[Bug 257314] FBSD 13 crash after some KDE parts crash supposing out of swap space

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=257314

--- Comment #6 from Mark Millard <marklmi26-fbsd@yahoo.com> ---
(In reply to Graham Perrin from comment #5)

Just for context . . .

Folks trying buildworld buildkernel on small armv7
and aarch64 boards with only, say, 1 GiBytes or 2
GiBytes of RAM, tend to see such kills from long
running, compute/memory-bound llvm compiles and links,
even when configured with RAM+swap being sufficient
so that swap does not run out.

The classic solution for such folks has been the
use of something like:

vm.pageout_oom_seq=120
vm.pfault_oom_attempts=-1

I build my own kernels, with sufficient messaging
added to indicate which of the 4 conditions initiated
the kill:

Sustained low free RAM (via 1 or more stays-runnable processes).
A sufficiently delayed pageout.
The swap blk uma zone was exhausted.
The swap pctrie uma zone was exhausted.

(Those are not the messages themselves, just the
summaries.)

I do that on everything from small arm boards to
a ThreadRipper 1950X that I have access to. If I
end up with such a kill, I want to know which
condition lead to it in order to figure out what
to do in the future. I, of course, also check on
if "swap_pager: out of swap space" or
"swp_pager_getswapspace(. . .): failed" messages
also happened.

So far as I know, actually running out of swap space
and getting the kills involves at least one of the 4
conditions as well: there is no separate condition
for out of swap space that initiates a kill in what I
found in the kernel.

Hopefully these sorts of notes are of some use to
Michael in getting control of the problem.

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