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From:   Pavel Tatashin <pasha.tatashin@oracle.com>
Date:   Mon, 2 Jul 2018 08:06:50 -0400
Message-ID: <CAGM2reaPhcWNhNW+i7kCysUr2tEMBour-GO_hkr4N-SrEvcx0w@mail.gmail.com>
Subject: Re: [PATCH v9 0/6] optimize memblock_next_valid_pfn and
 early_pfn_valid on arm and arm64
To:     mhocko@kernel.org
Cc:     Jia He <hejianet@gmail.com>, linux@armlinux.org.uk,
        Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
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        AKASHI Takahiro <takahiro.akashi@linaro.org>,
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On Mon, Jul 2, 2018 at 7:40 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Fri 29-06-18 10:29:17, Jia He wrote:
> > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > where possible") tried to optimize the loop in memmap_init_zone(). But
> > there is still some room for improvement.
>
> It would be great to shortly describe those optimization from high level
> POV.
>
> >
> > Patch 1 introduce new config to make codes more generic
> > Patch 2 remain the memblock_next_valid_pfn on arm and arm64
> > Patch 3 optimizes the memblock_next_valid_pfn()
> > Patch 4~6 optimizes the early_pfn_valid()
> >
> > As for the performance improvement, after this set, I can see the time
> > overhead of memmap_init() is reduced from 27956us to 13537us in my
> > armv8a server(QDF2400 with 96G memory, pagesize 64k).
>
> So this is 13ms saving when booting 96G machine. Is this really worth
> the additional code? Are there any other benefits?

While 0.0144s for 96G is definitely small, I think the time is
proportional to the number of pages since memmap_init() loops through
all the pages. If base pages were changed to 4K, I bet the time would
increase 16 times: 0.23s on given machine, in other words around 2s
per 1T of memory.

I agree, a high level description of optimization is needed, and also
an explanation of why it would not work on other arches that support
memblock.

Pavel