oh well, that went fast. actually with having low values for buffers (around 100MB) with caches
around 20G or so, the performance was nevertheless super-low, I really had to drop
the caches right now. This is the first time I see it with caches >10G happening, but hopefully
this also provides a clue for you.
Just after starting the stats I reset from previously defer to madvise - I suspect that this somehow
caused the rapid reaction, since a few minutes later I saw that the free RAM jumped from 5GB to 10GB,
after that I went afk, returning to the pc since my monitoring systems went crazy telling me about downtime.
If you think changing /sys/kernel/mm/transparent_hugepage/defrag back to its default, while it was
on defer now for days, was a mistake, then please tell me.