Johannes Berg <johannes@xxxxxxxxxxxxxxxx> writes: > On Sun, 2019-12-22 at 18:24 +0100, Toke Høiland-Jørgensen wrote: >> Didn't have a chance to >> do anything other than compile-test it yet, but wanted to get it out >> before the holidays (which I almost managed, since technically my >> holiday started two days ago)... > > Didn't help you much, I at least was already on vacation by then too > :P Yeah, well, I did say "almost". But at least it gave *me* peace of mind over the holidays :) >> @@ -1948,6 +1978,7 @@ void ieee80211_sta_update_pending_airtime(struct ieee80211_local *local, >> tx_pending, 0); >> } >> >> + > > nit, what's that doing here? :) Pining for the fjords? >> +#define IEEE80211_RECIPROCAL_DIVISOR 0x100000000ULL >> +#define IEEE80211_RECIPROCAL_SHIFT 32 > > Could we live with less precision and use 32-bit arithmetic only? That > might help 32-bit systems? > > This is basically a 32.32 (31.32 for signed) fixed point number, right? > So I guess I'm asking if we could live with 16.16 (or 15.16), or > similar. Hmm, not sure. For the per-station weights, probably; I expect that in most cases individual station weights won't be big enough to cause rounding. However, the weight sum is a different matter. We go above a 10% rounding error once that goes above 2^13, which is certainly not unrealistic. The worst-case error is 50% if the weight sum happens to land at 2^15+1. The impact of a rounding error ends up being that a station's next transmission is delayed longer than it should be. As long as the rounding error is constant (i.e., the same set of stations keeps being active), this should cancel out, I guess; but since stations tend to cycle between being active and not, I fear it could end up impacting fairness to a measurable degree. So IDK; we could say we'll live with this in the interest of performance? Or we could decide the performance hit is worth keeping precision? Or do a middle ground thing where we use 32-bit arithmetic for the per-station weights, but go to 64-bit for the weight sum? I don't really have a good grip on how much of a performance impact we're talking about here, so I'm not sure which I prefer... > I think overall this looks good. I guess you should subject it to some > testing since I can't. Heh, yeah, testing is definitely needed :) I'm hoping Yibo will take it for a spin. If not, I'll try to see if I can get my old testbed to work; but I seem to recall there being a hardware issue with it, and I don't have physical access anymore, so it may be beyond rescue... -Toke
