From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 3278D6B000C for ; Fri, 19 Oct 2018 04:54:39 -0400 (EDT) Received: by mail-ed1-f69.google.com with SMTP id w42-v6so20146401edd.0 for ; Fri, 19 Oct 2018 01:54:39 -0700 (PDT) Received: from outbound-smtp16.blacknight.com (outbound-smtp16.blacknight.com. [46.22.139.233]) by mx.google.com with ESMTPS id a18-v6si538727ejt.250.2018.10.19.01.54.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 19 Oct 2018 01:54:37 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail04.blacknight.ie [81.17.254.17]) by outbound-smtp16.blacknight.com (Postfix) with ESMTPS id 3F8FB1C226A for ; Fri, 19 Oct 2018 09:54:37 +0100 (IST) Date: Fri, 19 Oct 2018 09:54:35 +0100 From: Mel Gorman Subject: Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed Message-ID: <20181019085435.GR5819@techsingularity.net> References: <20181017063330.15384-1-aaron.lu@intel.com> <20181017063330.15384-3-aaron.lu@intel.com> <20181017104427.GJ5819@techsingularity.net> <20181017131059.GA9167@intel.com> <20181017135807.GL5819@techsingularity.net> <20181017145904.GC9167@intel.com> <20181018111632.GM5819@techsingularity.net> <20181019055703.GA2401@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20181019055703.GA2401@intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Aaron Lu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Huang Ying , Dave Hansen , Kemi Wang , Tim Chen , Andi Kleen , Michal Hocko , Vlastimil Babka , Matthew Wilcox , Daniel Jordan , Tariq Toukan , Jesper Dangaard Brouer On Fri, Oct 19, 2018 at 01:57:03PM +0800, Aaron Lu wrote: > > > > I don't think this is the right way of thinking about it because it's > > possible to have the system split in such a way so that the migration > > scanner only encounters unmovable pages before it meets the free scanner > > where unmerged buddies were in the higher portion of the address space. > > Yes it is possible unmerged pages are in the higher portion. > > My understanding is, when the two scanners meet, all unmerged pages will > be either used by the free scanner as migrate targets or sent to merge > by the migration scanner. > It's not guaranteed if the lower portion of the address space consisted entirely of pages that cannot migrate (because they are unmovable or because migration failed due to pins). It's actually a fundamental limitation of compaction that it can miss migration and compaction opportunities due to how the scanners are implemented. It was designed that way to avoid pageblocks being migrated unnecessarily back and forth but the downside is missed opportunities. > > You either need to keep unmerged buddies on a separate list or search > > the order-0 free list for merge candidates prior to compaction. > > > > > > It's needed to form them efficiently but excessive reclaim or writing 3 > > > > to drop_caches can also do it. Be careful of tying lazy buddy too > > > > closely to compaction. > > > > > > That's the current design of this patchset, do you see any immediate > > > problem of this? Is it that you are worried about high-order allocation > > > success rate using this design? > > > > I've pointed out what I see are the design flaws but yes, in general, I'm > > worried about the high order allocation success rate using this design, > > the reliance on compaction and the fact that the primary motivation is > > when THP is disabled. > > When THP is in use, zone lock contention is pretty much nowhere :-) > > I'll see what I can get with 'address space range' lock first and will > come back to 'lazy buddy' if it doesn't work out. Thank you and > Vlastimil for all the suggestions. My pleasure. -- Mel Gorman SUSE Labs