block, bfq: increase idling for weight-raised queues
If a sync bfq_queue has a higher weight than some other queue, and remains temporarily empty while in service, then, to preserve the bandwidth share of the queue, it is necessary to plug I/O dispatching until a new request arrives for the queue. In addition, a timeout needs to be set, to avoid waiting for ever if the process associated with the queue has actually finished its I/O. Even with the above timeout, the device is however not fed with new I/O for a while, if the process has finished its I/O. If this happens often, then throughput drops and latencies grow. For this reason, the timeout is kept rather low: 8 ms is the current default. Unfortunately, such a low value may cause, on the opposite end, a violation of bandwidth guarantees for a process that happens to issue new I/O too late. The higher the system load, the higher the probability that this happens to some process. This is a problem in scenarios where service guarantees matter more than throughput. One important case are weight-raised queues, which need to be granted a very high fraction of the bandwidth. To address this issue, this commit lower-bounds the plugging timeout for weight-raised queues to 20 ms. This simple change provides relevant benefits. For example, on a PLEXTOR PX-256M5S, with which gnome-terminal starts in 0.6 seconds if there is no other I/O in progress, the same applications starts in - 0.8 seconds, instead of 1.2 seconds, if ten files are being read sequentially in parallel - 1 second, instead of 2 seconds, if, in parallel, five files are being read sequentially, and five more files are being written sequentially Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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@ -2545,6 +2545,8 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd)
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if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
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bfq_symmetric_scenario(bfqd))
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sl = min_t(u64, sl, BFQ_MIN_TT);
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else if (bfqq->wr_coeff > 1)
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sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);
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bfqd->last_idling_start = ktime_get();
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hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),
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