How does netbsd decide which swap partitions to use in which order?

swapinfo gives me this:

     Device      512-blocks       Used  Available   Capacity
     /dev/wd0b        40320       7528      32792      19%
     /dev/sd0b       122880      56968      65912      46%
     /dev/wd0e        32760          8      32752       0%
     Total           195960      64504     131456      33%

I wonder why much of sd0b is used when wd0b still has space.  (Perhaps
it is just my usage patterns?  But I don't recall starting and then
killing anything which would use those 32MB.)

I would like all the space on wd0 to be used first, since that disk is
faster than sd0.

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