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[96.255.20.138]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8cb8d122906sm1023135085a.51.2026.02.24.10.49.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Feb 2026 10:49:23 -0800 (PST) Date: Tue, 24 Feb 2026 13:49:21 -0500 From: Gregory Price To: Joshua Hahn Cc: Michal Hocko , Johannes Weiner , Kaiyang Zhao , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Roman Gushchin , Shakeel Butt , Muchun Song , Waiman Long , Chen Ridong , Tejun Heo , Michal Koutny , Axel Rasmussen , Yuanchu Xie , Wei Xu , Qi Zheng , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [RFC PATCH 0/6] mm/memcontrol: Make memcg limits tier-aware Message-ID: References: <20260224161357.2622501-1-joshua.hahnjy@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260224161357.2622501-1-joshua.hahnjy@gmail.com> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 685B740012 X-Stat-Signature: e1nf8ia8731rm9ns8fm4p3a8fg7cj91p X-HE-Tag: 1771958965-768836 X-HE-Meta: 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 XhDwZszX iFicGnh0BVOrpZv9/M3eGz4DoM9AEkOI9BULF67ABcnpZh+PIKtL/aenXdMoa5Xj4EifykilOIySmopUG8oliCD4bQqSJEntazGbW5pdEGCNUXodNTdNWLxXreSjBuQldjKfkyZoTd6dxmPk/X2AANiAPRPKX+b/Ki7w7gMRKBGvW1xZXQTvqYhmIT2+Xtu65RU4EHztdGtc2ZxSpTusyWG1HQYa57qRCYLnlVURy55xiY65htE3uvqkGBTqBCYnYykoaE8t6PfD9ZbcpLBYi8gJ2/XKZNW/CXzg4BxdocpHEwHVotfAcK5xrVBtLHlNRkv/anUVa7opsef0SVyNzSg8arrgxw5zIagIeYAHSbEAIOnVTwQJeOS9pWk5VxeX5mxutPnrLkoM/ydHLffhq8+yLRA== 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: List-Subscribe: List-Unsubscribe: On Tue, Feb 24, 2026 at 08:13:56AM -0800, Joshua Hahn wrote: > ... snip ... Just injecting a few points here (disclosure: I have been in the development loop for this feature) > > > Otherwise promotions would make sure to that we have the most active > > memory in the top tier. > Yes / No. This makes the assumption that you always want this. Barring a minimum Quality of Service mechanism (as Joshua explains) this reduces the usefulness of a secondary tier of memory. Services will just prefer not to be deployed to these kinds of machines because the performance variance is too high. > > > Is this typical in real life configurations? > > I would say so. I think that the two examples above are realistic > scenarios that cloud providers and hyperscalers might face on tiered systems. > The answer is unequivocally yes. Lacking tier-awareness is actually a huge blocker for deploying mixed workloads on large, dense memory systems with multiple tiers (2+). Technically we're already at 4-ish tiers: DDR, CXL, ZSWAP, SWAP. We have zswap/swap controls in cgroups already, we just lack that same control for coherent memory tiers. This tries to use the existing nobs (max/high/low/min) to do what they already do - just proportionally. ~Gregory