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    SubjectRe: [PATCH 2/3] mm: make the threshold of enabling THP configurable
    On Tue, Jun 21, 2011 at 01:16:00AM +0800, Cong Wang wrote:
    > ??? 2011???06???21??? 00:59, Mel Gorman ??????:
    > >On Tue, Jun 21, 2011 at 12:34:29AM +0800, Amerigo Wang wrote:
    > >>Don't hard-code 512M as the threshold in kernel, make it configruable,
    > >>and set 512M by default.
    > >>
    > >
    > >I'm not seeing the gain here either. This is something that is going to
    > >be set by distributions and probably never by users. If the default of
    > >512 is incorrect, what should it be? Also, the Kconfig help message has
    > >spelling errors.
    > >
    > Sorry for spelling errors, I am not an English speaker.
    > Hard-coding is almost never a good thing in kernel, enforcing 512
    > is not good either. Since the default is still 512, I don't think this
    > will affect much users.
    > I do agree to improve the help message, like Dave mentioned in his reply,
    > but I don't like enforcing a hard-coded number in kernel.
    > BTW, why do you think 512 is suitable for *all* users?

    Fragmentation avoidance benefits from tuning min_free_kbytes to a higher
    value and minimising fragmentation-related problems is crucial if THP is
    to allocate its necessary pages.

    THP tunes min_free_kbytes automatically and this value is in part
    related to the number of zones. At 512M on a single node machine, the
    recommended min_free_kbytes is close to 10% of memory which is barely
    tolerable as it is. At 256M, it's 17%, at 128M, it's 34% so tuning the
    value lower has diminishing returns as the performance impact of giving
    up such a high percentage of free memory is not going to be offset by
    reduced TLB misses. Tuning it to a higher value might make some sense
    if the higher min_free_kbytes was a problem but it would be much more
    rational to tune it as a sysctl than making it a compile-time decision.

    Mel Gorman
    SUSE Labs

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