The semi collector is simple. It is mostly useful as a first
collector to try out, to make sure that a mutator correctly records all
roots: because semi moves every live object on every collection, it is
very effective at shaking out mutator bugs.
If your embedder chooses to not precisely record roots, for example
instead choosing to conservatively scan the stack, then the semi-space
collector is not for you: semi requires precise roots.
For more on semi-space collectors, see https://wingolog.org/archives/2022/12/10/a-simple-semi-space-collector.
Whippet's semi collector incorporates a large-object space, which
marks objects in place instead of moving. Otherwise, semi generally
requires a heap size at least twice as large as the maximum live heap
size, and performs best with ample heap sizes; between 3× and 5× is
best.
The semi-space collector doesn't support multiple mutator threads. If you want a whole-heap copying collector for a multi-threaded mutator, look at pcc.