Re: [PATCH V6 4/5] blk-mq-sched: improve dispatching from sw queue

[Bart Van Assche]

On Thu, 2017-10-12 at 18:01 +0800, Ming Lei wrote:
> Even EWMA approach isn't good on SCSI-MQ too, because
> some SCSI's .cmd_per_lun is very small, such as 3 on
> lpfc and qla2xxx, and one full flush will trigger
> BLK_STS_RESOURCE easily.
> 
> So I suggest to use the way of q->queue_depth first, since we
> don't get performance degrade report on other devices(!q->queue_depth)
> with blk-mq. We can improve this way in the future if we
> have better approach.

Measurements have shown that even with this patch series applied sequential
I/O performance is still below that of the legacy block and SCSI layers. So
this patch series is not the final solution. (See also John Garry's e-mail
of October 10th - https://lkml.org/lkml/2017/10/10/401). I have been
wondering what could be causing that performance difference. Maybe it's
because requests can reside for a while in the hctx dispatch queue and hence
are unvisible for the scheduler while in the hctx dispatch queue? Should we
modify blk_mq_dispatch_rq_list() such that it puts back requests that have
not been accepted by .queue_rq() onto the scheduler queue(s) instead of to
the hctx dispatch queue? If we would make that change, would it allow us to
drop patch "blk-mq-sched: improve dispatching from sw queue"?

Bart.