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Date:   Wed, 19 Apr 2023 03:01:05 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, Kaiyang Zhao <kaiyang2@cs.cmu.edu>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Rientjes <rientjes@google.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [RFC PATCH 03/26] mm: make pageblock_order 2M per default
Message-ID: <20230419000105.matz43p6ihrqmado@box.shutemov.name>
References: <20230418191313.268131-1-hannes@cmpxchg.org>
 <20230418191313.268131-4-hannes@cmpxchg.org>
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On Tue, Apr 18, 2023 at 03:12:50PM -0400, Johannes Weiner wrote:
> pageblock_order can be of various sizes, depending on configuration,
> but the default is MAX_ORDER-1.

Note that MAX_ORDER got redefined in -mm tree recently.

> Given 4k pages, that comes out to
> 4M. This is a large chunk for the allocator/reclaim/compaction to try
> to keep grouped per migratetype. It's also unnecessary as the majority
> of higher order allocations - THP and slab - are smaller than that.

This seems way to x86-specific. Other arches have larger THP sizes. I
believe 16M is common.

Maybe define it as min(MAX_ORDER, PMD_ORDER)?

> Before subsequent patches increase the effort that goes into
> maintaining migratetype isolation, it's important to first set the
> defrag block size to what's likely to have common consumers.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/pageblock-flags.h | 4 ++--
>  mm/page_alloc.c                 | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> index 5f1ae07d724b..05b6811f8cee 100644
> --- a/include/linux/pageblock-flags.h
> +++ b/include/linux/pageblock-flags.h
> @@ -47,8 +47,8 @@ extern unsigned int pageblock_order;
>  
>  #else /* CONFIG_HUGETLB_PAGE */
>  
> -/* If huge pages are not used, group by MAX_ORDER_NR_PAGES */
> -#define pageblock_order		(MAX_ORDER-1)
> +/* Manage fragmentation at the 2M level */
> +#define pageblock_order		ilog2(2U << (20 - PAGE_SHIFT))
>  
>  #endif /* CONFIG_HUGETLB_PAGE */
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ac03571e0532..5e04a69f6a26 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7634,7 +7634,7 @@ static inline void setup_usemap(struct zone *zone) {}
>  /* Initialise the number of pages represented by NR_PAGEBLOCK_BITS */
>  void __init set_pageblock_order(void)
>  {
> -	unsigned int order = MAX_ORDER - 1;
> +	unsigned int order = ilog2(2U << (20 - PAGE_SHIFT));
>  
>  	/* Check that pageblock_nr_pages has not already been setup */
>  	if (pageblock_order)
> -- 
> 2.39.2
> 
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov