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Tue, 28 Nov 2023 00:42:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51620 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231540AbjK1Fm3 (ORCPT ); Tue, 28 Nov 2023 00:42:29 -0500 Received: from mail-ej1-x62e.google.com (mail-ej1-x62e.google.com [IPv6:2a00:1450:4864:20::62e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC84CC4 for ; Mon, 27 Nov 2023 21:42:34 -0800 (PST) Received: by mail-ej1-x62e.google.com with SMTP id a640c23a62f3a-a02ba1f500fso719335866b.0 for ; Mon, 27 Nov 2023 21:42:34 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1701150153; x=1701754953; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=/q5Ao6kkaSp6ntRr5jYTJ0UF0vBlRA6OurkdKyp1VkQ=; b=aAMuUkOchABJYb6Q0bDMA7m8QZWvNRiYeVA3AQ8ex7sK8kM9KXGU2+FiKPl/wO7nCu bcM/Ronm0E1VOxP7Ft2vp/fZ2VYYRNPWPigLTUYOM8as7Fxuks9+gBdAlsVzWfu2fFvb EkhbxP4E3CyO6WKN4cxqGveDBgzHtC/YRXYhkj0ghJc0yNpFdb0sIRXKBaDV3ugp/u0l z0rCGx85CKsFzpJTwpxunoFirJ7XFxNogAhDKTIlTK8f1eQfj35xPnHCUJnUYvrDa2aZ KTc+XbzqbVc6sTbycm/oqz3A8uMcjV9iItmLDAQPUfkEEcuFi88B4m2rp+nnS51Km86G AuVQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1701150153; x=1701754953; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=/q5Ao6kkaSp6ntRr5jYTJ0UF0vBlRA6OurkdKyp1VkQ=; b=OHZZI2nvIryqrTEDrmuIfFJbT+k71dQWcQ3jDWAuYudDlnHKIaw9Nhx5KoyteNK0eY OXatLjffqOP2TtmBV6r6IMzYRbLb8Z1BfKViqfumHCsFg0Z8JRRNVttmPATd5ZMsidWy VKG27xjQCu+BzU/X6YHKa0PWjWYUXhVys/BvgbLGmPDh5N7fuH2uEyCYJ6MpvQLBMTns NdHOkW4QZwoOCTzvnN69Y6Zwr6Xe9B2Q/UzDhgb/IWYrPcLGPUY416NJLWEVsF7Z+RTE tKFH7xIdMgOTXhfvLQH4pKzrzzlRBt+xRw7/uH3fwPlFv73laSvM7OusnBfUYzKBRG2t Ho5w== X-Gm-Message-State: AOJu0YzCZ6zythwObpQ0g8jrYijGrbyAP9CxJ21bK5fvsvewV2DpBAKV 9KSAMhnEpkor/u/pt27tjr9xc0FBB3To/BbCir59sg== X-Received: by 2002:a17:906:eb17:b0:a03:6fd8:f14b with SMTP id mb23-20020a170906eb1700b00a036fd8f14bmr10117355ejb.28.1701150153208; Mon, 27 Nov 2023 21:42:33 -0800 (PST) MIME-Version: 1.0 References: <87msv58068.fsf@yhuang6-desk2.ccr.corp.intel.com> <87h6l77wl5.fsf@yhuang6-desk2.ccr.corp.intel.com> <87bkbf7gz6.fsf@yhuang6-desk2.ccr.corp.intel.com> <87msuy5zuv.fsf@yhuang6-desk2.ccr.corp.intel.com> <87fs0q5xsq.fsf@yhuang6-desk2.ccr.corp.intel.com> <87bkbe5tha.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: <87bkbe5tha.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Yosry Ahmed Date: Mon, 27 Nov 2023 21:41:54 -0800 Message-ID: Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space To: "Huang, Ying" Cc: Minchan Kim , Chris Li , Michal Hocko , Liu Shixin , Yu Zhao , Andrew Morton , Sachin Sant , Johannes Weiner , Kefeng Wang , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-8.4 required=5.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on pete.vger.email Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (pete.vger.email [0.0.0.0]); Mon, 27 Nov 2023 21:42:50 -0800 (PST) On Mon, Nov 27, 2023 at 9:39=E2=80=AFPM Huang, Ying = wrote: > > Yosry Ahmed writes: > > > On Mon, Nov 27, 2023 at 8:05=E2=80=AFPM Huang, Ying wrote: > >> > >> Yosry Ahmed writes: > >> > >> > On Mon, Nov 27, 2023 at 7:21=E2=80=AFPM Huang, Ying wrote: > >> >> > >> >> Yosry Ahmed writes: > >> >> > >> >> > On Mon, Nov 27, 2023 at 1:32=E2=80=AFPM Minchan Kim wrote: > >> >> >> > >> >> >> On Mon, Nov 27, 2023 at 12:22:59AM -0800, Chris Li wrote: > >> >> >> > On Mon, Nov 27, 2023 at 12:14=E2=80=AFAM Huang, Ying wrote: > >> >> >> > > > I agree with Ying that anonymous pages typically have dif= ferent page > >> >> >> > > > access patterns than file pages, so we might want to treat= them > >> >> >> > > > differently to reclaim them effectively. > >> >> >> > > > One random idea: > >> >> >> > > > How about we put the anonymous page in a swap cache in a d= ifferent LRU > >> >> >> > > > than the rest of the anonymous pages. Then shrinking again= st those > >> >> >> > > > pages in the swap cache would be more effective.Instead of= having > >> >> >> > > > [anon, file] LRU, now we have [anon not in swap cache, ano= n in swap > >> >> >> > > > cache, file] LRU > >> >> >> > > > >> >> >> > > I don't think that it is necessary. The patch is only for a= special use > >> >> >> > > case. Where the swap device is used up while some pages are= in swap > >> >> >> > > cache. The patch will kill performance, but it is used to a= void OOM > >> >> >> > > only, not to improve performance. Per my understanding, we = will not use > >> >> >> > > up swap device space in most cases. This may be true for ZR= AM, but will > >> >> >> > > we keep pages in swap cache for long when we use ZRAM? > >> >> >> > > >> >> >> > I ask the question regarding how many pages can be freed by th= is patch > >> >> >> > in this email thread as well, but haven't got the answer from = the > >> >> >> > author yet. That is one important aspect to evaluate how valua= ble is > >> >> >> > that patch. > >> >> >> > >> >> >> Exactly. Since swap cache has different life time with