Sultan, thanks for the advice.

I set this watermark boost factor to zero as you suggested, and then
decided to try a stupid simple benchmark of my storage -- my swap is a
zfs dataset on nvme. zfs means it'll go slower than raw nvme block
access:

$ dd if=ubuntu-18.04.4-desktop-amd64.iso of=foo bs=$((1024*1024)) conv=fsync
2028+1 records in
2028+1 records out
2126544896 bytes (2.1 GB, 2.0 GiB) copied, 6.91399 s, 308 MB/s

And this is a sequential workload with large blocks rather than random
with pages, but it gives the flavour of the speed well enough.

Anyway, my system hung again when re-focusing firefox even after
changing the watermark boost factor to zero. It was unusable for ten or
twenty seconds. I have loads of free memory, it shouldn't be in swap:

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           15Gi        11Gi       1.6Gi       643Mi       2.2Gi       2.8Gi
Swap:         2.0Gi        23Mi       2.0Gi

Thanks

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https://bugs.launchpad.net/bugs/1861359

Title:
  swap storms kills interactive use

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