Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism

[Li Zhe]

On Mon, 12 Jan 2026 20:52:12 +0100, david@xxxxxxxxxx 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.

Thanks,
Zhe