1341MB swap in use with half gig of free memory

[bob prohaska]

In watching wwwchromium (very) slowly compile on a Pi3 running
r362742-current something strange showed up: It had half a gig 
of "free" memory, but still has over a gig of swap in use. The 
swap device is saturated.

Here's snippet of top output:

last pid: 77187;  load averages:  0.56,  0.80,  0.82                   up 4+19:29:15  14:39:56
39 processes:  1 running, 38 sleeping
CPU:  0.1% user,  0.0% nice,  0.2% system,  0.1% interrupt, 99.6% idle
Mem: 80M Active, 63M Inact, 3508K Laundry, 176M Wired, 97M Buf, 580M Free
Swap: 3547M Total, 1341M Used, 2206M Free, 37% Inuse, 1188K In

It isn't entirely stuck, every few minutes free memory wanders down to 
20MB and one core reaches 100% use, but in general it seems to keep
considerable free memory in preference to reducing swap use. That would
seem to be unhelpful in this use case.  

The machine has a 1TB mechanical hard disk with a single root partition
and single swap partition. Make is running without explicit -j value,
and appears to be attempting -j4. Earlier in the build it seemed to be
running more rapidly, but over the last day or so it has gone from mostly
busy to mostly idle with not much change in swap use. There are no errors
or warnings on the console, and no complaint about swap quantitiy on boot.

Obviously it's possible to use -j or MAX_JOBS_NUMBER, but reluctance to
keep all memory in use makes me wonder if something else is amiss.
Looking at man tuning revealed nothing I recognized as relevant. 

Thanks for reading!

bob prohaska