Marvin has bisected a small (5%) performance regression in ext4 due to commit
077d0c2c78df ("ext4: make mb_optimize_scan performance mount option work with
extents"). The bisection result looks like:

              good-27b38686  bad-077d0c2c
Hmean disk-1    537.83         529.10
Hmean disk-5   3433.28        3431.71
...
Hmean disk-25 16999.09       16290.18 *  -3.67%*
Hmean disk-29 19162.99       18385.46 *  -4.73%*
Hmean disk-33 21859.13       20793.95 *  -4.54%*

The regression was reported on laurel1. Likely the overhead of mballoc has
somewhat increased due to rbtree searching of best group to use.