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: * https://imgur.com/a/6IZWicU IO subsided from 100% busy in page cache population at 300MB/s on a single SATA drive down to under 100MB/s. This sort of regression doesn't seem like a good thing.
