Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755644Ab1FJARM (ORCPT ); Thu, 9 Jun 2011 20:17:12 -0400 Received: from smtp-out.google.com ([216.239.44.51]:37397 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752171Ab1FJARJ convert rfc822-to-8bit (ORCPT ); Thu, 9 Jun 2011 20:17:09 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=Cfzzp1nd2DPtjzq9/avoVZMtX6VRDtg2OOpP9e22f3s9se61cn/FHrycjNnBHgKOpA 2ZS6zOOndg0R9laikgGw== MIME-Version: 1.0 In-Reply-To: <20110609233154.GA26745@cmpxchg.org> References: <20110602075028.GB20630@cmpxchg.org> <20110602175142.GH28684@cmpxchg.org> <20110608153211.GB27827@cmpxchg.org> <20110609083503.GC11603@cmpxchg.org> <20110609183637.GC20333@cmpxchg.org> <20110609233154.GA26745@cmpxchg.org> Date: Thu, 9 Jun 2011 17:17:05 -0700 Message-ID: Subject: Re: [patch 0/8] mm: memcg naturalization -rc2 From: Ying Han To: Johannes Weiner Cc: Hiroyuki Kamezawa , KAMEZAWA Hiroyuki , Daisuke Nishimura , Balbir Singh , Michal Hocko , Andrew Morton , Rik van Riel , Minchan Kim , KOSAKI Motohiro , Mel Gorman , Greg Thelen , Michel Lespinasse , "linux-mm@kvack.org" , linux-kernel Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7729 Lines: 169 On Thu, Jun 9, 2011 at 4:31 PM, Johannes Weiner wrote: > On Thu, Jun 09, 2011 at 03:30:27PM -0700, Ying Han wrote: >> On Thu, Jun 9, 2011 at 11:36 AM, Johannes Weiner wrote: >> > On Thu, Jun 09, 2011 at 10:36:47AM -0700, Ying Han wrote: >> >> On Thu, Jun 9, 2011 at 1:35 AM, Johannes Weiner wrote: >> >> > On Wed, Jun 08, 2011 at 08:52:03PM -0700, Ying Han wrote: >> >> >> On Wed, Jun 8, 2011 at 8:32 AM, Johannes Weiner wrote: >> >> >> > I guess it would make much more sense to evaluate if reclaiming from >> >> >> > memcgs while there are others exceeding their soft limit is even a >> >> >> > problem. ?Otherwise this discussion is pretty pointless. >> >> >> >> >> >> AFAIK it is a problem since it changes the spec of kernel API >> >> >> memory.soft_limit_in_bytes. That value is set per-memcg which all the >> >> >> pages allocated above that are best effort and targeted to reclaim >> >> >> prior to others. >> >> > >> >> > That's not really true. ?Quoting the documentation: >> >> > >> >> > ? ?When the system detects memory contention or low memory, control groups >> >> > ? ?are pushed back to their soft limits. If the soft limit of each control >> >> > ? ?group is very high, they are pushed back as much as possible to make >> >> > ? ?sure that one control group does not starve the others of memory. >> >> > >> >> > I am language lawyering here, but I don't think it says it won't touch >> >> > other memcgs at all while there are memcgs exceeding their soft limit. >> >> >> >> Well... :) I would say that the documentation of soft_limit needs lots >> >> of work especially after lots of discussions we have after the LSF. >> >> >> >> The RFC i sent after our discussion has the following documentation, >> >> and I only cut & paste the content relevant to our conversation here: >> >> >> >> What is "soft_limit"? >> >> The "soft_limit was introduced in memcg to support over-committing the >> >> memory resource on the host. Each cgroup can be configured with >> >> "hard_limit", where it will be throttled or OOM killed by going over >> >> the limit. However, the allocation can go above the "soft_limit" as >> >> long as there is no memory contention. The "soft_limit" is the kernel >> >> mechanism for re-distributing spare memory resource among cgroups. >> >> >> >> What we have now? >> >> The current implementation of softlimit is based on per-zone RB tree, >> >> where only the cgroup exceeds the soft_limit the most being selected >> >> for reclaim. >> >> >> >> It makes less sense to only reclaim from one cgroup rather than >> >> reclaiming all cgroups based on calculated propotion. This is required >> >> for fairness. >> >> >> >> Proposed design: >> >> round-robin across the cgroups where they have memory allocated on the >> >> zone and also exceed the softlimit configured. >> >> >> >> there was a question on how to do zone balancing w/o global LRU. This >> >> could be solved by building another cgroup list per-zone, where we >> >> also link cgroups under their soft_limit. We won't scan the list >> >> unless the first list being exhausted and >> >> the free pages is still under the