OOM at low page cache?

[John Moser <john.r.moser@gmail.com>]

Why is there no tunable to OOM at low page cache?

I have no swap configured.  I have 16GB RAM.  If Chrome or Gimp or some
other stupid program goes off the deep end and eats up my RAM, I hit
some 15.5GB or 15.75GB usage and stay there for about 40 minutes.  Every
time the program tries to do something to eat more RAM, it cranks disk
hard; the disk starts thrashing, the mouse pointer stops moving, and
nothing goes on.  It's like swapping like crazy, except you're reading
library files instead of paged anonymous RAM.

If only I could tell the system to OOM kill at 512MB or 1GB or 95%
non-evictable RAM, it would recover on its own.  As-is, I need to wait
or trigger the OOM killer by sysrq.

Am I just the only person in the world who's ever had that problem?  Or
is it a matter of questions fast popping up when you try to do this
*and* enable paging to disk?  (In my experience, that's a matter of too
much swap space:  if you have 16GB RAM and your computer dies at 15.25GB
usage, your swap space should be no larger than 750MB plus inactive
working RAM; obviously, your computer can't handle paging 750MB back and
forth.  If you make it 8GB wide and you start swap thrashing at 2GB
usage, you have too much swap available).

I guess you could try to detect excessive swap and page cache thrashing,
but that's complex; if anyone really wanted to do that, it would be done
by now.  A low-barrier OOM is much simpler.