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[96.255.20.42]) by smtp.gmail.com with ESMTPSA id af79cd13be357-820c8ac45a3sm1040371885a.5.2025.09.16.15.02.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 16 Sep 2025 15:02:32 -0700 (PDT) Date: Tue, 16 Sep 2025 18:02:29 -0400 From: Gregory Price To: David Rientjes Cc: Matthew Wilcox , Bharata B Rao , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Jonathan.Cameron@huawei.com, dave.hansen@intel.com, hannes@cmpxchg.org, mgorman@techsingularity.net, mingo@redhat.com, peterz@infradead.org, raghavendra.kt@amd.com, riel@surriel.com, sj@kernel.org, weixugc@google.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, dave@stgolabs.net, nifan.cxl@gmail.com, xuezhengchu@huawei.com, yiannis@zptcorp.com, akpm@linux-foundation.org, david@redhat.com, byungchul@sk.com, kinseyho@google.com, joshua.hahnjy@gmail.com, yuanchu@google.com, balbirs@nvidia.com, alok.rathore@samsung.com Subject: Re: [RFC PATCH v2 0/8] mm: Hot page tracking and promotion infrastructure Message-ID: References: <20250910144653.212066-1-bharata@amd.com> <7e3e7327-9402-bb04-982e-0fb9419d1146@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7e3e7327-9402-bb04-982e-0fb9419d1146@google.com> On Tue, Sep 16, 2025 at 12:45:52PM -0700, David Rientjes wrote: > > I'm hoping to share some of this data in the coming months. > > > > I've yet to see any strong indication that a complex hotness/movement > > system is warranted (yet) - but that may simply be because we have > > local cards with no switching involved. So far LRU-based promotion and > > demotion has been sufficient. > > ... > > I've been pretty focused on the promotion story here rather than demotion > because of how responsive it needs to be. Harvesting the page table > accessed bits or waiting on a sliding window through NUMA Balancing (even > NUMAB=2) is not as responsive as needed for very fast promotion to top > tier memory, hence things like the CHMU (or PEBS or IBS etc). > I feel the need to throw out there that we need to set some kind of baseline for comparison that isn't simply comparing new hotness tracking stuff against "Doing Nothing". For example, if we assume MGLRU is the default, we probably want to compare against some kind of simplistic system that is essentially: if tier0 has bandwidth room, and if tier1 is bandwidth pressured, then promote a chunk from tier1 youngest generation LRU ("hottest") and demote a chunk from tier0 older LRU ("coldest") [if there's no space available]. Active bandwidth utilization numbers are still a little hard to come by, but a system like above could be implemented largely in userland with a few tweaks to reclaim. ~Gregory