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 :) -- All Rights Reversed.
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