From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f171.google.com (mail-wi0-f171.google.com [209.85.212.171]) by kanga.kvack.org (Postfix) with ESMTP id 79E416B007D for ; Thu, 26 Jun 2014 11:12:07 -0400 (EDT) Received: by mail-wi0-f171.google.com with SMTP id n15so1263941wiw.4 for ; Thu, 26 Jun 2014 08:12:04 -0700 (PDT) Received: from zene.cmpxchg.org (zene.cmpxchg.org. [2a01:238:4224:fa00:ca1f:9ef3:caee:a2bd]) by mx.google.com with ESMTPS id bb8si33443582wib.1.2014.06.26.08.11.32 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 26 Jun 2014 08:11:32 -0700 (PDT) Date: Thu, 26 Jun 2014 11:11:11 -0400 From: Johannes Weiner Subject: Re: [PATCH 5/6] mm: page_alloc: Reduce cost of dirty zone balancing Message-ID: <20140626151111.GB30849@cmpxchg.org> References: <1403683129-10814-1-git-send-email-mgorman@suse.de> <1403683129-10814-6-git-send-email-mgorman@suse.de> <20140625163528.11368b86ef7d0a38cf9d1255@linux-foundation.org> <20140626084314.GE10819@suse.de> <20140626143738.GS7331@cmpxchg.org> <20140626145632.GG10819@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140626145632.GG10819@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Andrew Morton , Linux Kernel , Linux-MM , Linux-FSDevel , Jens Axboe , Jeff Moyer , Dave Chinner On Thu, Jun 26, 2014 at 03:56:32PM +0100, Mel Gorman wrote: > On Thu, Jun 26, 2014 at 10:37:38AM -0400, Johannes Weiner wrote: > > On Thu, Jun 26, 2014 at 09:43:14AM +0100, Mel Gorman wrote: > > > On Wed, Jun 25, 2014 at 04:35:28PM -0700, Andrew Morton wrote: > > > > On Wed, 25 Jun 2014 08:58:48 +0100 Mel Gorman wrote: > > > > > > > > > @@ -325,7 +321,14 @@ static unsigned long zone_dirty_limit(struct zone *zone) > > > > > */ > > > > > bool zone_dirty_ok(struct zone *zone) > > > > > { > > > > > - unsigned long limit = zone_dirty_limit(zone); > > > > > + unsigned long limit = zone->dirty_limit_cached; > > > > > + struct task_struct *tsk = current; > > > > > + > > > > > + if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { > > > > > + limit = zone_dirty_limit(zone); > > > > > + zone->dirty_limit_cached = limit; > > > > > + limit += limit / 4; > > > > > + } > > > > > > > > Could we get a comment in here explaining what we're doing and why > > > > PF_LESS_THROTTLE and rt_task control whether we do it? > > > > > > > > > > /* > > > * The dirty limits are lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) > > > * and real-time tasks to prioritise their allocations. > > > * PF_LESS_THROTTLE tasks may be cleaning memory and rt tasks may be > > > * blocking tasks that can clean pages. > > > */ > > > > > > That's fairly weak though. It would also seem reasonable to just delete > > > this check and allow PF_LESS_THROTTLE and rt_tasks to fall into the slow > > > path if dirty pages are already fairly distributed between zones. > > > Johannes, any objection to that limit raising logic being deleted? > > > > I copied that over from global_dirty_limits() such that the big > > picture and the per-zone picture have the same view - otherwise these > > tasks fall back to first fit zone allocations before global limits > > start throttling dirtiers and waking up the flushers. This increases > > the probability of reclaim running into dirty pages. > > > > Would you remove it from global_dirty_limits() as well? > > > > On that note, I don't really understand why global_dirty_limits() > > raises the *background* limit for less-throttle/rt tasks, shouldn't it > > only raise the dirty limit? Sure, the throttle point is somewhere > > between the two limits, but we don't really want to defer waking up > > the flushers for them. > > All of which is fair enough and is something that should be examined on > a rainy day (shouldn't take too long in Ireland). I'm not going to touch > it within this series though. It's outside the scope of what I'm trying > to do here -- restore performance of tiobench and bonnie++ to as close to > 3.0 levels as possible. The series is tripping up enough on the fair zone > and CFQ aspects as it is without increasing the scope :( You asked to remove it, I'm just asking follow-up questions ;-) I agree that this is out-of-scope for your patches and it probably should be left alone for now. However, I do like your comment and wouldn't mind including it in this change. Would everybody be okay with that? -- 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