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    SubjectRe: [RFC 0/4] Transparent on-demand struct page initialization embedded in the buddy allocator
    On Fri, Jul 12, 2013 at 10:27:56AM +0200, Ingo Molnar wrote:
    >
    > * Robin Holt <holt@sgi.com> wrote:
    >
    > > [...]
    > >
    > > With this patch, we did boot a 16TiB machine. Without the patches, the
    > > v3.10 kernel with the same configuration took 407 seconds for
    > > free_all_bootmem. With the patches and operating on 2MiB pages instead
    > > of 1GiB, it took 26 seconds so performance was improved. I have no feel
    > > for how the 1GiB chunk size will perform.
    >
    > That's pretty impressive.

    And WRONG!

    That is a 15x speedup in the freeing of memory at the free_all_bootmem
    point. That is _NOT_ the speedup from memmap_init_zone. I forgot to
    take that into account as Nate pointed out this morning in a hallway
    discussion. Before, on the 16TiB machine, memmap_init_zone took 1152
    seconds. After, it took 50. If it were a straight 1/512th, we would
    have expected that 1152 to be something more on the line of 2-3 so there
    is still significant room for improvement.

    Sorry for the confusion.

    > It's still a 15x speedup instead of a 512x speedup, so I'd say there's
    > something else being the current bottleneck, besides page init
    > granularity.
    >
    > Can you boot with just a few gigs of RAM and stuff the rest into hotplug
    > memory, and then hot-add that memory? That would allow easy profiling of
    > remaining overhead.

    Nate and I will be working on other things for the next few hours hoping
    there is a better answer to the first question we asked about there
    being a way to test a page other than comparing against all zeroes to
    see if it has been initialized.

    Thanks,
    Robin


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