On Thu, Mar 01, 2018 at 10:54:17AM +0530, Kashyap Desai wrote: > > -----Original Message----- > > From: Laurence Oberman [mailto:loberman@xxxxxxxxxx] > > Sent: Wednesday, February 28, 2018 9:52 PM > > To: Ming Lei; Kashyap Desai > > Cc: Jens Axboe; linux-block@xxxxxxxxxxxxxxx; Christoph Hellwig; Mike > > Snitzer; > > linux-scsi@xxxxxxxxxxxxxxx; Hannes Reinecke; Arun Easi; Omar Sandoval; > > Martin K . Petersen; James Bottomley; Christoph Hellwig; Don Brace; Peter > > Rivera > > Subject: Re: [PATCH V3 8/8] scsi: megaraid: improve scsi_mq performance > > via > > .host_tagset > > > > On Wed, 2018-02-28 at 23:21 +0800, Ming Lei wrote: > > > On Wed, Feb 28, 2018 at 08:28:48PM +0530, Kashyap Desai wrote: > > > > Ming - > > > > > > > > Quick testing on my setup - Performance slightly degraded (4-5% > > > > drop)for megaraid_sas driver with this patch. (From 1610K IOPS it > > > > goes to > > > > 1544K) > > > > I confirm that after applying this patch, we have #queue = #numa > > > > node. > > > > > > > > ls -l > > > > /sys/devices/pci0000:80/0000:80:02.0/0000:83:00.0/host10/target10:2 > > > > :23/10: > > > > 2:23:0/block/sdy/mq > > > > total 0 > > > > drwxr-xr-x. 18 root root 0 Feb 28 09:53 0 drwxr-xr-x. 18 root root 0 > > > > Feb 28 09:53 1 > > > > > > OK, thanks for your test. > > > > > > As I mentioned to you, this patch should have improved performance on > > > megaraid_sas, but the current slight degrade might be caused by > > > scsi_host_queue_ready() in scsi_queue_rq(), I guess. > > > > > > With .host_tagset enabled and use per-numa-node hw queue, request can > > > be queued to lld more frequently/quick than single queue, then the > > > cost of > > > atomic_inc_return(&host->host_busy) may be increased much meantime, > > > think about millions of such operations, and finally slight IOPS drop > > > is observed when the hw queue depth becomes half of .can_queue. > > > > > > > > > > > > > > > I would suggest to skip megaraid_sas driver changes using > > > > shared_tagset until and unless there is obvious gain. If overall > > > > interface of using shared_tagset is commit in kernel tree, we will > > > > investigate (megaraid_sas > > > > driver) in future about real benefit of using it. > > > > > > I'd suggest to not merge it until it is proved that performance can be > > > improved in real device. > > Noted. > > > > > > > I will try to work to remove the expensive atomic_inc_return(&host- > > > >host_busy) > > > from scsi_queue_rq(), since it isn't needed for SCSI_MQ, once it is > > > done, will ask you to test again. > > Ming - Do you mean removing host_busy stats from scsi_queue_rq() will still > provide correct value in host_busy whenever IO reach to LLD ? The host queue depth has been respected by blk-mq already before calling scsi_queue_rq(), so not necessary to do it again in scsi_queue_rq(), but this counter is needed in error handler, so we have to figure out one way to not break error handler. Also megaraid_sas driver need to be checked if there is host wide lock used in .queuecommand or completion path. Thanks, Ming
