The scheduling of block <span class="nobreak">I/O</span> requests has long
been a challenge for operating-system kernels. For many years, the
performance characteristics of rotating drives meant that putting
considerable resources into request ordering was worthwhile. In a world
with fast, solid-state drives, scheduling is more concerned with enforcing
fairness between competing users while being fast enough to keep up with
drives that can perform millions of <span class="nobreak">I/O</span>
operations per second. The blk-iocost <span class="nobreak">I/O</span>
controller was designed for the solid-state world and generally performs
well, but there is always a desire to do better. <a
href="https://lwn.net/ml/all/[email protected]">This patch
series</a> from Tao Cui aims to make blk-iocost more flexible by enabling
the loading of a BPF program to make cost decisions.