From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934630Ab0KQI1v (ORCPT ); Wed, 17 Nov 2010 03:27:51 -0500 Received: from mail30g.wh2.ocn.ne.jp ([220.111.41.239]:4109 "HELO mail30g.wh2.ocn.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S934603Ab0KQI1t (ORCPT ); Wed, 17 Nov 2010 03:27:49 -0500 From: Bruno Randolf To: Jouni Malinen Subject: Re: [PATCH v7 3/3] nl80211/mac80211: Report signal average Date: Wed, 17 Nov 2010 17:28:06 +0900 User-Agent: KMail/1.13.5 (Linux/2.6.35-22-generic; KDE/4.5.1; x86_64; ; ) Cc: linville@tuxdriver.com, randy.dunlap@oracle.com, peterz@infradead.org, blp@cs.stanford.edu, linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org, Lars_Ericsson@telia.com, stefanr@s5r6.in-berlin.de, kosaki.motohiro@jp.fujitsu.com, akpm@linux-foundation.org, kevin.granade@gmail.com References: <20101112024901.28522.21895.stgit@localhost6.localdomain6> <20101112030035.28522.75318.stgit@localhost6.localdomain6> <20101116093743.GA21872@jm.kir.nu> In-Reply-To: <20101116093743.GA21872@jm.kir.nu> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201011171728.06801.br1@einfach.org> X-SF-Loop: 1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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