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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by MN1PEPF000793FF.mail.protection.outlook.com (10.167.249.201) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.569.4 via Frontend Transport; Thu, 29 Oct 2026 09:35:02 +0000 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.53; Thu, 29 Oct 2026 04:35:02 -0500 Received: from [192.168.0.152] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.53 via Frontend Transport; Thu, 29 Oct 2026 04:34:56 -0500 Message-ID: <0676993f-c04c-4e4e-9f8d-1650c8fd6d6e@amd.com> Date: Thu, 29 Oct 2026 15:04:51 +0530 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/3] sched/numa: make free-space tiering promotion bypass configurable To: Li Zhe , , , , , , , , , , , , , , , , , CC: , , References: <20261028041253.74839-1-lizhe.67@bytedance.com> <20261028041253.74839-2-lizhe.67@bytedance.com> Content-Language: en-US From: Bharata B Rao In-Reply-To: <20261028041253.74839-2-lizhe.67@bytedance.com> Content-Type: text/plain; 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This preserves the > existing policy of filling available fast memory before relying on > hotness and rate-limit control. > > That is not always the desired policy. Some deployments may intentionally > keep part of a workload's memory on CXL-attached memory, for example to > preserve a desired DRAM/CXL capacity split or bandwidth distribution. With > the free-space bypass, short cold accesses and bursts of hot accesses can > refill the fast tier and pull the workload away from that distribution > whenever DRAM has free space. > > Redis testing exercises this policy issue. On a CXL system with node0 as > DRAM and node3 as slow-tier memory, a Redis test was run with > memory-tiering-only mode enabled. The test used a Redis dataset with 12 > million 4KB values. It reclaimed memory until about 85% of Redis > anonymous memory was on node3, used a Redis benchmark configuration with > a 10% hot-key working set, and then issued a short 256MB access burst to > cold keys outside that working set. The cold-key burst mainly touches > memory outside the hot working set, which is expected to remain mostly on > the slow tier. Can you please clarify if 85% of redis memory ended up on lower tier naturally due to overflow from toptier? I ask this question because, if it is a natural overflow/demotion, then it will be hard to find head room in the toptier to allow promotions that bypass hotness or rate limit checks." Regards, Bharata.