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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768299334; bh=deRUAWIkgjHzXVV3cTmMvD4gr7oy47IkI84pINNcAqQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=JKchUiTqrBXmedCjGI0bAxn9puHSObsRiojxEBeoJJxbhHibVzSCf/lyzADGXCpoy n1RMGbqC6tGw97/p1n93+GR5kJpE3NTQL0qYksCYAT+ZIhKeCrm0zahbCGScUyxerz MqwJSCf7hocs3WwkqnwrrmqMej+7D4Y39Xsr581gkUSdzH5svy4pIlXoctvzEudqD2 hTQ1WLTwFbQMVmxxAG56nxOBnUq5/jWTiKaE3tUwqqGmDGh4YTNyVYz0hL8oKt6vyE 6WmhxjeMQPdzfVEmwd0FE+Hc29ax7w7YJSuKRQa9Ga2+mdi8NjeeFA9xCt9/+axWWf S2ssqlJLg1OXg== Message-ID: <7963534f-cce8-4330-8a67-3f31bd6b2166@kernel.org> Date: Tue, 13 Jan 2026 11:15:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism To: Li Zhe Cc: akpm@linux-foundation.org, ankur.a.arora@oracle.com, fvdl@google.com, joao.m.martins@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, mjguzik@gmail.com, muchun.song@linux.dev, osalvador@suse.de, raghavendra.kt@amd.com References: <20260113063736.29694-1-lizhe.67@bytedance.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 1B5AD180010 X-Rspamd-Server: rspam06 X-Stat-Signature: aktx5dsqxi91yzms39tb36y8mmtzxzds X-Rspam-User: X-HE-Tag: 1768299334-976832 X-HE-Meta: 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 3J/CTPWe PyyvMdEVtVpfKfkWKZjGXBYK5y+mlUbQ1RZuhS4PLMLZz4lQl3ZvZ4V9IvkwwDsGSu5DphlK0tz+s6AaGLNMEh5H1T3ImCUEdP3k0IWV7BT0W4NdJ4AyAPyMd1jqMXeMdCzSEjJB433jcNFjCYdgOMteq8yseSIxFFbK1RfB4F0yqhR5VKKzkra1BVKhgDFyv8wiOgJ24qvi0Vet6EGxixdqVEdkMC0HilwiYYfM8TM4uhX2KTiuvBhx1EXGExVYVEhpIxH4i5+oSMaPHq3bMz4hr5w== 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 1/13/26 07:37, Li Zhe wrote: > On Mon, 12 Jan 2026 20:52:12 +0100, david@kernel.org wrote: > >>> As for concern (4), I believe it is orthogonal to this patchset, and >>> the cover letter already contains a performance comparison that >>> demonstrates the additional benefit. >>> >>>> I did see some comments in [1] about QEMU supporting user-mode >>>> parallel zero-page operations; I'm just not sure what the current >>>> state of that support looks like, or what the corresponding benchmark >>>> numbers are. >>> >>> As noted above, QEMU already employs a parallel page-touch mechanism, >>> yet the elapsed time remains noticeable. I am not deeply familiar with >>> QEMU; please correct me if I am mistaken. >> >> I implemented some part of the parallel preallocation support in QEMU. >> >> With QEMU, you can specify the number of threads and even specify the >> NUMA-placement of these threads. So you can pretty much fine-tune that >> for an environment. >> >> You still pre-zero all hugetlb pages at VM startup time, just in >> parallel though. So you pay some price at APP startup time. > > Hi David, > > Thank you for the comprehensive explanation. > > You are absolutely correct: QEMU's parallel preallocation is performed > only during VM start-up. We submitted this patch series mainly > because we observed that, even with the existing parallel mechanism, > launching large-size VMs still incurs prohibitive delays. (Bringing up > a 2 TB VM still requires more than 40 seconds for zeroing) > >> If you know that you will run such a VM (or something else) later, you >> could pre-zero the memory from user space by using a hugetlb-backed file >> and supplying that to QEMU as memory backend for the VM. Then, you can >> start your VM without any pre-zeroing. >> >> I guess that approach should work universally. Of course, there are >> limitations, as you would have to know how much memory an app needs, and >> have a way to supply that memory in form of a file to that app. > > Regarding user-space pre-zeroing, I agree that it is feasible once the > VM's memory footprint is known. We evaluated this approach internally; > however, in production environments, it is almost impossible to predict > the exact amount of memory a VM will require. Of course, you could preallocate to the expected maximum and then truncate the file to the size you need :) -- Cheers David