[ Upstream commit 778c02a236a8728bb992de10ed1f12c0be5b7b0e ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- block/bfq-iosched.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 3b44bd28fc45..7d45ac451745 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -2282,6 +2282,8 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd) if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 && bfq_symmetric_scenario(bfqd)) sl = min_t(u64, sl, BFQ_MIN_TT); + else if (bfqq->wr_coeff > 1) + sl = max_t(u32, sl, 20ULL * NSEC_PER_MSEC);
bfqd->last_idling_start = ktime_get(); hrtimer_start(&bfqd->idle_slice_timer, ns_to_ktime(sl),