On Tue November 16 2010 18:37:43 Jouni Malinen wrote: > On Fri, Nov 12, 2010 at 12:00:35PM +0900, Bruno Randolf wrote: > > Extend nl80211 to report an exponential weighted moving average (EWMA) of > > the signal value. Since the signal value usually fluctuates between > > different packets, an average can be more useful than the value of the > > last packet. > > > > This uses the recently added generic EWMA library function. > > Isn't that generic EWMA library function making this much more CPU > intensive than necessary? I would assume the compiler is not able to > optimize the unsigned long division in ewma_add() when the weight value > is "hidden" in this way through ewma_init() call. While generic library > functions can be nice, it could be useful to have an option for using an > inline version of ewma_add that takes in avg->weight as one of the > arguments to allow cases like weight=8 here to be implemented using > shift right instead of unsigned long division especially when done on > hot path.. > > > @@ -1158,6 +1158,7 @@ ieee80211_rx_h_sta_process(struct ieee80211_rx_data > > *rx) > > > > sta->last_signal = status->signal; > > > > + ewma_add(&sta->avg_signal, -status->signal); > > This one here would be done for every frame received and by using a > function call that ends up doing a potentially expensive division. > > > @@ -241,6 +241,8 @@ struct sta_info *sta_info_alloc(struct > > ieee80211_sub_if_data *sdata, + ewma_init(&sta->avg_signal, 1000, 8); > > The divisor (weight) is set to 8 here which should allow for quite a bit > more efficient ewma_add implementation if it were to be done in a way > that makes it easier for the compiler to see what value is being used > (e.g., passing this 8 to ewma_add_inline()) to allow shift right to be > used.. > > > Or am I missing something here and we have a compiler that is actually > able to take care of this kind of optimizations by itself in the current > ewma library design? Or is the unsigned long division with the expected > weight values going to be fast enough to not justify caring about it > even on any of the embedded boards anyway? I understand that this could be more efficient, but if it matters or not - honestly, I don't know. Can a more knowledgeable person than me comment on this? bruno -- 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
