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Subject: Re: [RFC PATCH 00/16] 1GB THP support on x86_64
To:     Roman Gushchin <guro@fb.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org,
        Rik van Riel <riel@surriel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Shakeel Butt <shakeelb@google.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        David Nellans <dnellans@nvidia.com>,
        linux-kernel@vger.kernel.org
References: <20200902180628.4052244-1-zi.yan@sent.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <8e677ead-206d-08dd-d73e-569bd3803e3b@redhat.com>
Date:   Tue, 8 Sep 2020 13:57:16 +0200
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On 03.09.20 18:30, Roman Gushchin wrote:
> On Thu, Sep 03, 2020 at 05:23:00PM +0300, Kirill A. Shutemov wrote:
>> On Wed, Sep 02, 2020 at 02:06:12PM -0400, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>>
>>> Hi all,
>>>
>>> This patchset adds support for 1GB THP on x86_64. It is on top of
>>> v5.9-rc2-mmots-2020-08-25-21-13.
>>>
>>> 1GB THP is more flexible for reducing translation overhead and increasing the
>>> performance of applications with large memory footprint without application
>>> changes compared to hugetlb.
>>
>> This statement needs a lot of justification. I don't see 1GB THP as viable
>> for any workload. Opportunistic 1GB allocation is very questionable
>> strategy.
> 
> Hello, Kirill!
> 
> I share your skepticism about opportunistic 1 GB allocations, however it might be useful
> if backed by an madvise() annotations from userspace application. In this case,
> 1 GB THPs might be an alternative to 1 GB hugetlbfs pages, but with a more convenient
> interface.

I have concerns if we would silently use 1~GB THPs in most scenarios
where be would have used 2~MB THP. I'd appreciate a trigger to
explicitly enable that - MADV_HUGEPAGE is not sufficient because some
applications relying on that assume that the THP size will be 2~MB
(especially, if you want sparse, large VMAs).

E.g., read via man page

"This  feature  is  primarily  aimed at applications that use large
mappings of data and access large regions of that memory at a time
(e.g., virtualization systems such as QEMU).  It  can  very  easily
waste  memory (e.g., a 2 MB mapping that only ever accesses 1 byte will
result in 2 MB of wired memory instead of one 4 KB page)."


Having that said, I consider having 1~GB THP - similar to 512~MP THP on
arm64 - useless in most setup and I am not sure if it is worth the
trouble. Just use hugetlbfs for the handful of applications where it
makes sense.

-- 
Thanks,

David / dhildenb