An RPI3 kernel panic while running -j4 buildworld with

3 GB  swap on microSD
No swap on USB flash
 sysctl vm.pageout_update_period=0

Buildworld was restarted after a prior OOM kill  with the -DNO_CLEAN 
option. Make clean was run in /usr/src/lib/clang/libllvm, but no other
cleanup/backtraking was done.

Kernel was at 334800 

Top showed 465 MB of memory in use