[PATCH RFC v1 5/5] scsi: storvsc: Hardware queues share blk_mq_tags

["Melanie Plageman (Microsoft)" <melanieplageman@gmail.com>]

Decouple the number of tags available from the number of hardware queues
by sharing a single blk_mq_tags amongst all hardware queues.

When storage latency is relatively high, having too many tags available
can harm the performance of mixed workloads.
By sharing blk_mq_tags amongst hardware queues, nr_requests can be set
to the appropriate number of tags for the device.

Signed-off-by: Melanie Plageman <melanieplageman@gmail.com>
---
As an example, on a 16-core VM coupled with a 1 TiB storage device having a
combined (VM + disk) max BW of 200 MB/s and IOPS of 5000, configured with 16
hardware queues and with nr_requests set to 56 and queue_depth set to 15, the
following fio job description illustrates the benefit of hardware queues sharing
blk_mq_tags:

[global]
time_based=1
ioengine=io_uring
direct=1
runtime=60

[read_hogs]
bs=16k
iodepth=10000
rw=randread
filesize=10G
numjobs=15
directory=/mnt/test

[wal]
bs=8k
iodepth=3
filesize=4G
rw=write
numjobs=1
directory=/mnt/test

with hctx_share_tags set, the "wal" job does 271 IOPS, averaging 13120 usec
completion latency and the "read_hogs" jobs average around 4700 IOPS.

without hctx_share_tags set, the "wal" job does 85 IOPS and averages around
45308 usec completion latency and the "read_hogs" job average around 4900 IOPS.

Note that reducing nr_requests to a number sufficient to increase WAL IOPS
results in unacceptably low IOPS for the random reads when only one random read
job is running.

 drivers/scsi/storvsc_drv.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 0ed764bcabab..5048e7fcf959 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -1997,6 +1997,7 @@ static struct scsi_host_template scsi_driver = {
 	.track_queue_depth =	1,
 	.change_queue_depth =	storvsc_change_queue_depth,
 	.per_device_tag_set =	1,
+	.hctx_share_tags = 1,
 };
 
 enum {
-- 
2.25.1