Re: [PATCH 4/6] blk-mq: use EWMA to estimate congestion threshold

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Thu, 2017-07-13 at 23:32 +0800, Ming Lei wrote:
> On Thu, Jul 13, 2017 at 02:56:38PM +0000, Bart Van Assche wrote:
> > hctx_may_queue() severely limits the queue depth if many LUNs are assoc=
iated
> > with the same SCSI host. I think that this is a performance regression
> > compared to scsi-sq and that this performance regression should be fixe=
d.
>=20
> IMO, it is hard to evaluate/compare perf between scsi-mq vs scsi-sq:
>=20
> 	- how many LUNs do you run IO on concurrently?
> 	- evaluate the perf on single LUN or multi LUN?
>=20
> BTW, active_queues is a runtime variable which accounts the actual active
> queues in use.

Hello Ming,

What I described can be verified easily by running fio against a single LUN
of a SCSI host with which a large number of LUNs are associated.

BTW, something I overlooked in my analysis of the active_queues variable is
that it is not only decremented if a queue is destroyed but also if a queue
is idle during (block layer request timeout) seconds. So the problem I
described will only occur if some software submits I/O requests periodicall=
y
to all SCSI LUNs with a shorter interval than the I/O timeout. I'm not sure
any Linux software does this today. As an example, the time between path
checks by the multipathd TUR checker typically exceeds the timeout of a
single TUR check.

Bart.=