From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 170D3C433DB for ; Tue, 16 Mar 2021 20:30:41 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 900EA64F73 for ; Tue, 16 Mar 2021 20:30:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 900EA64F73 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 1DF5E6B0036; Tue, 16 Mar 2021 16:30:40 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1B4956B006C; Tue, 16 Mar 2021 16:30:40 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 056656B006E; Tue, 16 Mar 2021 16:30:40 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0005.hostedemail.com [216.40.44.5]) by kanga.kvack.org (Postfix) with ESMTP id DFCE16B0036 for ; Tue, 16 Mar 2021 16:30:39 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 96DDE2494 for ; Tue, 16 Mar 2021 20:30:39 +0000 (UTC) X-FDA: 77926880598.04.1598875 Received: from mail-il1-f176.google.com (mail-il1-f176.google.com [209.85.166.176]) by imf16.hostedemail.com (Postfix) with ESMTP id 273998019143 for ; Tue, 16 Mar 2021 20:30:39 +0000 (UTC) Received: by mail-il1-f176.google.com with SMTP id r7so13986715ilb.0 for ; Tue, 16 Mar 2021 13:30:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=/c3T5C9/0/6gu9M2ypjdfLKBZClamiOtsmCTUreEKsA=; b=OmDimc8N673Wag+Yx1Wf9NQMO42Jz4Pr3b8j3CSUeW/Pu7A0fVMRLofNpib26uq5y5 3ooarYhRfIIrj2dZx/UCNFX6HVp1wEq6AJdClkjnHdh77GTEV9zSDyjZ2zwBG1E/295N jiJlqMeea96paZwIntvd1fjIC3uKQoMfhDQqH6+PCrZ79eVA9nAeAl9GU3q7gYTgPVH5 bl54JgS9mvVkhB+2YPfwvomujUChvx08ESL1K41gz4+Z4B0bLMSo3bjvlBrzWnK1nqjo 2Bo4opPofuB79lfl2TqCPiDeQPFR0UL1N8CKEUS/2zNJrE0Bg8ramlB5YHtKGMM8oTYc RZ9w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=/c3T5C9/0/6gu9M2ypjdfLKBZClamiOtsmCTUreEKsA=; b=Ruu0mMj9DckUQCwQewuKMltK12Pvqzby5CX7z2TaVwBXvTk7b6hlh7d6M/j5bGJhNm vSdliw8STWmOqhttwRYNzJ/UrpfFIg7LI8XvJFfwpFsT699kHfJMXs1/n00p01r4Jqvl FK9ivRye3mlpH90guCZ0qWNYbd5mM/5+3RVRHlwxei3/rZs5BtCYrUuv4f8AaH92fUFh hH/3mA+HAqTacX1wl3rPQ9tc1ZObvnHD7RFIYnQdaH/F+GQAxTO6j69uiWezmY09iCUN ho3YKTJ0JlXh1w9Xo48g02uQbHb2/w8MD32DNfLLlHeh3hvGpajG4Xjt/ZBP5/Q0CNkY XZIQ== X-Gm-Message-State: AOAM532aDo5Foqy3+Z+yY6BajQg+5QT53vwBxG9rXHpSob2xIAiM/Uzh WGlNrU2ZQZ9v0XbO0f50dcEiTw== X-Google-Smtp-Source: ABdhPJyYkTC4Mb8WYn+CFmJApuiYLuvsFUiv5pC+AS2yBXMSsCD2JgLZY6FdwHCKayGJuZFBmPhjWQ== X-Received: by 2002:a05:6e02:1be2:: with SMTP id y2mr5049431ilv.20.1615926638332; Tue, 16 Mar 2021 13:30:38 -0700 (PDT) Received: from google.com ([2620:15c:183:200:85db:6a0d:7a4d:5606]) by smtp.gmail.com with ESMTPSA id r3sm9889684ilq.42.2021.03.16.13.30.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Mar 2021 13:30:37 -0700 (PDT) Date: Tue, 16 Mar 2021 14:30:33 -0600 From: Yu Zhao To: Dave Hansen Cc: linux-mm@kvack.org, Alex Shi , Andrew Morton , Dave Hansen , Hillf Danton , Johannes Weiner , Joonsoo Kim , Matthew Wilcox , Mel Gorman , Michal Hocko , Roman Gushchin , Vlastimil Babka , Wei Yang , Yang Shi , Ying Huang , linux-kernel@vger.kernel.org, page-reclaim@google.com Subject: Re: [PATCH v1 00/14] Multigenerational LRU Message-ID: References: <20210313075747.3781593-1-yuzhao@google.com> <5f621dd6-4bbd-dbf7-8fa1-d63d9a5bfc16@intel.com> <7378f56e-4bc0-51d0-4a61-26aa6969c0de@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7378f56e-4bc0-51d0-4a61-26aa6969c0de@intel.com> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 273998019143 X-Stat-Signature: 8bpcyppstj4hkz5rdkifd6n5o3g5mna4 Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf16; identity=mailfrom; envelope-from=""; helo=mail-il1-f176.google.com; client-ip=209.85.166.176 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1615926639-831336 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 Tue, Mar 16, 2021 at 07:50:23AM -0700, Dave Hansen wrote: > On 3/15/21 7:24 PM, Yu Zhao wrote: > > On Mon, Mar 15, 2021 at 11:00:06AM -0700, Dave Hansen wrote: > >> How bad does this scanning get in the worst case if there's a lot of > >> sharing? > > > > Actually the improvement is larger when there is more sharing, i.e., > > higher map_count larger improvement. Let's assume we have a shmem > > page mapped by two processes. To reclaim this page, we need to make > > sure neither PTE from the two sets of page tables has the accessed > > bit. The current page reclaim uses the rmap, i.e., rmap_walk_file(). > > It first looks up the two VMAs (from the two processes mapping this > > shmem file) in