I/O

iotop connects task-level kernel accounting with a system-wide storage investigation. It can identify tasks reporting substantial IO or waiting, but those are different observations. A task blocked behind other work may be a victim rather than the source of contention. Kernel counters do not provide perfect attribution for every filesystem, cache, or storage layer. ...

blk-mq provides CPU- or node-local software staging queues and hardware dispatch queues. Scheduling and merging can occur before dispatch, while driver and device behavior also affect service. Software submission order does not guarantee completion order. A scheduler can influence how competing requests enter the device without controlling every later decision in the storage stack. ...