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    SubjectRe: [PATCH 0/8] Use memory compaction instead of lumpy reclaim during high-order allocations
    On Wed, Nov 17, 2010 at 03:46:41PM -0800, Andrew Morton wrote:
    > On Wed, 17 Nov 2010 16:22:41 +0000
    > Mel Gorman <> wrote:
    > > Huge page allocations are not expected to be cheap but lumpy reclaim
    > > is still very disruptive.
    > Huge pages are boring. Can we expect any benefit for the
    > stupid-nic-driver-which-does-order-4-GFP_ATOMIC-allocations problem?

    Yes. Specifically, while GFP_ATOMIC allocations still cannot enter compaction
    (although with asynchronous migration, it's closer), kswapd will react
    faster. As a result, it should be harder to trigger allocation failures.

    Huge pages are simply the worst case in terms of allocation latency which
    is why I tend to focus testing on them. That, and I don't have a suitable
    pair of machines with one of these order-4-atomic-stupid-nics.

    > > I haven't pushed hard on the concept of lumpy compaction yet and right
    > > now I don't intend to during this cycle. The initial prototypes did not
    > > behave as well as expected and this series improves the current situation
    > > a lot without introducing new algorithms. Hence, I'd like this series to
    > > be considered for merging.
    > Translation: "Andrew, wait for the next version"? :)

    Preferably do not wait unless review reveals a major flaw. Lumpy compaction
    in its initial prototype versions simply did not work out as a good policy
    modification and requires much deeper thought. This series was effective
    at getting latencies down to the level I expected lumpy compaction to.
    If I do make lumpy compaction work properly, its effect will be to reduce
    scanning rates but the latencies are likely to be similar.

    > > I'm hoping that this series also removes the
    > > necessity for the "delete lumpy reclaim" patch from the THP tree.
    > Now I'm sad. I read all that and was thinking "oh goody, we get to
    > delete something for once". But no :(
    > If you can get this stuff to work nicely, why can't we remove lumpy
    > reclaim?

    Ultimately we should be able to. Lumpy reclaim is still there for the
    !CONFIG_COMPACTION case and to have an option if we find that compaction
    behaves badly for some reason.

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

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