Date:   Wed, 5 Jul 2023 21:38:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 0/5] variable-order, large folios for anonymous memory

On 03.07.23 15:53, Ryan Roberts wrote:
> Hi All,
> 
> This is v2 of a series to implement variable order, large folios for anonymous
> memory. The objective of this is to improve performance by allocating larger
> chunks of memory during anonymous page faults. See [1] for background.
> 
> I've significantly reworked and simplified the patch set based on comments from
> Yu Zhao (thanks for all your feedback!). I've also renamed the feature to
> VARIABLE_THP, on Yu's advice.
> 
> The last patch is for arm64 to explicitly override the default
> arch_wants_pte_order() and is intended as an example. If this series is accepted
> I suggest taking the first 4 patches through the mm tree and the arm64 change
> could be handled through the arm64 tree separately. Neither has any build
> dependency on the other.
> 
> The one area where I haven't followed Yu's advice is in the determination of the
> size of folio to use. It was suggested that I have a single preferred large
> order, and if it doesn't fit in the VMA (due to exceeding VMA bounds, or there
> being existing overlapping populated PTEs, etc) then fallback immediately to
> order-0. It turned out that this approach caused a performance regression in the
> Speedometer benchmark. With my v1 patch, there were significant quantities of
> memory which could not be placed in the 64K bucket and were instead being
> allocated for the 32K and 16K buckets. With the proposed simplification, that
> memory ended up using the 4K bucket, so page faults increased by 2.75x compared
> to the v1 patch (although due to the 64K bucket, this number is still a bit
> lower than the baseline). So instead, I continue to calculate a folio order that
> is somewhere between the preferred order and 0. (See below for more details).
> 
> The patches are based on top of v6.4 plus Matthew Wilcox's set_ptes() series
> [2], which is a hard dependency. I have a branch at [3].
> 
> 
> Changes since v1 [1]
> --------------------
> 
>    - removed changes to arch-dependent vma_alloc_zeroed_movable_folio()
>    - replaced with arch-independent alloc_anon_folio()
>        - follows THP allocation approach
>    - no longer retry with intermediate orders if allocation fails
>        - fallback directly to order-0
>    - remove folio_add_new_anon_rmap_range() patch
>        - instead add its new functionality to folio_add_new_anon_rmap()
>    - remove batch-zap pte mappings optimization patch
>        - remove enabler folio_remove_rmap_range() patch too
>        - These offer real perf improvement so will submit separately
>    - simplify Kconfig
>        - single FLEXIBLE_THP option, which is independent of arch
>        - depends on TRANSPARENT_HUGEPAGE
>        - when enabled default to max anon folio size of 64K unless arch
>          explicitly overrides
>    - simplify changes to do_anonymous_page():
>        - no more retry loop
> 
> 
> Performance
> -----------
> 
> Below results show 3 benchmarks; kernel compilation with 8 jobs, kernel
> compilation with 80 jobs, and speedometer 2.0 (a javascript benchmark running in
> Chromium). All cases are running on Ampere Altra with 1 NUMA node enabled,
> Ubuntu 22.04 and XFS filesystem. Each benchmark is repeated 15 times over 5
> reboots and averaged.
> 
> 'anonfolio-lkml-v1' is the v1 patchset at [1]. 'anonfolio-lkml-v2' is this v2
> patchset. 'anonfolio-lkml-v2-simple-order' is anonfolio-lkml-v2 but with the
> order selection simplification that Yu Zhao suggested - I'm trying to justify
> here why I did not follow the advice.
> 
> 
> Kernel compilation with 8 jobs:
> 
> | kernel                         |   real-time |   kern-time |   user-time |
> |:-------------------------------|------------:|------------:|------------:|
> | baseline-4k                    |        0.0% |        0.0% |        0.0% |
> | anonfolio-lkml-v1              |       -5.3% |      -42.9% |       -0.6% |
> | anonfolio-lkml-v2-simple-order |       -4.4% |      -36.5% |       -0.4% |
> | anonfolio-lkml-v2              |       -4.8% |      -38.6% |       -0.6% |
> 
> We can see that the simple-order approach is responsible for a regression of
> 0.4%.
> 
> 
> Kernel compilation with 80 jobs:
> 
> | kernel                         |   real-time |   kern-time |   user-time |
> |:-------------------------------|------------:|------------:|------------:|
> | baseline-4k                    |        0.0% |        0.0% |        0.0% |
> | anonfolio-lkml-v1              |       -4.6% |      -45.7% |        1.4% |
> | anonfolio-lkml-v2-simple-order |       -4.7% |      -40.2% |       -0.1% |
> | anonfolio-lkml-v2              |       -5.0% |      -42.6% |       -0.3% |
> 
> simple-order costs 0.3 % here. v2 is actually performing higher than v1 due to
> fixing the v1 regression on user-time.
> 
> 
> Speedometer 2.0:
> 
> | kernel                         |   runs_per_min |
> |:-------------------------------|---------------:|
> | baseline-4k                    |           0.0% |
> | anonfolio-lkml-v1              |           0.7% |
> | anonfolio-lkml-v2-simple-order |          -0.9% |
> | anonfolio-lkml-v2              |           0.5% |
> 
> simple-order regresses performance by 0.9% vs the baseline, for a total negative
> swing of 1.6% vs v1. This is fixed by keeping the more complex order selection
> mechanism from v1.
> 
> 
> The remaining (kernel time) performance gap between v1 and v2 for the above
> benchmarks is due to the removal of the "batch zap" patch in v2. Adding that
> back in gives us the performance back. I intend to submit that as a separate
> series once this series is accepted.
> 
> 
> [1] https://lore.kernel.org/linux-mm/20230626171430.3167004-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/linux-mm/20230315051444.3229621-1-willy@infradead.org/
> [3] https://gitlab.arm.com/linux-arm/linux-rr/-/tree/features/granule_perf/anonfolio-lkml_v2
> 
> Thanks,
> Ryan

Hi Ryan,

is page migration already working as expected (what about page 
compaction?), and do we handle migration -ENOMEM when allocating a 
target page: do we split an fallback to 4k page migration?

-- 
Cheers,

David / dhildenb

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