Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754779AbbDNLRT (ORCPT ); Tue, 14 Apr 2015 07:17:19 -0400 Received: from ud10.udmedia.de ([194.117.254.50]:38372 "EHLO mail.ud10.udmedia.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752464AbbDNLRJ (ORCPT ); Tue, 14 Apr 2015 07:17:09 -0400 Date: Tue, 14 Apr 2015 13:17:06 +0200 From: Markus Trippelsdorf To: Ingo Molnar Cc: Linus Torvalds , "H. Peter Anvin" , Denys Vlasenko , "Paul E. McKenney" , Jason Low , Peter Zijlstra , Davidlohr Bueso , Tim Chen , Aswin Chandramouleeswaran , LKML , Borislav Petkov , Andy Lutomirski , Brian Gerst , Thomas Gleixner , Peter Zijlstra Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries Message-ID: <20150414111706.GC398@x4> References: <5527CD92.1080901@zytor.com> <20150411144135.GB31416@x4> <20150412101422.GA2862@gmail.com> <20150413162308.GB398@x4> <20150413172624.GC398@x4> <20150413190914.GA398@x4> <20150414053821.GA28321@gmail.com> <20150414082346.GB398@x4> <20150414091615.GA10345@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150414091615.GA10345@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3427 Lines: 81 On 2015.04.14 at 11:16 +0200, Ingo Molnar wrote: > > * Markus Trippelsdorf wrote: > > > On 2015.04.14 at 07:38 +0200, Ingo Molnar wrote: > > > > > > Just to make sure, could you please also apply the 3 alignment patches > > > attached below? There's a lot of noise from extra alignment. > > > > Here's an updated table: > > > > text data bss dec filename > > 8746230 970072 802816 10519118 ./vmlinux gcc-5 (lto) > > 9202488 978512 811008 10992008 ./vmlinux gcc-5 > > 8036915 970296 802816 9810027 ./vmlinux gcc-5 (lto -fno-guess-branch-probability) > > 8593615 978512 811008 10383135 ./vmlinux gcc-5 (-fno-guess-branch-probability) > > 8202614 970072 802816 9975502 ./vmlinux gcc-5 (lto + Ingo's patch) > > 8801016 978512 811008 10590536 ./vmlinux gcc-5 (Ingo's patch) > > 8733943 952088 798720 10484751 ./vmlinux gcc-5 (lto + -malign-data=abi) > > 9186105 958320 806912 10951337 ./vmlinux gcc-5 (-malign-data=abi) > > 8190327 952088 798720 9941135 ./vmlinux gcc-5 (lto + Ingo's patch + -malign-data=abi) > > 8784633 958320 806912 10549865 ./vmlinux gcc-5 (Ingo's patch + -malign-data=abi) > > > > For the "lto + Ingo's patch + -malign-data=abi" combination there is a > > 10% text size reduction. > > Lets rename "Ingo's patch" to "code_align=1". The interesting one > would be to compare: > > code_align=1 + -fno-guess-branch-probability > vs. > lto + code_align=1 + -fno-guess-branch-probability > > I'd expect LTO to still be in the 5% reduction range. Yes: text data bss dec 7886231 970296 802816 9659343 lto + code_align=1 + -fno-guess-branch-probability 8398284 978512 811008 10187804 code_align=1 + -fno-guess-branch-probability > > -malign-data is a new option for gcc-5 that controls how the > > compiler aligns variables. "abi" aligns variables according to psABI > > and give the tightest packing. > > I'm not so sure about that one, our data access patterns are usually a > lot more well thought out than our code alignment (which is really > mostly compiler controlled). It also gives limited savings: > > 9202488 vmlinux gcc-5 > 9186105 vmlinux gcc-5 (-malign-data=abi) > > Which is 0.1%. I've got a handful of options in that size range: > > + # Reduces vmlinux size by 0.25%: > + KBUILD_CFLAGS += -fno-caller-saves > + > + # Reduces vmlinux size by 1.10%: > + KBUILD_CFLAGS += -fno-inline-small-functions > + > + # Reduces vmlinux size by about 0.95%: > + KBUILD_CFLAGS += -fno-tree-ch > > but obviously they are more obscure and thus riskier. Find below an > updated "Ingo's combo patch". It gives more than 10% savings here on > x86 defconfig using gcc 4.9, without LTO. Well obviously, if you do not care about performance you can reduce the text size further and further. But what is interesting is to keep the performance up (or even increase it) and still reduce the text size. And that is what the "lto + Ingo's patch + -malign-data=abi" kernel hopefully achieves (, but it would need further benchmarking to confirm this claim). -- Markus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/