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    SubjectRe: Reclaim regression after 1c30844d2dfe
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    On Fri, 2020-02-07 at 14:54 -0800, Ivan Babrou wrote:
    > This change from 5.5 times:
    >
    > * https://github.com/torvalds/linux/commit/1c30844d2dfe
    >
    > > mm: reclaim small amounts of memory when an external fragmentation
    > > event occurs
    >
    > Introduced undesired effects in our environment.
    >
    > * NUMA with 2 x CPU
    > * 128GB of RAM
    > * THP disabled
    > * Upgraded from 4.19 to 5.4
    >
    > Before we saw free memory hover at around 1.4GB with no spikes. After
    > the upgrade we saw some machines decide that they need a lot more
    > than
    > that, with frequent spikes above 10GB, often only on a single numa
    > node.
    >
    > We can see kswapd quite active in balance_pgdat (it didn't look like
    > it slept at all):
    >
    > $ ps uax | fgrep kswapd
    > root 1850 23.0 0.0 0 0 ? R Jan30 1902:24
    > [kswapd0]
    > root 1851 1.8 0.0 0 0 ? S Jan30 152:16
    > [kswapd1]
    >
    > This in turn massively increased pressure on page cache, which did
    > not
    > go well to services that depend on having a quick response from a
    > local cache backed by solid storage.
    >
    > Here's how it looked like when I zeroed vm.watermark_boost_factor:

    We have observed the same thing, even on single node systems.

    I have some hacky patches to apply the watermark_boost thing on
    a per pgdat basis, which seems to resolve the issue, but I have
    not yet found the time to get the locking for that correct.

    Given how rare the watermark boosting is, maybe the answer is
    just to use atomics? Not sure :)

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