A bit dirty patch to collect and print latency histogram for disk
access. Example outputs:

kirr@deco:~/src/tools/fs/ioping$ ./ioping -i 100ms -D .
4 KiB <<< . (ext4 /dev/sda3): request=1 time=128.7 us (warmup)
4 KiB <<< . (ext4 /dev/sda3): request=2 time=262.9 us
4 KiB <<< . (ext4 /dev/sda3): request=3 time=282.2 us
...
4 KiB <<< . (ext4 /dev/sda3): request=170 time=259.5 us
^C
--- . (ext4 /dev/sda3) ioping statistics ---
169 requests completed in 45.8 ms, 676 KiB read, 3.69 k iops, 14.4 MiB/s
generated 170 requests in 17.0 s, 680 KiB, 10 iops, 40.0 KiB/s
min/avg/max/mdev = 142.9 us / 270.9 us / 378.1 us / 28.8 us
< 142.9 us      0       |
< 165.7 us      0       |
< 188.5 us      0       |
< 211.2 us      0       |
< 234.0 us      0       |
< 256.8 us      21      | ******
< 279.6 us      29      | ********
< 302.3 us      8       | **
< 325.1 us      5       | *
< 347.9 us      4       | *
< 370.7 us      1       |
<       +∞      1       |

kirr@deco:~/src/tools/fs/ioping$ ./ioping -R -S 1024M -k .

--- . (ext4 /dev/sda3) ioping statistics ---
17.1 k requests completed in 2.90 s, 66.7 MiB read, 5.89 k iops, 23.0 MiB/s
generated 17.1 k requests in 3.00 s, 66.8 MiB, 5.70 k iops, 22.2 MiB/s
min/avg/max/mdev = 119.4 us / 169.8 us / 3.83 ms / 260.2 us
< 298.2 us      16623   | ************************************************
< 651.4 us      110     |
< 1.00 ms       64      |
< 1.36 ms       35      |
< 1.71 ms       37      |
< 2.06 ms       33      |
< 2.42 ms       20      |
< 2.77 ms       17      |
< 3.12 ms       19      |
< 3.48 ms       25      |
< 3.83 ms       4       |
<       +∞      0       |