Pi3 out of swap at < 50%

[bob prohaska]

Running make -j4 buildworld on a Pi3 at r329360 tends to result
in  messages similar to:

pid 33492 (llvm-tblgen), uid 0, was killed: out of swap space
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 11764, size: 28672
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 16080, size: 4096
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 15721, size: 20480
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 28139, size: 65536
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 40544, size: 4096
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 58384, size: 65536
pid 49735 (c++), uid 0, was killed: out of swap space
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 28470, size: 4096
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 24889, size: 8192
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 28736, size: 4096
swap_pager: indefinite wait buffer: bufobj: 0, blkno: 29736, size: 4096

even though swap usage appears to be less than one GB out of two available.

So far it's been possible to use the -DNO_CLEAN option with make to pick
up where things left off and I've backed down to -j3 to see if that helps.

There are two 1 GB swap partitions, one on USB flash and one on the microSD
card and both appear to be in use according to swapinfo. There are no warnings
in the boot messages and no explicit changes have been made to configuration
files apart from adding the swap entries to /etc/fstab.

World and kernel are about two weeks out of sync, so top isn't perfectly
up-to-date with this kernel; could that account for the mismatch between
apparent swap usage and the "out of swap" messages?

Thanks for reading, and any ideas!

bob prohaska