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[195.135.220.15]) by mx.google.com with ESMTPS id h14si342505edk.382.2019.04.22.23.33.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Apr 2019 23:33:58 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz 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 ED2F8AD72; Tue, 23 Apr 2019 06:33:57 +0000 (UTC) Subject: Re: [PATCH] mm: Do not boost watermarks to avoid fragmentation for the DISCONTIG memory model To: Mel Gorman , Andrew Morton Cc: Mikulas Patocka , James Bottomley , linux-parisc@vger.kernel.org, linux-mm@kvack.org, LKML References: <20190419094335.GJ18914@techsingularity.net> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190419094335.GJ18914@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 4/19/19 11:43 AM, Mel Gorman wrote: > Mikulas Patocka reported that 1c30844d2dfe ("mm: reclaim small amounts > of memory when an external fragmentation event occurs") "broke" memory > management on parisc. The machine is not NUMA but the DISCONTIG model > creates three pgdats even though it's a UMA machine for the following > ranges > > 0) Start 0x0000000000000000 End 0x000000003fffffff Size 1024 MB > 1) Start 0x0000000100000000 End 0x00000001bfdfffff Size 3070 MB > 2) Start 0x0000004040000000 End 0x00000040ffffffff Size 3072 MB > > From his own report > > With the patch 1c30844d2, the kernel will incorrectly reclaim the > first zone when it fills up, ignoring the fact that there are two > completely free zones. Basiscally, it limits cache size to 1GiB. > > For example, if I run: > # dd if=/dev/sda of=/dev/null bs=1M count=2048 > > - with the proper kernel, there should be "Buffers - 2GiB" > when this command finishes. With the patch 1c30844d2, buffers > will consume just 1GiB or slightly more, because the kernel was > incorrectly reclaiming them. > > The page allocator and reclaim makes assumptions that pgdats really > represent NUMA nodes and zones represent ranges and makes decisions > on that basis. Watermark boosting for small pgdats leads to unexpected > results even though this would have behaved reasonably on SPARSEMEM. > > DISCONTIG is essentially deprecated and even parisc plans to move to > SPARSEMEM so there is no need to be fancy, this patch simply disables > watermark boosting by default on DISCONTIGMEM. > > Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") > Reported-by: Mikulas Patocka > Tested-by: Mikulas Patocka > Signed-off-by: Mel Gorman Acked-by: Vlastimil Babka > --- > Documentation/sysctl/vm.txt | 16 ++++++++-------- > mm/page_alloc.c | 13 +++++++++++++ > 2 files changed, 21 insertions(+), 8 deletions(-) > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt > index 6af24cdb25cc..3f13d8599337 100644 > --- a/Documentation/sysctl/vm.txt > +++ b/Documentation/sysctl/vm.txt > @@ -866,14 +866,14 @@ The intent is that compaction has less work to do in the future and to > increase the success rate of future high-order allocations such as SLUB > allocations, THP and hugetlbfs pages. > > -To make it sensible with respect to the watermark_scale_factor parameter, > -the unit is in fractions of 10,000. The default value of 15,000 means > -that up to 150% of the high watermark will be reclaimed in the event of > -a pageblock being mixed due to fragmentation. The level of reclaim is > -determined by the number of fragmentation events that occurred in the > -recent past. If this value is smaller than a pageblock then a pageblocks > -worth of pages will be reclaimed (e.g. 2MB on 64-bit x86). A boost factor > -of 0 will disable the feature. > +To make it sensible with respect to the watermark_scale_factor > +parameter, the unit is in fractions of 10,000. The default value of > +15,000 on !DISCONTIGMEM configurations means that up to 150% of the high > +watermark will be reclaimed in the event of a pageblock being mixed due > +to fragmentation. The level of reclaim is determined by the number of > +fragmentation events that occurred in the recent past. If this value is > +smaller than a pageblock then a pageblocks worth of pages will be reclaimed > +(e.g. 2MB on 64-bit x86). A boost factor of 0 will disable the feature. > > ============================================================= > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index cfaba3889fa2..86c3806f1070 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -266,7 +266,20 @@ compound_page_dtor * const compound_page_dtors[] = { > > int min_free_kbytes = 1024; > int user_min_free_kbytes = -1; > +#ifdef CONFIG_DISCONTIGMEM > +/* > + * DiscontigMem defines memory ranges as separate pg_data_t even if the ranges > + * are not on separate NUMA nodes. Functionally this works but with > + * watermark_boost_factor, it can reclaim prematurely as the ranges can be > + * quite small. By default, do not boost watermarks on discontigmem as in > + * many cases very high-order allocations like THP are likely to be > + * unsupported and the premature reclaim offsets the advantage of long-term > + * fragmentation avoidance. > + */ > +int watermark_boost_factor __read_mostly; > +#else > int watermark_boost_factor __read_mostly = 15000; > +#endif > int watermark_scale_factor = 10; > > static unsigned long nr_kernel_pages __initdata; >