Using threads for in-flight IOs means burning up lots of memory with stacks, and burning ever more CPU in context switches as IOs complete. In practice this mea

Using threads for in-flight IOs means burning up lots of memory with stacks, and burning ever more CPU in context switches as IOs complete. In practice this means using some kind of state machine (either explicit or in the form of coroutines) to pick up where logic paused at the initiation of the IO. The logic with respect to locking and concurrency control doesn't change; the costs in memory and CPU do. The CPU cost is increasing all the time due to all the cache attacks being revealed and the mitigations blowing away various cache contents, which is especially damaging in VM contexts like AWS where the mitigations can't be reasonably disabled.