page cach= e, they > >> >> >> would be usually dropped when pages are unmapped(unless they are= shared > >> >> >> with others but anon is usually exclusive private) so I wonder h= ow much > >> >> >> memory we can save. > >> >> > > >> >> > I think the point of this patch is not saving memory, but rather > >> >> > avoiding an OOM condition that will happen if we have no swap spa= ce > >> >> > left, but some pages left in the swap cache. Of course, the OOM > >> >> > avoidance will come at the cost of extra work in reclaim to swap = those > >> >> > pages out. > >> >> > > >> >> > The only case where I think this might be harmful is if there's p= lenty > >> >> > of pages to reclaim on the file LRU, and instead we opt to chase = down > >> >> > the few swap cache pages. So perhaps we can add a check to only s= et > >> >> > sc->swapcache_only if the number of pages in the swap cache is mo= re > >> >> > than the number of pages on the file LRU or similar? Just make su= re we > >> >> > don't chase the swapcache pages down if there's plenty to scan on= the > >> >> > file LRU? > >> >> > >> >> The swap cache pages can be divided to 3 groups. > >> >> > >> >> - group 1: pages have been written out, at the tail of inactive LRU= , but > >> >> not reclaimed yet. > >> >> > >> >> - group 2: pages have been written out, but were failed to be recla= imed > >> >> (e.g., were accessed before reclaiming) > >> >> > >> >> - group 3: pages have been swapped in, but were kept in swap cache.= The > >> >> pages may be in active LRU. > >> >> > >> >> The main target of the original patch should be group 1. And the p= ages > >> >> may be cheaper to reclaim than file pages. > >> >> > >> >> Group 2 are hard to be reclaimed if swap_count() isn't 0. > >> >> > >> >> Group 3 should be reclaimed in theory, but the overhead may be high= . > >> >> And we may need to reclaim the swap entries instead of pages if the= pages > >> >> are hot. But we can start to reclaim the swap entries before the s= wap > >> >> space is run out. > >> >> > >> >> So, if we can count group 1, we may use that as indicator to scan a= non > >> >> pages. And we may add code to reclaim group 3 earlier. > >> >> > >> > > >> > My point was not that reclaiming the pages in the swap cache is more > >> > expensive that reclaiming the pages in the file LRU. In a lot of > >> > cases, as you point out, the pages in the swap cache can just be > >> > dropped, so they may be as cheap or cheaper to reclaim than the page= s > >> > in the file LRU. > >> > > >> > My point was that scanning the anon LRU when swap space is exhausted > >> > to get to the pages in the swap cache may be much more expensive, > >> > because there may be a lot of pages on the anon LRU that are not in > >> > the swap cache, and hence are not reclaimable, unlike pages in the > >> > file LRU, which should mostly be reclaimable. > >> > > >> > So what I am saying is that maybe we should not do the effort of > >> > scanning the anon LRU in the swapcache_only case unless there aren't= a > >> > lot of pages to reclaim on the file LRU (relatively). For example, i= f > >> > we have a 100 pages in the swap cache out of 10000 pages in the anon > >> > LRU, and there are 10000 pages in the file LRU, it's probably not > >> > worth scanning the anon LRU. > >> > >> For group 1 pages, they are at the tail of the anon inactive LRU, so t= he > >> scan overhead is low too. For example, if number of group 1 pages is > >> 100, we just need to scan 100 pages to reclaim them. We can choose to > >> stop scanning when the number of the non-group-1 pages reached some > >> threshold. > >> > > > > We should still try to reclaim pages in groups 2 & 3 before OOMing > > though. Maybe the motivation for this patch is group 1, but I don't > > see why we should special case them. Pages in groups 2 & 3 should be > > roughly equally cheap to reclaim. They may have higher refault cost, > > but IIUC we should still try to reclaim them before OOMing. > > The scan cost of group 3 may be high, you may need to scan all anonymous > pages to identify them. The reclaim cost of group 2 may be high, it may > just cause trashing (shared pages that are accessed by just one > process). So I think that we can allow reclaim group 1 in all cases. > Try to reclaim swap entries for group 3 during normal LRU scanning after > more than half of swap space of limit is used. As a last resort before > OOM, try to reclaim group 2 and group 3. Or, limit scan count for group > 2 and group 3. It would be nice if this can be done auto-magically without having to keep track of the groups separately. > > BTW, in some situation, OOM is not the worst situation. For example, > trashing may kill interaction latency, while killing the memory hog (may > be caused by memory leak) saves system response time. I agree that in some situations OOMs are better than thrashing, it's not an easy problem.