[PATCH BUGFIX IMPROVEMENT 1/8] block, bfq: increase idling for weight-raised queues

From: Paolo Valente
Date: Thu Mar 07 2019 - 11:26:58 EST

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

Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
block/bfq-iosched.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 4c592496a16a..eb658de3cc40 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2545,6 +2545,8 @@ static void bfq_arm_slice_timer(struct bfq_data *bfqd)
if (BFQQ_SEEKY(bfqq) && bfqq->wr_coeff == 1 &&
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),