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 X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6832BC3A59F for ; Mon, 26 Aug 2019 12:06:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3D2DA2184D for ; Mon, 26 Aug 2019 12:06:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1566821194; bh=ZXqKUrKoc3XvWsVOR9tThsSaHUCIG5VakBqZox5EhWU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=JRpYcVNme4w024eK2guIzags6nK4FJKHXggcBGxtejrXOTKmisSCfCxF1bM3GCsAn tGpVTDj34iZGHkYvjbe0KpZ/duaoJiMuctICynYSj+1Er6W75l6NRsZQynP9DqRPap FsoywMesSGNXgWQb4wNlIe3FJXph01sC/FfZhvz0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731579AbfHZMGd (ORCPT ); Mon, 26 Aug 2019 08:06:33 -0400 Received: from mx2.suse.de ([195.135.220.15]:43858 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726945AbfHZMGd (ORCPT ); Mon, 26 Aug 2019 08:06:33 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id ED07DAFCC; Mon, 26 Aug 2019 12:06:31 +0000 (UTC) Date: Mon, 26 Aug 2019 14:06:30 +0200 From: Michal Hocko To: Johannes Weiner Cc: Minchan Kim , Andrew Morton , LKML , linux-mm , Miguel de Dios , Wei Wang , Mel Gorman , Nicholas Piggin Subject: Re: [RFC PATCH] mm: drop mark_page_access from the unmap path Message-ID: <20190826120630.GI7538@dhcp22.suse.cz> References: <20190730123935.GB184615@google.com> <20190730125751.GS9330@dhcp22.suse.cz> <20190731054447.GB155569@google.com> <20190731072101.GX9330@dhcp22.suse.cz> <20190806105509.GA94582@google.com> <20190809124305.GQ18351@dhcp22.suse.cz> <20190809183424.GA22347@cmpxchg.org> <20190812080947.GA5117@dhcp22.suse.cz> <20190812150725.GA3684@cmpxchg.org> <20190813105143.GG17933@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190813105143.GG17933@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 13-08-19 12:51:43, Michal Hocko wrote: > On Mon 12-08-19 11:07:25, Johannes Weiner wrote: > > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote: [...] > > > > Maybe the refaults will be fine - but latency expectations around > > > > mapped page cache certainly are a lot higher than unmapped cache. > > > > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with > > > > the lock batching, I'd prefer that. > > > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case > > > but this is a kind of change that might have other subtle side effects. > > > E.g. will-it-scale has noticed a regression [1], likely because the > > > critical section is shorter and the overal throughput of the operation > > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose > > > much sleep over it normally but we have already seen real regressions > > > when the locking pattern has changed in the past so I would by a bit > > > cautious. > > > > I'm much more concerned about fundamentally changing the aging policy > > of mapped page cache then about the lock breaking scheme. With locking > > we worry about CPU effects; with aging we worry about additional IO. > > But the later is observable and debuggable little bit easier IMHO. > People are quite used to watch for major faults from my experience > as that is an easy metric to compare. > > > > As I've said, this RFC is mostly to open a discussion. I would really > > > like to weigh the overhead of mark_page_accessed and potential scenario > > > when refaults would be visible in practice. I can imagine that a short > > > lived statically linked applications have higher chance of being the > > > only user unlike libraries which are often being mapped via several > > > ptes. But the main problem to evaluate this is that there are many other > > > external factors to trigger the worst case. > > > > We can discuss the pros and cons, but ultimately we simply need to > > test it against real workloads to see if changing the promotion rules > > regresses the amount of paging we do in practice. > > Agreed. Do you see other option than to try it out and revert if we see > regressions? We would get a workload description which would be helpful > for future regression testing when touching this area. We can start > slower and keep it in linux-next for a release cycle to catch any > fallouts early. > > Thoughts? ping... -- Michal Hocko SUSE Labs