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    SubjectRe: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression
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    On 12/14/18 10:04 PM, David Rientjes wrote:
    > On Wed, 12 Dec 2018, Vlastimil Babka wrote:

    ...

    > Reclaim likely could be deterministically useful if we consider a redesign
    > of how migration sources and targets are determined in compaction.
    >
    > Has anybody tried a migration scanner that isn't linearly based, rather
    > finding the highest-order free page of the same migratetype, iterating the
    > pages of its pageblock, and using this to determine whether the actual
    > migration will be worthwhile or not?

    Not exactly that AFAIK, but a year ago in my series [1] patch 6 made
    migration scanner 'prescan' the block of requested order before actually
    trying to isolate anything for migration.

    > I could imagine pageblock_skip being
    > repurposed for this as the heuristic.
    >
    > Finding migration targets would be more tricky, but if we iterate the
    > pages of the pageblock for low-order free pages and find them to be mostly
    > used, that seems more appropriate than just pushing all memory to the end
    > of the zone?

    Agree. That was patch 8/8 of the same series [1].

    > It would be interesting to know if anybody has tried using the per-zone
    > free_area's to determine migration targets and set a bit if it should be
    > considered a migration source or a migration target. If all pages for a
    > pageblock are not on free_areas, they are fully used.

    Repurposing/adding a new pageblock bit was in my mind to help multiple
    compactors not undo each other's work in the scheme where there's no
    free page scanner, but I didn't implement it yet.

    >>> otherwise we fail and defer because it wasn't able
    >>> to make a hugepage available.
    >>
    >> Note that THP fault compaction doesn't actually defer itself, which I
    >> think is a weakness of the current implementation and hope that patch 3
    >> in my series from yesterday [1] can address that. Because defering is
    >> the general feedback mechanism that we have for suppressing compaction
    >> (and thus associated reclaim) in cases it fails for any reason, not just
    >> the one you mention. Instead of inspecting failure conditions in detail,
    >> which would be costly, it's a simple statistical approach. And when
    >> compaction is improved to fail less, defering automatically also happens
    >> less.
    >>
    >
    > I couldn't get the link to work, unfortunately, I don't think the patch
    > series made it to LKML :/ I do see it archived for linux-mm, though, so
    > I'll take a look, thanks!

    Yeah I forgot to Cc: LKML, but you were also in direct To: so you should
    have received them directly. Also the abovementioned series, but that's
    year ago. My fault for not returning to it after being done with the
    Meltdown fun. I hope to do that soon.

    [1] https://marc.info/?l=linux-mm&m=151315560308753

    >> [1] https://lkml.kernel.org/r/20181211142941.20500-1-vbabka@suse.cz
    >>

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