block, bfq: handle bursts of queue activations
Many popular I/O-intensive services or applications spawn or reactivate many parallel threads/processes during short time intervals. Examples are systemd during boot or git grep. These services or applications benefit mostly from a high throughput: the quicker the I/O generated by their processes is cumulatively served, the sooner the target job of these services or applications gets completed. As a consequence, it is almost always counterproductive to weight-raise any of the queues associated to the processes of these services or applications: in most cases it would just lower the throughput, mainly because weight-raising also implies device idling. To address this issue, an I/O scheduler needs, first, to detect which queues are associated with these services or applications. In this respect, we have that, from the I/O-scheduler standpoint, these services or applications cause bursts of activations, i.e., activations of different queues occurring shortly after each other. However, a shorter burst of activations may be caused also by the start of an application that does not consist in a lot of parallel I/O-bound threads (see the comments on the function bfq_handle_burst for details). In view of these facts, this commit introduces: 1) an heuristic to detect (only) bursts of queue activations caused by services or applications consisting in many parallel I/O-bound threads; 2) the prevention of device idling and weight-raising for the queues belonging to these bursts. Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@fb.com>
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