I believe there are some high concurrency hash tables out there where the data structure contains two tables, and so each get results in a constant number of fe

I believe there are some high concurrency hash tables out there where the data structure contains two tables, and so each get results in a constant number of fetches, but the count is almost always greater than one. But if it avoids concurrency issues that ends up being acceptable. I remember Cliff Click presenting one that was lockless and if I'm recalling correctly, where capacity growth operations could happen in parallel with reads. And possibly writes.