linux-kernel - Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86

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Date:   Fri, 2 Oct 2020 09:50:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>, Zi Yan <ziy@...dia.com>
Cc:     linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64

>>> - huge page sizes controllable by the userspace?
>>
>> It might be good to allow advanced users to choose the page sizes, so they
>> have better control of their applications.
> 
> Could you elaborate more? Those advanced users can use hugetlb, right?
> They get a very good control over page size and pool preallocation etc.
> So they can get what they need - assuming there is enough memory.
> 

I am still not convinced that 1G THP (TGP :) ) are really what we want
to support. I can understand that there are some use cases that might
benefit from it, especially:

"I want a lot of memory, give me memory in any granularity you have, I
absolutely don't care - but of course, more TGP might be good for
performance." Say, you want a 5GB region, but only have a single 1GB
hugepage lying around. hugetlbfs allocation will fail.

But then, do we really want to optimize for such (very special?) use
cases via " 58 files changed, 2396 insertions(+), 460 deletions(-)" ?

I think gigantic pages are a sparse resource. Only selected applications
*really* depend on them and benefit from them. Let these special
applications handle it explicitly.

Can we have a summary of use cases that would really benefit from this
change?

-- 
Thanks,

David / dhildenb