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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

> 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.

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.

-- 
Cheers

David