12.0-ALPHA5 - ZFS default ARC max apparently forcing system to run out of memory

I'm running 12.0-ALPHA5 on a laptop which has 32GB RAM and 2GB swap.
I've found it running out of memory when building ports via synth: I
think I've also seen it when running a buildworld. Johannes on
FreeBSDDesktop suggested it might be related to ZFS, and setting
vfs.zfs.arc_max to 8GB *does* appear to have resolved the problem.


Shortly after running out of memory (with |"swap_pager_getswapspace(32):
failed" messages)|, the first few lines of 'top' were:


Mem: 4335M Active, 4854M Inact, 7751M Laundry, 9410M Wired, 48K Buf,
5332M Free

ARC: 5235M Total, 4169M MFU, 497M MRU, 172K Anon, 97M Header, 471M Other

     3479M Compressed, 5930M Uncompressed, 1.70:1 Ratio

Swap: 2048M Total, 2009M Used, 39M Free, 98% Inuse



I've not seen this happen before on ZFS systems, so is it a regression
in 12?


-- 
Rebecca Cran