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X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Hi Marcelo, On 2026-04-17 06:51, Marcelo Tosatti wrote: > Is this to improve boot time of very large guests ? It depends a bit on how you define "boot time". It won't make a difference if you measure the time it takes to create the VM until you start the vCPUs. It may even make a huge difference if you measure the time until a user can first log in. What it will do is reduce the time the guest spends on writing memory, mostly in the background, to zero out all pages it has gotten from the hypervisor. The main advantage (other than saving CPU cycles that are effectively wasted on an unneeded operation) is that if you don't pre-allocate all the VMs memory, then the VM won't touch all pages after boot, keeping the RSS down until the memory is actually used. That's for example useful if you overcommit/oversubscribe VM memory. Cheers, Florian