While RocksDB allows for num_shard_bits to be set for the block cache, MyRocks does not expose this.
This is a feature request for MyRocks to expose an option that allows me to set it.

There can be serious perf problems when the block cache is too small. With intra-L0 compaction there can be large SSTs in the L0, large SSTs imply large filter blocks and reading a large object into a small cache shard can evict everything from that shard.

A blog post on this is here.