Re: [bug report] shared tags causes IO hang and performance drop

[John Garry <john.garry@huawei.com>]

Hi Ming,

I'll have a look.

BTW, are you intentionally using scsi_debug over null_blk? null_blk 
supports shared sbitmap as well, and performance figures there are 
generally higher than scsi_debug for similar fio settings.

Thanks,
john

EOM


>> 		IOPs
>> mq-deadline	usr=21.72%, sys=44.18%,		423K
>> none		usr=23.15%, sys=74.01%		450K
> Today I re-run the scsi_debug test on two server hardwares(32cores, dual
> numa nodes), and the CPU utilization issue can be reproduced, follow
> the test result:
> 
> 1) randread test on ibm-x3850x6[*] with deadline
> 
>                |IOPS    | FIO CPU util
> ------------------------------------------------
> hosttags      | 94k    | usr=1.13%, sys=14.75%
> ------------------------------------------------
> non hosttags  | 124k   | usr=1.12%, sys=10.65%,
> 
> 
> 2) randread test on ibm-x3850x6[*] with none
>                |IOPS    | FIO CPU util
> ------------------------------------------------
> hosttags      | 120k   | usr=0.89%, sys=6.55%
> ------------------------------------------------
> non hosttags  | 121k   | usr=1.07%, sys=7.35%
> ------------------------------------------------
> 
>   *:
>   	- that is the machine Yanhui reported VM cpu utilization increased by 20%
> 	- kernel: latest linus tree(v5.12-rc7, commit: 7f75285ca57)
> 	- also run same test on another 32cores machine, IOPS drop isn't
> 	  observed, but CPU utilization is increased obviously
> 
> 3) test script
> #/bin/bash
> 
> run_fio() {
> 	RTIME=$1
> 	JOBS=$2
> 	DEVS=$3
> 	BS=$4
> 
> 	QD=64
> 	BATCH=16
> 
> 	fio --bs=$BS --ioengine=libaio \
> 		--iodepth=$QD \
> 	    --iodepth_batch_submit=$BATCH \
> 		--iodepth_batch_complete_min=$BATCH \
> 		--filename=$DEVS \
> 		--direct=1 --runtime=$RTIME --numjobs=$JOBS --rw=randread \
> 		--name=test --group_reporting
> }
> 
> SCHED=$1
> 
> NRQS=`getconf _NPROCESSORS_ONLN`
> 
> rmmod scsi_debug
> modprobe scsi_debug host_max_queue=128 submit_queues=$NRQS virtual_gb=256
> sleep 2
> DEV=`lsscsi | grep scsi_debug | awk '{print $6}'`
> echo $SCHED >/sys/block/`basename $DEV`/queue/scheduler
> echo 128 >/sys/block/`basename $DEV`/device/queue_depth
> run_fio 20 16 $DEV 8K
> 
> 
> rmmod scsi_debug
> modprobe scsi_debug max_queue=128 submit_queues=1 virtual_gb=256
> sleep 2
> DEV=`lsscsi | grep scsi_debug | awk '{print $6}'`
> echo $SCHED >/sys/block/`basename $DEV`/queue/scheduler
> echo 128 >/sys/block/`basename $DEV`/device/queue_depth
> run_fio 20 16 $DEV 8k