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DM4PR12MB6446.namprd12.prod.outlook.com (2603:10b6:8:be::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7677.30; Mon, 17 Jun 2024 11:40:27 +0000 Received: from CH2PR12MB4262.namprd12.prod.outlook.com ([fe80::3bdb:bf3d:8bde:7870]) by CH2PR12MB4262.namprd12.prod.outlook.com ([fe80::3bdb:bf3d:8bde:7870%2]) with mapi id 15.20.7677.030; Mon, 17 Jun 2024 11:40:27 +0000 Message-ID: Date: Mon, 17 Jun 2024 17:10:18 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/5] Enhancements to Page Migration with Batch Offloading via DMA To: Matthew Wilcox Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bharata@amd.com, raghavendra.kodsarathimmappa@amd.com, Michael.Day@amd.com, dmaengine@vger.kernel.org, vkoul@kernel.org References: <20240614221525.19170-1-shivankg@amd.com> Content-Language: en-US From: "Garg, Shivank" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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 I8OjKrx0 /S+bCBrqE7+YQ+mqiZ/6sMXC9cWKJU/jCnakIF6aG6e112wjd8JJrkzlojCfb2Fa3vzqUol0ZPLB5D2MoEmKOF/paaf9UujTYx4XbRhYS31DovtRfYUGGD9H5KNZUO8Sr9lgew/I+A/OsTLm/PBIz6ZooxwFRcbiqqPt8Ug5QlpdzlAXVd0++fpoo9GlO8gKBTvXE9idRof7WBsKd/xV1jjGacjxfwmJG+Jmo4oB+/bIE9cQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000012, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Matthew, On 6/15/2024 9:32 AM, Matthew Wilcox wrote: > On Sat, Jun 15, 2024 at 03:45:20AM +0530, Shivank Garg wrote: > > You haven't measured the important thing though -- what's the cost > _to userspace_? When the CPU does the copy, the data is now > cache-hot in that CPU's cache. When the DMA engine does the copy, > it's not cache-hot in any CPU. > > Now, this may not be a big problem. I don't think we do anything to > ensure that the CPU that is going to access the folio in userspace > is the one which does the copy. > > But your methodology is wrong. You're right about importance of measuring the cost to userspace. I initially focused on analyzing the folio_copy overheads within migrate_pages to identify potential optimizations opportunities using DMA hardware accelerators. To address this, I'm planning extend my experiments to measure the cost to userspace specifically related to cache-hotness. This will involve the accessing the migrated pages after the migration process is complete, and measuring the resulting latency to read/write. This approach of DMA-offloading could possibly help in scenarios involving bulk data copying with workload size >> cache capacity or incurs a large shootdown overhead. The userspace cost analysis will provide a more comprehensive picture of page-migration using CPU v/s DMA-offloading. I appreciate your feedback. Shivank