high_wmark. >> >> >> >> Since the per-zone memcg list design is being replaced by your >> >> patchset, some of the details doesn't apply. But the concept still >> >> remains where we would like to scan some memcgs first (above >> >> soft_limit) . >> > >> > I think the most important thing we wanted was to round-robin scan all >> > soft limit excessors instead of just the biggest one. ?I understood >> > this is the biggest fault with soft limits right now. >> > >> > We came up with maintaining a list of excessors, rather than a tree, >> > and from this particular implementation followed naturally that this >> > list is scanned BEFORE we look at other memcgs at all. >> > >> > This is a nice to have, but it was never the primary problem with the >> > soft limit implementation, as far as I understood. >> > >> >> > It would be a lie about the current code in the first place, which >> >> > does soft limit reclaim and then regular reclaim, no matter the >> >> > outcome of the soft limit reclaim cycle. ?It will go for the soft >> >> > limit first, but after an allocation under pressure the VM is likely >> >> > to have reclaimed from other memcgs as well. >> >> > >> >> > I saw your patch to fix that and break out of reclaim if soft limit >> >> > reclaim did enough. ?But this fix is not much newer than my changes. >> >> >> >> My soft_limit patch was developed in parallel with your patchset, and >> >> most of that wouldn't apply here. >> >> Is that what you are referring to? >> > >> > No, I meant that the current behaviour is old and we are only changing >> > it only now, so we are not really breaking backward compatibility. >> > >> >> > The second part of this is: >> >> > >> >> > ? ?Please note that soft limits is a best effort feature, it comes with >> >> > ? ?no guarantees, but it does its best to make sure that when memory is >> >> > ? ?heavily contended for, memory is allocated based on the soft limit >> >> > ? ?hints/setup. Currently soft limit based reclaim is setup such that >> >> > ? ?it gets invoked from balance_pgdat (kswapd). >> >> >> >> We had patch merged which add the soft_limit reclaim also in the global ttfp. >> >> >> >> memcg-add-the-soft_limit-reclaim-in-global-direct-reclaim.patch >> >> >> >> > It's not the pages-over-soft-limit that are best effort. ?It says that >> >> > it tries its best to take soft limits into account while reclaiming. >> >> Hmm. Both cases are true. The best effort pages I referring to means >> >> "the page above the soft_limit are targeted to reclaim first under >> >> memory contention" >> > >> > I really don't know where you are taking this from. ?That is neither >> > documented anywhere, nor is it the current behaviour. >> >> I got the email from andrew on may 27 and you were on the cc-ed :) >> Anyway, i just forwarded you that one. > > I wasn't asking about this patch at all... ?This is the conversation: > > Me: > >> >> > It's not the pages-over-soft-limit that are best effort. ?It says that >> >> > it tries its best to take soft limits into account while reclaiming. > > You: > >> >> Hmm. Both cases are true. The best effort pages I referring to means >> >> "the page above the soft_limit are targeted to reclaim first under >> >> memory contention" > > Me: > >> > I really don't know where you are taking this from. ?That is neither >> > documented anywhere, nor is it the current behaviour. > > And this is still my question. > > Current: scan up to all pages of the biggest soft limit offender, then > reclaim from random memcgs (because of the global LRU). agree. > > After my patch: scan all memcgs according to their size, with double > the pressure on those over their soft limit. agree. > > Please tell me exactly how my patch regresses existing behaviour, a > user interface, a documented feature, etc. > Ok, thank you for clarifying it. Now i understand what's the confusion here. I agree that your patch doesn't regress from what we have now currently. What i referred earlier was the improvement from the current design. So we were comparing to two targets. Please go ahead with your patch, and I don't have problem with that now. I will propose the soft_limit reclaim improvement as separate thread. Thanks --Ying > If you have an even better idea, please propose it. > -- 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/