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Fri, 19 Oct 2018 15:00:56 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserv0022.oracle.com (8.14.4/8.14.4) with ESMTP id w9JF0tY3018294 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 19 Oct 2018 15:00:56 GMT Received: from abhmp0011.oracle.com (abhmp0011.oracle.com [141.146.116.17]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id w9JF0rWe027937; Fri, 19 Oct 2018 15:00:53 GMT Received: from ca-dmjordan1.us.oracle.com (/10.211.9.48) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 19 Oct 2018 08:00:53 -0700 Date: Fri, 19 Oct 2018 08:00:53 -0700 From: Daniel Jordan To: Mel Gorman , Aaron Lu , Vlastimil Babka Cc: 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 , Daniel Jordan , Tariq Toukan , Jesper Dangaard Brouer Subject: Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed Message-ID: <20181019150053.iaubsdtcsi64mqb7@ca-dmjordan1.us.oracle.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181019085435.GR5819@techsingularity.net> User-Agent: NeoMutt/20180323-268-5a959c X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9050 signatures=668683 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=2 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=661 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1810190132 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.