Shard per core has challenges. Scylla solves it partially by making the client shard aware (topology aware), so the client submits requests directly to the right cpu core using a different port. This way no data gets blocked due to hot keys or imbalance in other shards. You do need to have a good key distribution. We're implementing a new mechanism to split key ranges dynamically. It will be a fast an parallel mechanism.

If the number of connections is not small, Scylla will crank up any other implementation with traditional locking.