linux-kernel - Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

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Message-ID: <8d5fdb17-c670-4814-8f48-4b90062668fc@redhat.com>
Date:   Tue, 5 Dec 2023 18:21:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Barry Song <21cnbao@...il.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

On 04.12.23 11:20, Ryan Roberts wrote:
> Hi All,
> 
> A new week, a new version, a new name... This is v8 of a series to implement
> multi-size THP (mTHP) for anonymous memory (previously called "small-sized THP"
> and "large anonymous folios"). Matthew objected to "small huge" so hopefully
> this fares better.
> 
> The objective of this is to improve performance by allocating larger chunks of
> memory during anonymous page faults:
> 
> 1) Since SW (the kernel) is dealing with larger chunks of memory than base
>     pages, there are efficiency savings to be had; fewer page faults, batched PTE
>     and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
>     overhead. This should benefit all architectures.
> 2) Since we are now mapping physically contiguous chunks of memory, we can take
>     advantage of HW TLB compression techniques. A reduction in TLB pressure
>     speeds up kernel and user space. arm64 systems have 2 mechanisms to coalesce
>     TLB entries; "the contiguous bit" (architectural) and HPA (uarch).
> 
> This version changes the name and tidies up some of the kernel code and test
> code, based on feedback against v7 (see change log for details).
> 
> By default, the existing behaviour (and performance) is maintained. The user
> must explicitly enable multi-size THP to see the performance benefit. This is
> done via a new sysfs interface (as recommended by David Hildenbrand - thanks to
> David for the suggestion)! This interface is inspired by the existing
> per-hugepage-size sysfs interface used by hugetlb, provides full backwards
> compatibility with the existing PMD-size THP interface, and provides a base for
> future extensibility. See [8] for detailed discussion of the interface.
> 
> This series is based on mm-unstable (715b67adf4c8).

I took a look at the core pieces. Some things might want some smaller 
tweaks, but nothing that should stop this from having fun in 
mm-unstable, and replacing the smaller things as we move forward.

-- 
Cheers,

David / dhildenb