From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 904A06B0003 for ; Sat, 20 Oct 2018 05:00:08 -0400 (EDT) Received: by mail-pf1-f200.google.com with SMTP id h76-v6so34393598pfd.10 for ; Sat, 20 Oct 2018 02:00:08 -0700 (PDT) Received: from mga05.intel.com (mga05.intel.com. [192.55.52.43]) by mx.google.com with ESMTPS id h185-v6si3782300pge.308.2018.10.20.02.00.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 20 Oct 2018 02:00:07 -0700 (PDT) Date: Sat, 20 Oct 2018 17:00:02 +0800 From: Aaron Lu Subject: Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed Message-ID: <20181020090002.GA13858@intel.com> 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> <20181019085435.GR5819@techsingularity.net> <20181019150053.iaubsdtcsi64mqb7@ca-dmjordan1.us.oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181019150053.iaubsdtcsi64mqb7@ca-dmjordan1.us.oracle.com> Sender: owner-linux-mm@kvack.org List-ID: To: Daniel Jordan Cc: Mel Gorman , Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Huang Ying , Dave Hansen , Kemi Wang , Tim Chen , Andi Kleen , Michal Hocko , Matthew Wilcox , Tariq Toukan , Jesper Dangaard Brouer On Fri, Oct 19, 2018 at 08:00:53AM -0700, Daniel Jordan wrote: > On Fri, Oct 19, 2018 at 09:54:35AM +0100, Mel Gorman wrote: > > 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. > > With the address space range idea, wouldn't the zone free_area require changes > too? I can't see how locking by address range could synchronize it as it > exists now otherwise, with per order/mt list heads. > > One idea is to further subdivide the free area according to how the locking > works and find some reasonable way to handle having to search for pages of a > given order/mt in multiple places. I plan to create one free_are per 'address space range'. The challenge will be how to quickly locate a free_area that has the required free page on allocation path. Other details like how big the address space range should be etc. will need to be explored with testing. I think this approach is worth a try because it wouldn't cause fragmentation.