Currently PageSize is hardcoded and is 4KB on everything but ARM64 macOS systems. This is used as the unit of decommitting pages and is not necessarily the same as the size of an Arena.

If this is wrong then we don't decommit unused arenas in chunks.

I don't know if the current values are correct or not, but we should probably just detect this on startup and not hardcode it.

After I filed this I remembered that is used to size the decommittedPages bitmap in TenuredChunkBase and calculate the number of arenas we get in a chunk. This is small enough that we can probably size this as if for 4KB pages and it won't make any difference.