Re: [PATCH V2 2/2] scsi: avoid to hold host-wide counter of host_busy for scsi_mq

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On 06/24/18 07:04, Ming Lei wrote:
It isn't necessary to check the host depth in scsi_queue_rq() any more
since it has been respected by blk-mq before calling scsi_queue_rq() via
getting driver tag.

Lots of LUNs may attach to same host, and per-host IOPS may reach millions
level, so we should avoid to this expensive atomic operations on the
hostwide counter in IO path.

This patch implemens scsi_host_busy() via blk_mq_tagset_busy_iter() for
reading the count of busy IOs for scsi_mq.

It is observed that IOPS is increased by 15% in IO test on scsi_debug
(32 LUNs, 32 submit queues, 1024 can_queue, libaio/dio) in one
dual-socket system.

Reviewed-by: Bart Van Assche <bart.vanassche@xxxxxxx>



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