Hash Table Occupancy

It is characteristic of hash tables that they are allocated a certain size, but always retain some empty records, so they are less than 100% occupied. A healthy amount of empty space is important for quick access to the DRAGEN hash table. Approximately 90% occupancy is a good upper bound. Empty space is important because records are pseudo-randomly placed in the hash table, resulting in an abnormally high number of records in some places. These congested regions can get quite large as the percentage of empty space approaches zero, and queries by the DRAGEN mapper for some seeds can become increasingly slow.