[dm-devel] [PATCH 1/5] block: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE

[Bart Van Assche]

On Wed, 2017-09-20 at 06:44 +0800, Ming Lei wrote:
> For this issue, it isn't same between SCSI and dm-rq.
> 
> We don't need to run queue in .end_io of dm, and the theory is
> simple, otherwise it isn't performance issue, and should be I/O hang.
> 
> 1) every dm-rq's request is 1:1 mapped to SCSI's request
> 
> 2) if there is any mapped SCSI request not finished, either
> in-flight or in requeue list or whatever, there will be one
> corresponding dm-rq's request in-flight
> 
> 3) once the mapped SCSI request is completed, dm-rq's completion
> path will be triggered and dm-rq's queue will be rerun because of
> SCHED_RESTART in dm-rq
> 
> So the hw queue of dm-rq has been run in dm-rq's completion path
> already, right? Why do we need to do it again in the hot path?

The measurement data in the description of patch 5/5 shows a significant
performance regression for an important workload, namely random I/O.
Additionally, the performance improvement for sequential I/O was achieved
for an unrealistically low queue depth. Sorry but given these measurement
results I don't see why I should spend more time on this patch series.

Bart.