The pale columns represent repeated consecutive reads from the same data file, which we assume involves reading from the disk cache.
The performance-enhancing effect of both the disk cache and the data reading strategy is clear.
Experiments were performed on a Dell Latitude E6320 laptop with 8 GB RAM, an Intel Core-i7 (2.70 GHz) multicore processor and a 500 GB SATA 7200rpm disk drive, using an Oracle Java Virtual Machine, version 1.7.0_07. Code is available as supplementary material to this paper.