the interval tree of this shmem file, then from each > > VMA, it goes through PGD/PUD/PMD to reach the PTE. The page can't be > > reclaimed if either of the PTEs has the accessed bit, therefore cost > > of the scanning is more than proportional to the number of accesses, > > when there is a lot sharing. > > > > Why this series makes it better? We track the usage of page tables. > > Specifically, we work alongside switch_mm(): if one of the processes > > above hasn't be scheduled since the last scan, we don't need to scan > > its page tables. So the cost is roughly proportional to the number of > > accesses, regardless of how many processes. And instead of scanning > > pages one by one, we do it in large batches. However, page tables can > > be very sparse -- this is not a problem for the rmap because it knows > > exactly where the PTEs are (by vma_address()). We only know ranges (by > > vma->vm_start/vm_end). This is where the accessed bit on non-leaf > > PMDs can be of help. > > That's an interesting argument. *But*, this pivoted into describing an > optimization. My takeaway from this is that large amounts of sharing > are probably only handled well if the processes doing the sharing are > not running constantly. > > > But I guess you are wondering what downsides are. Well, we haven't > > seen any (yet). We do have page cache (non-shmem) heavy workloads, > > but not at a scale large enough to make any statistically meaningful > > observations. We are very interested in working with anybody who has > > page cache (non-shmem) heavy workloads and is willing to try out this > > series. > > I would also be very interested to see some synthetic, worst-case > micros. Maybe take a few thousand processes with very sparse page > tables that all map some shared memory. They wake up long enough to > touch a few pages, then go back to sleep. > > What happens if we do that? I'm not saying this is a good workload or > that things must behave well, but I do find it interesting to watch the > worst case. It is a reasonable request, thank you. I've just opened a bug to cover this case (a large sparse shared shmem) and we'll have something soon. > I think it would also be very worthwhile to include some research in > this series about why the kernel moved away from page table scanning. > What has changed? Are the workloads we were concerned about way back > then not around any more? Has faster I/O or larger memory sizes with a > stagnating page size changed something? Sure. Hugh also suggested this too but I personally found that ancient pre-2.4 history too irrelevant (and uninteresting) to the modern age and decided to spare audience of the boredom. > >> I'm kinda surprised by this, but my 16GB laptop has a lot more page > >> cache than I would have guessed: > >> > >>> Active(anon): 4065088 kB > >>> Inactive(anon): 3981928 kB > >>> Active(file): 2260580 kB > >>> Inactive(file): 3738096 kB > >>> AnonPages: 6624776 kB > >>> Mapped: 692036 kB > >>> Shmem: 776276 kB > >> > >> Most of it isn't mapped, but it's far from all being used for text. > > > > We have categorized two groups: > > 1) average users that haven't experienced memory pressure since > > their systems have booted. The booting process fills up page cache > > with one-off file pages, and they remain until users experience > > memory pressure. This can be confirmed by looking at those counters > > of a freshly rebooted and idle system. My guess this is the case for > > your laptop. > > It's been up ~12 days. There is ~10GB of data in swap, and there's been > a lot of scanning activity which I would associate with memory pressure: > > > SwapCached: 1187596 kB > > SwapTotal: 51199996 kB > > SwapFree: 40419428 kB > ... > > nr_vmscan_write 24900719 > > nr_vmscan_immediate_reclaim 115535 > > pgscan_kswapd 320831544 > > pgscan_direct 23396383 > > pgscan_direct_throttle 0 > > pgscan_anon 127491077 > > pgscan_file 216736850 > > slabs_scanned 400469680 > > compact_migrate_scanned 1092813949 > > compact_free_scanned 4919523035 > > compact_daemon_migrate_scanned 2372223 > > compact_daemon_free_scanned 20989310 > > unevictable_pgs_scanned 307388545 10G swap + 8G anon rss + 6G file rss, hmm... an interesting workload. The file rss does seem a bit high to me, my wild speculation is there have been git/make activities in addition to a VM?