From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2F43EC433EF for ; Mon, 4 Apr 2022 09:36:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354629AbiDDJiq (ORCPT ); Mon, 4 Apr 2022 05:38:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42934 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1354650AbiDDJil (ORCPT ); Mon, 4 Apr 2022 05:38:41 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5B73D3BFBC; Mon, 4 Apr 2022 02:36:36 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 0F7091F37E; Mon, 4 Apr 2022 09:36:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1649064995; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=LpYoL6AgjzJZ4tpGv3/xyzDAd+WQcpBXKtpN+zEEZ6M=; b=URZHSoRiL8o7b+yMQLyfEwpcnDVBQxC25DaiTXuXyRDL0IVDsBLTG0R5+BoVe4h2RwHe+8 846rB9SKsGl/yDW1VzKHMBpdDywbpp7qqlqIszK75ScfGaGQ/nc4UbHX9VFyfjHiaxZssh AyAChWqakgt161TVz0sPrchh0q2dSMg= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id CC8C7A3B92; Mon, 4 Apr 2022 09:36:34 +0000 (UTC) Date: Mon, 4 Apr 2022 11:36:33 +0200 From: Michal Hocko To: Zhaoyang Huang Cc: Suren Baghdasaryan , "zhaoyang.huang" , Andrew Morton , Johannes Weiner , Vladimir Davydov , "open list:MEMORY MANAGEMENT" , LKML , cgroups mailinglist , Ke Wang Subject: Re: [RFC PATCH] cgroup: introduce dynamic protection for memcg Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 04-04-22 11:32:28, Michal Hocko wrote: > On Mon 04-04-22 17:23:43, Zhaoyang Huang wrote: > > On Mon, Apr 4, 2022 at 5:07 PM Zhaoyang Huang wrote: > > > > > > On Mon, Apr 4, 2022 at 4:51 PM Michal Hocko wrote: > > > > > > > > On Mon 04-04-22 10:33:58, Zhaoyang Huang wrote: > > > > [...] > > > > > > One thing that I don't understand in this approach is: why memory.low > > > > > > should depend on the system's memory pressure. It seems you want to > > > > > > allow a process to allocate more when memory pressure is high. That is > > > > > > very counter-intuitive to me. Could you please explain the underlying > > > > > > logic of why this is the right thing to do, without going into > > > > > > technical details? > > > > > What I want to achieve is make memory.low be positive correlation with > > > > > timing and negative to memory pressure, which means the protected > > > > > memcg should lower its protection(via lower memcg.low) for helping > > > > > system's memory pressure when it's high. > > > > > > > > I have to say this is still very confusing to me. The low limit is a > > > > protection against external (e.g. global) memory pressure. Decreasing > > > > the protection based on the external pressure sounds like it goes right > > > > against the purpose of the knob. I can see reasons to update protection > > > > based on refaults or other metrics from the userspace but I still do not > > > > see how this is a good auto-magic tuning done by the kernel. > > > > > > > > > The concept behind is memcg's > > > > > fault back of dropped memory is less important than system's latency > > > > > on high memory pressure. > > > > > > > > Can you give some specific examples? > > > For both of the above two comments, please refer to the latest test > > > result in Patchv2 I have sent. I prefer to name my change as focus > > > transfer under pressure as protected memcg is the focus when system's > > > memory pressure is low which will reclaim from root, this is not > > > against current design. However, when global memory pressure is high, > > > then the focus has to be changed to the whole system, because it > > > doesn't make sense to let the protected memcg out of everybody, it > > > can't > > > do anything when the system is trapped in the kernel with reclaiming work. > > Does it make more sense if I describe the change as memcg will be > > protect long as system pressure is under the threshold(partially > > coherent with current design) and will sacrifice the memcg if pressure > > is over the threshold(added change) > > No, not really. For one it is still really unclear why there should be any > difference in the semantic between global and external memory pressure > in general. The low limit is always a protection from the external > pressure. And what should be the actual threshold? Amount of the reclaim > performed, effectivness of the reclaim or what? Btw. you might want to have a look at http://lkml.kernel.org/r/20220331084151.2600229-1-yosryahmed@google.com where a new interface to allow pro-active memory reclaim is discussed. I think that this might turn out to be a better fit then an automagic kernel manipulation with a low limit. It will require a user agent to drive the reclaim though. -- Michal Hocko SUSE Labs