From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f173.google.com (mail-wi0-f173.google.com [209.85.212.173]) by kanga.kvack.org (Postfix) with ESMTP id E23856B00B9 for ; Tue, 4 Nov 2014 12:47:05 -0500 (EST) Received: by mail-wi0-f173.google.com with SMTP id n3so10151036wiv.0 for ; Tue, 04 Nov 2014 09:47:05 -0800 (PST) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id yu10si1391532wjc.112.2014.11.04.09.47.04 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Nov 2014 09:47:04 -0800 (PST) Date: Tue, 4 Nov 2014 12:46:50 -0500 From: Johannes Weiner Subject: Re: [patch 1/3] mm: embed the memcg pointer directly into struct page Message-ID: <20141104174650.GA19061@phnom.home.cmpxchg.org> References: <1414898156-4741-1-git-send-email-hannes@cmpxchg.org> <54589017.9060604@jp.fujitsu.com> <20141104132701.GA18441@phnom.home.cmpxchg.org> <20141104134110.GD22207@dhcp22.suse.cz> <20141104140937.GA18602@phnom.home.cmpxchg.org> <20141104150039.GF22207@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141104150039.GF22207@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Kamezawa Hiroyuki , Andrew Morton , Vladimir Davydov , Tejun Heo , David Miller , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org On Tue, Nov 04, 2014 at 04:00:39PM +0100, Michal Hocko wrote: > On Tue 04-11-14 09:09:37, Johannes Weiner wrote: > > On Tue, Nov 04, 2014 at 02:41:10PM +0100, Michal Hocko wrote: > > > On Tue 04-11-14 08:27:01, Johannes Weiner wrote: > > > > From: Johannes Weiner > > > > Subject: [patch] mm: move page->mem_cgroup bad page handling into generic code fix > > > > > > > > Remove obsolete memory saving recommendations from the MEMCG Kconfig > > > > help text. > > > > > > The memory overhead is still there. So I do not think it is good to > > > remove the message altogether. The current overhead might be 4 or 8B > > > depending on the configuration. What about > > > " > > > Note that setting this option might increase fixed memory > > > overhead associated with each page descriptor in the system. > > > The memory overhead depends on the architecture and other > > > configuration options which have influence on the size and > > > alignment on the page descriptor (struct page). Namely > > > CONFIG_SLUB has a requirement for page alignment to two words > > > which in turn means that 64b systems might not see any memory > > > overhead as the additional data fits into alignment. On the > > > other hand 32b systems might see 8B memory overhead. > > > " > > > > What difference does it make whether this feature maybe costs an extra > > pointer per page or not? These texts are supposed to help decide with > > the selection, but this is not a "good to have, if affordable" type of > > runtime debugging option. You either need cgroup memory accounting > > and limiting or not. There is no possible trade-off to be had. > > If you are compiling the kernel for your specific usecase then it > is clear. You enable only what you really need/want. But if you are > providing a pre-built kernel and considering which features to enable > then an information about overhead might be useful. You can simply > disable the feature for memory restricted kernel flavors. > > > Slub and numa balancing don't mention this, either, simply because > > this cost is negligible or irrelevant when it comes to these knobs. > > I agree that the overhead seems negligible but does it hurt us to > mention it though? Yes, it's fairly misleading. What about the instructions it adds to the fault hotpaths? The additional memory footprint of each cgroup created? You're adding 9 lines to point out one specific cost aspect, when the entire feature is otherwise summed up in two lines. The per-page overhead of memcg is not exceptional or unexpected if you know what it does - which you should when you enable it, even as a distributor - and such a gross overrepresentation in the help text is more confusing than helpful. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org