On Mon, Oct 09, 2017 at 11:04:39PM +0800, Ming Lei wrote:

Hi John,

On Mon, Oct 09, 2017 at 01:09:22PM +0100, John Garry wrote:

quoted

On 30/09/2017 11:27, Ming Lei wrote:

quoted

Hi Jens,

In Red Hat internal storage test wrt. blk-mq scheduler, we
found that I/O performance is much bad with mq-deadline, especially
about sequential I/O on some multi-queue SCSI devcies(lpfc, qla2xxx,
SRP...)

Turns out one big issue causes the performance regression: requests
are still dequeued from sw queue/scheduler queue even when ldd's
queue is busy, so I/O merge becomes quite difficult to make, then
sequential IO degrades a lot.

This issue becomes one of mains reasons for reverting default SCSI_MQ
in V4.13.

The 1st patch takes direct issue in blk_mq_request_bypass_insert(),
then we can improve dm-mpath's performance in part 2, which will
be posted out soon.

The 2nd six patches improve this situation, and brings back
some performance loss.

With this change, SCSI-MQ sequential I/O performance is
improved much, Paolo reported that mq-deadline performance
improved much[2] in his dbench test wrt V2. Also performanc
improvement on lpfc/qla2xx was observed with V1.[1]

Please consider it for V4.15.

[1] http://marc.info/?l=linux-block&m=150151989915776&w=2
[2] https://marc.info/?l=linux-block&m=150217980602843&w=2

I tested this series for the SAS controller on HiSilicon hip07 platform as I
am interested in enabling MQ for this driver. Driver is
./drivers/scsi/hisi_sas/.

So I found that that performance is improved when enabling default SCSI_MQ
with this series vs baseline. However, it is still not as a good as when
default SCSI_MQ is disabled.

Here are some figures I got with fio:
4.14-rc2 without default SCSI_MQ
read, rw, write IOPS	
952K, 133K/133K, 800K

4.14-rc2 with default SCSI_MQ
read, rw, write IOPS	
311K, 117K/117K, 320K

This series* without default SCSI_MQ
read, rw, write IOPS	
975K, 132K/132K, 790K

This series* with default SCSI_MQ
read, rw, write IOPS	
770K, 164K/164K, 594K

Thanks for testing this patchset!

Looks there is big improvement, but the gap compared with
block legacy is not small too.

quoted

Please note that hisi_sas driver does not enable mq by exposing multiple
queues to upper layer (even though it has multiple queues). I have been
playing with enabling it, but my performance is always worse...

* I'm using
https://github.com/ming1/linux/commits/blk_mq_improve_scsi_mpath_perf_V5.1,
as advised by Ming Lei.

Could you test on the following branch and see if it makes a
difference?

	https://github.com/ming1/linux/commits/blk_mq_improve_scsi_mpath_perf_V6.1_test

Hi John,

Please test the following branch directly:

https://github.com/ming1/linux/tree/blk_mq_improve_scsi_mpath_perf_V6.2_test

And code is simplified and cleaned up much in V6.2, then only two extra
patches(top 2) are needed against V6 which was posted yesterday.

Please test SCSI_MQ with mq-deadline, which should be the default
mq scheduler on your HiSilicon SAS.

-- 
Ming