[PATCH BUGFIX/IMPROVEMENT 1/6] block, bfq: use half slice_idle as a threshold to check short ttime

From: Paolo Valente
Date: Fri Jan 22 2021 - 14:19:40 EST

The value of the I/O plugging (idling) timeout is used also as the
think-time threshold to decide whether a process has a short think
time. In this respect, a good value of this timeout for rotational
drives is un the order of several ms. Yet, this is often too long a
time interval to be effective as a think-time threshold. This commit
mitigates this problem (by a lot, according to tests), by halving the

Tested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
block/bfq-iosched.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 9e4eb0fc1c16..eb2ca32d5b63 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5238,12 +5238,13 @@ static void bfq_update_has_short_ttime(struct bfq_data *bfqd,

/* Think time is infinite if no process is linked to
- * bfqq. Otherwise check average think time to
- * decide whether to mark as has_short_ttime
+ * bfqq. Otherwise check average think time to decide whether
+ * to mark as has_short_ttime. To this goal, compare average
+ * think time with half the I/O-plugging timeout.
if (atomic_read(&bic->icq.ioc->active_ref) == 0 ||
(bfq_sample_valid(bfqq->ttime.ttime_samples) &&
- bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle))
+ bfqq->ttime.ttime_mean > bfqd->bfq_slice_idle>>1))
has_short_ttime = false;

state_changed = has_short_ttime != bfq_bfqq_has_short_ttime(bfqq);