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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 38614C433F5 for ; Fri, 18 Mar 2022 12:15:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6EDBB8D0002; Fri, 18 Mar 2022 08:15:51 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 69D4D8D0001; Fri, 18 Mar 2022 08:15:51 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 565148D0002; Fri, 18 Mar 2022 08:15:51 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0214.hostedemail.com [216.40.44.214]) by kanga.kvack.org (Postfix) with ESMTP id 435438D0001 for ; Fri, 18 Mar 2022 08:15:51 -0400 (EDT) Received: from smtpin28.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id E4AC8A0FA7 for ; Fri, 18 Mar 2022 12:15:50 +0000 (UTC) X-FDA: 79257403260.28.CC227B3 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf16.hostedemail.com (Postfix) with ESMTP id 3027B180023 for ; Fri, 18 Mar 2022 12:15:50 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 4C7C2B820FD; Fri, 18 Mar 2022 12:15:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 99A57C340E8; Fri, 18 Mar 2022 12:15:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1647605747; bh=apeeG6WHRgatQTif+1EPcuy7+7eZkIEix8HhadU5MZc=; h=From:To:Cc:Subject:Date:In-Reply-To:From; b=bGScCB0Xn69LYU84wjG6fNIR0WKtN3pDC/uAs8BxGSt85dKp+W/t+Nkuq6vZYY426 OE8DzopDzg5ogmjolQrR4JAXnHOHe3DsBwLhoJteKuFmE5LdcHD74W6+O04vfvtiA8 teeWJJyD1q1s3a7esrs/Q4XPvtCivFsqj4Gda6RNqriKtkrd86/0q6B1P77E2waQFb 5x3c42ObpRFegAKi8UKolYmT/wvskMpv69bVBe1dZc3zhID3sf5QV3M72q0cwVmkdm wD0NC/FHfk9FztZ4q4PvDR8KJJQduo8KertujFzAOzNijAGiGT9ehtw/6rVjNhw2I8 i3LXuf6VVzknw== From: sj@kernel.org To: Baolin Wang Cc: sj@kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/damon: Make the sampling more accurate Date: Fri, 18 Mar 2022 12:15:43 +0000 Message-Id: <20220318121543.26861-1-sj@kernel.org> X-Mailer: git-send-email 2.17.1 In-Reply-To: <6cb97421-ab4a-2520-2503-10fec548edd0@linux.alibaba.com> X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 3027B180023 X-Stat-Signature: 98ggd95o3eaem9bfepdicqgtsw61o6ni Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=bGScCB0X; dmarc=pass (policy=none) header.from=kernel.org; spf=pass (imf16.hostedemail.com: domain of sj@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=sj@kernel.org X-HE-Tag: 1647605750-565420 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 Fri, 18 Mar 2022 19:58:07 +0800 Baolin Wang wrote: > > > On 3/18/2022 6:49 PM, sj@kernel.org wrote: > > On Fri, 18 Mar 2022 18:01:19 +0800 Baolin Wang wrote: > > > >> > >> On 3/18/2022 5:40 PM, sj@kernel.org wrote: > >>> Hi Baolin, > >>> > >>> On Fri, 18 Mar 2022 17:23:13 +0800 Baolin Wang wrote: > >>> > >>>> When I try to sample the physical address with DAMON to migrate pages > >>>> on tiered memory system, I found it will demote some cold regions mistakenly. > >>>> Now we will choose an physical address in the region randomly, but if > >>>> its corresponding page is not an online LRU page, we will ignore the > >>>> accessing status in this cycle of sampling, and actually will be treated > >>>> as a non-accessed region. Suppose a region including some non-LRU pages, > >>>> it will be treated as a cold region with a high probability, and may be > >>>> merged with adjacent cold regions, but there are some pages may be > >>>> accessed we missed. > >>>> > >>>> So instead of ignoring the access status of this region if we did not find > >>>> a valid page according to current sampling address, we can use last valid > >>>> sampling address to help to make the sampling more accurate, then we can do > >>>> a better decision. > >>> > >>> Well... Offlined pages are also a valid part of the memory region, so treating > >>> those as not accessed and making the memory region containing the offlined > >>> pages looks colder seems legal to me. IOW, this approach could make memory > >>> regions containing many non-online-LRU pages as hot. > >> > >> IMO I don't think this is a problem, since if this region containing > >> many non-online-LRU pages is treated as hot, which means threre are aome > >> pages are hot, right? We can find them and promote them to fast memory > >> (or do other schemes). Meanwhile, for non-online-LRU pages, we can > >> filter them and do nothing for them, since we can not get a valid page > >> struct for them. > > > > For some of DAMOS actions that you mentioned, that could make sense. However, > > that wouldn't make much sense for some other cases, especially for manual > > DAMON-based access pattern profiling. > > I am not sure about this case, could you elaborate on how this can worse > the case you mentioned? For an example, let's suppose a user using DAMON to know the working set size of the system. And further suppose there is a region that containing many offlined pages and one online hot page. With this patch, once DAMON sampled the one hot page, the entire region will be reported as hot, though the other offlined pages has not accessed. As a result, the user will think the working set size is bigger than real. > > Like you said as below, we can split the regions to separate the hot > pages out of the hot regions containing some offline or non-lru pages, > that is also a benefit to improve the regions adjustment. > > > After all, we already have a mechanism for this case: adaptive regions > > adjustment (or, regions split/merge). That mechanism will eventually separate > > out hot oneline-LRU pages in the memory regions. Before the region is > > adjusted, reporting the whole region as hot looks like a right result to me. > > Of course, I admit that it could take too much time to converge to the optimal > > regions, and there are many rooms for improvement of the regions adjustment > > mechanism. I think we should pursue the direction (improving the regions > > adjustment mechanism). > > Yes, agree. > > > FYI, I have some rough ideas for improving the mechanism including partitioning > > regions into more than 2 sub-regions if we belive it is not making a good > > progress. Nevertheless, I'd like to first make a methodology for evaluating > > current accuracy. For that, I am planning to implement a page-granularity > > access monitoring. > > Great, I think the page-granularity monitoring will be more suitable for > tiered memory system, which can reduce redundant demotion and promotion. > However, I still concern the overhead if the monitoring is a > page-granularity, especially for a large memory size. Sure. It's main purpose for now is only to be compared with DAMON for evaluating DAMON's accuracy. Someone who has small-enough memory size of huge-enough CPU resource could use that for their product, of course. > Anyway, I'd like to help to test or review the new page-granularity > monitoring when you're ready to send out. Thanks. So glad to hear that and appreciate always for your help! Thanks, SJ