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    SubjectRe: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT_HUGEPAGE enabled
    On Tue, Nov 09, 2010 at 10:11:45PM +0100, Andrea Arcangeli wrote:
    > On Tue, Nov 09, 2010 at 03:20:33PM +0900, KOSAKI Motohiro wrote:
    > > > From: Andrea Arcangeli <aarcange@redhat.com>
    > > >
    > > > With transparent hugepage support we need compaction for the "defrag" sysfs
    > > > controls to be effective.
    > > >
    > > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    > > > ---
    > > >
    > > > diff --git a/mm/Kconfig b/mm/Kconfig
    > > > --- a/mm/Kconfig
    > > > +++ b/mm/Kconfig
    > > > @@ -305,6 +305,7 @@ config NOMMU_INITIAL_TRIM_EXCESS
    > > > config TRANSPARENT_HUGEPAGE
    > > > bool "Transparent Hugepage Support"
    > > > depends on X86 && MMU
    > > > + select COMPACTION
    > > > help
    > > > Transparent Hugepages allows the kernel to use huge pages and
    > > > huge tlb transparently to the applications whenever possible.
    > >
    > > I dislike this. THP and compaction are completely orthogonal. I think
    > > you are talking only your performance recommendation. I mean I dislike
    > > Kconfig 'select' hell and I hope every developers try to avoid it as
    > > far as possible.
    >
    > At the moment THP hangs the system if COMPACTION isn't selected

    Just to confirm - by hang, you mean grinds to a slow pace as opposed to
    coming to a complete stop and having to restart?

    If so then

    Acked-by: Mel Gorman <mel@csn.ul.ie>

    > (please try yourself if you don't believe), as without COMPACTION
    > lumpy reclaim wouldn't be entirely disabled. So at the moment it's not
    > orthogonal. When lumpy will be removed from the VM (like I tried
    > multiple times to achieve) I can remove the select COMPACTION in
    > theory, but then 99% of THP users would be still doing a mistake in
    > disabling compaction, even if the mistake won't return in fatal
    > runtime but just slightly degraded performance.
    >

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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