not entirely correct. In the situation I'm thinking about,
> the user (the user space network manager is acting on behalf of him)
> thinks exactly on the lines of "I want my battery to last that long, or
> longer." The device is in the users pocket, WLAN associated. He does not
> care about latency at all, so the network manager sets a large enough
> latency value that allows the WLAN subsystem to sacrifice all latency to
> just reduce power consumption.

But in that case you also don't care about latency, obviously. So you
can just say "don't care about latency", and get all the power savings
that are possible in such a situation.

Basically there's a trade-off here between power consumption and
latency. I'm arguing that one should control latency, and power
consumption follows "best effort", but you seem to also be arguing that
in some situations one should control power consumption, and latency
gets bad.

> > > The ps_timeout could be calculated based on the latency too, I guess.
> > > I'm just not aware of any simple formula to do this.
> > 
> > But you did just base it on that?
> 
> Yeah, sorry, I intended to say "based on beacon interval and latency."

Which might actually make sense, because if for instance required
latency >> beacon interval, there's not much gain from dynamic power
saving timeouts.

> > It just seems to me that you're putting the power consumption
> > requirements after the quality of service demands, which would seem
> > wrong?
> 
> I'm sorry, I don't understand this statement (literally). To argue
> anyway: I'm thinking I'm binding power consumption requirements together
> with QoS demands. :)

Yeah, but I have a feeling you're thinking about power consumption too
much. I understand that is a goal, but shouldn't the goal be stated as

"provide the lowest power consumption under the latency QoS
constraints"?

johannes