In some instances, particularly with swap on zram, swap used will be high
while there is still a lot of memory available. FB OOMD handles this by
thresholding kills to X% of total swap usage. Let's do the same thing here.
Anecdotally with these thresholds and my laptop which is exclusively swap
on zram I can sit at 0K / 4G swap free with most of memory free and
systemd-oomd doesn't kill anything.
Partially addresses aggressive kill behavior from
https://bugzilla.redhat.com/show_bug.cgi?id=1941170
oomd.conf has two parameters with fractionals: SwapUsedLimit= and
DefaultMemoryPressureLimit=, but one accepts permyriads, the other only
percentages, for no apparent reason. One carries the "Percent" in the
name, the other doesn't.
Let's clean this up: always accept permyriads, and drop the suffix,
given that it is misleading.
I figure we should internally try to focus on scaling everything
relative to UINT32_MAX, and if that isn't in the cards at least 10000,
but never permille nor percent unless there's a really really good
reason for it (e.g. interface defined by someone else).
