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block, bfq: boost the throughput on NCQ-capable flash-based devices
This patch boosts the throughput on NCQ-capable flash-based devices, while still preserving latency guarantees for interactive and soft real-time applications. The throughput is boosted by just not idling the device when the in-service queue remains empty, even if the queue is sync and has a non-null idle window. This helps to keep the drive's internal queue full, which is necessary to achieve maximum performance. This solution to boost the throughput is a port of commits a68bbdd and f7d7b7a for CFQ. As already highlighted in a previous patch, allowing the device to prefetch and internally reorder requests trivially causes loss of control on the request service order, and hence on service guarantees. Fortunately, as discussed in detail in the comments on the function bfq_bfqq_may_idle(), if every process has to receive the same fraction of the throughput, then the service order enforced by the internal scheduler of a flash-based device is relatively close to that enforced by BFQ. In particular, it is close enough to let service guarantees be substantially preserved. Things change in an asymmetric scenario, i.e., if not every process has to receive the same fraction of the throughput. In this case, to guarantee the desired throughput distribution, the device must be prevented from prefetching requests. This is exactly what this patch does in asymmetric scenarios. Signed-off-by: Paolo Valente <[email protected]> Signed-off-by: Arianna Avanzini <[email protected]> Signed-off-by: Jens Axboe <[email protected]>
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