From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754318AbaKDPBH (ORCPT ); Tue, 4 Nov 2014 10:01:07 -0500 Received: from cantor2.suse.de ([195.135.220.15]:51786 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753641AbaKDPAl (ORCPT ); Tue, 4 Nov 2014 10:00:41 -0500 Date: Tue, 4 Nov 2014 16:00:39 +0100 From: Michal Hocko To: Johannes Weiner Cc: Kamezawa Hiroyuki , Andrew Morton , Vladimir Davydov , Tejun Heo , David Miller , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [patch 1/3] mm: embed the memcg pointer directly into struct page Message-ID: <20141104150039.GF22207@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141104140937.GA18602@phnom.home.cmpxchg.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? -- Michal Hocko SUSE Labs