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Howlett" , Zi Yan , Jonathan Corbet , Shuah Khan , Sean Christopherson , Paolo Bonzini , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory Date: Thu, 23 Apr 2026 17:26:24 -0700 Message-ID: <20260424002625.89857-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: 9gip3cctyythasuqnygsqosjzrkc47go X-Rspam-User: X-Rspamd-Queue-Id: 7B6FC4000C X-Rspamd-Server: rspam05 X-HE-Tag: 1776990395-323383 X-HE-Meta: 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 4CM2E1t7 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 23 Apr 2026 14:57:34 -0400 Peter Xu wrote: > On Thu, Apr 23, 2026 at 07:08:00PM +0100, Kiryl Shutsemau wrote: > > On Thu, Apr 23, 2026 at 10:50:06AM -0400, Peter Xu wrote: [...] > > > - Whether we have explored other approaches on page hotness tracking [...] > > DAMON is built around sampling. It is good for working set estimation, > > but I don't think it is directly useful for eviction decision. It can > > miss hot pages. LRU rotation will also loose info. > > Exactly. If we need to collect ACCESS bit (or anything similar) for > eviction accuracy pusrpose, IIUC we need per-page info, we can't estimate > by sampling. That's a fair argument. Nonetheless, there are some companies who use DAMON [1] for a similar eviction purpose on their products. Also, page level accuracy issue was indeed concerns from many people. DAMON therefore provides page level DAMOS filter [2]. The idea is finding a large region of cold pages in low overhead first, then do page level access recheck on page of the region using the filter, just before doing the eviction. DAMON-based memory tiering also uses it [3], to avoid wrongly promoting/demoting cold/hot pages in DAMON-claimed hot/cold regions. The evaluation result was not very bad, and a few more users reported positive test results. Also, DAMON can be used for page level monitoring [5] and open to changes for users. Actually a work [6] for making DAMON-based page level monitoring more lightweight is ongoing. I understand no one fits all and the decision is up to each user :) Nevertheless, I will be happy to help if you have any question or request for DAMON. [1] https://cdn.amazon.science/ee/a4/41ff11374f2f865e5e24de11bd17/resource-management-in-aurora-serverless.pdf [2] https://origin.kernel.org/doc/html/latest/mm/damon/design.html#filters [3] https://github.com/damonitor/damo/blob/next/scripts/mem_tier.sh#L40 [4] https://www.phoronix.com/news/DAMON-Self-Tuned-Memory-Tiering [5] https://origin.kernel.org/doc/html/latest/mm/damon/faq.html#can-i-simply-monitor-page-granularity [6] https://lore.kernel.org/20260423004211.7037-1-akinobu.mita@gmail.com Thanks, SJ [...]