The original formula was inaccurate:
dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
For write requests, when we assign a tags from sched_tags,
data->shallow_depth will be passed to sbitmap_find_bit,
see the following code:
nr = sbitmap_find_bit_in_word(&sb->map[index],
min_t (unsigned int,
__map_depth(sb, index),
depth),
alloc_hint, wrap);
The smaller of data->shallow_depth and __map_depth(sb, index)
will be used as the maximum range when allocating bits.
For a mmc device (one hw queue, deadline I/O scheduler):
q->nr_requests = sched_tags = 128, so according to the previous
calculation method, dd->async_depth = data->shallow_depth = 96,
and the platform is 64bits with 8 cpus, sched_tags.bitmap_tags.sb.shift=5,
sb.maps[]=32/32/32/32, 32 is smaller than 96, whether it is a read or
a write I/O, tags can be allocated to the maximum range each time,
which has not throttling effect.
In addition, refer to the methods of bfg/kyber I/O scheduler,
limit ratiois are calculated base on sched_tags.bitmap_tags.sb.shift.
This patch can throttle write requests really.
Fixes: 07757588e507 ("block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests")
Signed-off-by: Zhiguo Niu <zhiguo.niu@xxxxxxxxxx>
---
block/mq-deadline.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 5839a027e0f0..7e043d4a78f8 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -620,8 +620,9 @@ static void dd_depth_updated(struct blk_mq_hw_ctx *hctx)
struct request_queue *q = hctx->queue;
struct deadline_data *dd = q->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;
+ unsigned int shift = tags->bitmap_tags.sb.shift;
- dd->async_depth = max(1UL, 3 * q->nr_requests / 4);
+ dd->async_depth = max(1U, 3 * (1U << shift) / 4);
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, dd->async_depth);
}
--
2.37.3
