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block: introduce the BFQ-v5r1 I/O sched for 3.7
Add the BFQ-v5r1 I/O scheduler to 3.7. The general structure is borrowed from CFQ, as much code. A (bfq_)queue is associated to each task doing I/O on a device, and each time a scheduling decision has to be taken a queue is selected and it is served until it expires. - Slices are given in the service domain: tasks are assigned budgets, measured in number of sectors. Once got the disk, a task must however consume its assigned budget within a configurable maximum time (by default, the maximum possible value of the budgets is automatically computed to comply with this timeout). This allows the desired latency vs "throughput boosting" tradeoff to be set. - Budgets are scheduled according to a variant of WF2Q+, implemented using an augmented rb-tree to take eligibility into account while preserving an O(log N) overall complexity. - A low-latency tunable is provided; if enabled, both interactive and soft real-time applications are guaranteed very low latency. - Latency guarantees are preserved also in presence of NCQ. - High throughput with flash-based devices, while still preserving latency guarantees. - Useful features borrowed from CFQ: cooperating-queues merging (with some additional optimizations with respect to the original CFQ version), static fallback queue for OOM. - BFQ supports full hierarchical scheduling, exporting a cgroups interface. Each node has a full scheduler, so each group can be assigned its own ioprio and an ioprio_class. - If the cgroups interface is used, weights can be explictly assigned, otherwise ioprio values are mapped to weights using the relation weight = IOPRIO_BE_NR - ioprio. - ioprio classes are served in strict priority order, i.e., lower priority queues are not served as long as there are higher priority queues. Among queues in the same class the bandwidth is distributed in proportion to the weights of each queue. A very thin extra bandwidth is however guaranteed to the Idle class, to prevent it from starving. Signed-off-by: Paolo Valente <firstname.lastname@example.org> Signed-off-by: Arianna Avanzini <email@example.com>
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