On 08/03/2017 10:48 AM, Paolo Valente wrote: > When a queue associated with a process remains empty, there are cases > where throughput gets boosted if the device is idled to await the > arrival of a new I/O request for that queue. Currently, BFQ assumes > that one of these cases is when the device has no internal queueing > (regardless of the properties of the I/O being served). Unfortunately, > this condition has proved to be too general. So, this commit refines it > as "the device has no internal queueing and is rotational". > > This refinement provides a significant throughput boost with random > I/O, on flash-based storage without internal queueing. For example, on > a HiKey board, throughput increases by up to 125%, growing, e.g., from > 6.9MB/s to 15.6MB/s with two or three random readers in parallel. > > This commit also refactors the code related to device idling, for the > following reason. Finding the change that provides the above large > improvement has been slightly more difficult than it had to be, > because the logic that decides whether to idle the device is still > scattered across three functions. Almost all of the logic is in the > function bfq_bfqq_may_idle, but (1) part of the decision is made in > bfq_update_idle_window, and (2) the function bfq_bfqq_must_idle may > switch off idling regardless of the output of bfq_bfqq_may_idle. In > addition, both bfq_update_idle_window and bfq_bfqq_must_idle make > their decisions as a function of parameters that are used, for similar > purposes, also in bfq_bfqq_may_idle. This commit addresses this issue > by moving all the logic into bfq_bfqq_may_idle. This should be split into two patches - do one refactor patch that doesn't change anything, then your functional change on top of that. -- Jens Axboe
