cfq-iosched: adapt slice to number of processes doing I/O
When the number of processes performing I/O concurrently increases, a fixed time slice per process will cause large latencies. This patch, if low_latency mode is enabled, will scale the time slice assigned to each process according to a 300ms target latency. In order to keep fairness among processes: * The number of active processes is computed using a special form of running average, that quickly follows sudden increases (to keep latency low), and decrease slowly (to have fairness in spite of rapid decreases of this value). To safeguard sequential bandwidth, we impose a minimum time slice (computed using 2*cfq_slice_idle as base, adjusted according to priority and async-ness). Signed-off-by: NCorrado Zoccolo <czoccolo@gmail.com> Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
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