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* About scsi device queue depth
@ 2021-01-11 16:21 John Garry
  2021-01-11 16:40 ` James Bottomley
                   ` (2 more replies)
  0 siblings, 3 replies; 22+ messages in thread
From: John Garry @ 2021-01-11 16:21 UTC (permalink / raw)
  To: Ming Lei, Bart Van Assche, Hannes Reinecke, Kashyap Desai,
	Martin K . Petersen, James E.J. Bottomley, Christoph Hellwig,
	linux-scsi, Sathya Prakash, Sreekanth Reddy,
	Suganath Prabu Subramani, PDL-MPT-FUSIONLINUX
  Cc: chenxiang

Hi,

I was looking at some IOMMU issue on a LSI RAID 3008 card, and noticed 
that performance there is not what I get on other SAS HBAs - it's lower.

After some debugging and fiddling with sdev queue depth in mpt3sas 
driver, I am finding that performance changes appreciably with sdev 
queue depth:

sdev qdepth	fio number jobs* 	1	10	20
16					1590	1654	1660
32					1545	1646	1654
64					1436	1085	1070
254 (default)				1436	1070	1050

fio queue depth is 40, and I'm using 12x SAS SSDs.

I got comparable disparity in results for fio queue depth = 128 and num 
jobs = 1:

sdev qdepth	fio number jobs* 	1	
16					1640
32					1618	
64					1577	
254 (default)				1437	

IO sched = none.

That driver also sets queue depth tracking = 1, but never seems to kick in.

So it seems to me that the block layer is merging more bios per request, 
as averge sg count per request goes up from 1 - > upto 6 or more. As I 
see, when queue depth lowers the only thing that is really changing is 
that we fail more often in getting the budget in 
scsi_mq_get_budget()->scsi_dev_queue_ready().

So initial sdev queue depth comes from cmd_per_lun by default or 
manually setting in the driver via scsi_change_queue_depth(). It seems 
to me that some drivers are not setting this optimally, as above.

Thoughts on guidance for setting sdev queue depth? Could blk-mq changed 
this behavior?

Thanks,
John

^ permalink raw reply	[flat|nested] 22+ messages in thread

end of thread, other threads:[~2021-01-13 15:41 UTC | newest]

Thread overview: 22+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-01-11 16:21 About scsi device queue depth John Garry
2021-01-11 16:40 ` James Bottomley
2021-01-11 17:11   ` John Garry
2021-01-12  6:35     ` James Bottomley
2021-01-12 10:27       ` John Garry
2021-01-12 16:40         ` Bryan Gurney
2021-01-12 16:47         ` James Bottomley
2021-01-12 17:20           ` Bryan Gurney
2021-01-11 17:31   ` Douglas Gilbert
2021-01-13  6:07   ` Martin K. Petersen
2021-01-13  6:36     ` Damien Le Moal
2021-01-12  1:42 ` Ming Lei
2021-01-12  8:56   ` John Garry
2021-01-12  9:06     ` Ming Lei
2021-01-12  9:23       ` John Garry
2021-01-12 11:44         ` Kashyap Desai
2021-01-13 12:17           ` John Garry
2021-01-13 13:34             ` Kashyap Desai
2021-01-13 15:39               ` John Garry
2021-01-12 17:44       ` John Garry
2021-01-12  7:23 ` Hannes Reinecke
2021-01-12  9:15   ` John Garry

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