When using small heaps (like less than 128M), the heap reserve tends to take up a relatively large portion of the heap. We have quite a lot of tests that explicitly set the heap size to small values (like -Xmx8M, -Xmx16M, etc). Today, these tests fail with OOME when using ZGC. While ZGC isn't really that focused on tiny/small heaps, we still want to make testing of ZGC easy without having to patch these tests to use a larger heap.

There are at least two things we can do when ZGC is given a small heap:
1) Dynamically scale the medium ZPage size, and even disable medium pages all together when using tiny heaps.
2) Stop using per-CPU small pages for allocations, and instead switch to using a single small page.