linux-kernel - Re: [PATCH] mm/hugetlb: optionally pre-zero hugetlb pages

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Message-ID: <ecb1c40c-45fe-4f8f-8831-5d2cb31f4c8e@redhat.com>
Date: Tue, 3 Dec 2024 13:17:10 +0100
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>, Frank van der Linden <fvdl@...gle.com>
Cc: Mateusz Guzik <mjguzik@...il.com>, linux-mm@...ck.org,
 akpm@...ux-foundation.org, Muchun Song <muchun.song@...ux.dev>,
 Miaohe Lin <linmiaohe@...wei.com>, Oscar Salvador <osalvador@...e.de>,
 Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: optionally pre-zero hugetlb pages

On 03.12.24 13:06, Michal Hocko wrote:
> On Mon 02-12-24 14:50:49, Frank van der Linden wrote:
>> On Mon, Dec 2, 2024 at 1:58 PM Mateusz Guzik <mjguzik@...il.com> wrote:
>>> Any games with "background zeroing" are notoriously crappy and I would
>>> argue one should exhaust other avenues before going there -- at the end
>>> of the day the cost of zeroing will have to get paid.
>>
>> I understand that the concept of background prezeroing has been, and
>> will be, met with some resistance. But, do you have any specific
>> concerns with the patch I posted? It's pretty well isolated from the
>> rest of the code, and optional.
> 
> The biggest concern I have is that the overhead is payed by everybody on
> the system - it is considered to be a system overhead regardless only
> part of the workload benefits from hugetlb pages. In other words the
> workload using those pages is not accounted for the use completely.
> 
> If the startup latency is a real problem is there a way to workaround
> that in the userspace by preallocating hugetlb pages ahead of time
> before those VMs are launched and hand over already pre-allocated pages?

In QEMU we support parallel preallocation, and even NUMA-aware 
preallocation, to reduce VM startup times with hugetlb.

We discussed improvements as referred to by Mateusz to further speed up 
preallocation, but for now parallel+NUMA-aware preallocation seems to do 
the trick.

-- 
Cheers,

David / dhildenb