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

[David Hildenbrand (Red Hat)]

On 1/13/26 07:37, Li Zhe wrote:
> 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.

Of course, you could preallocate to the expected maximum and then 
truncate the file to the size you need :)

--
Cheers

David