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

Bart Van Assche
2017-07-13T14:56:38Z

On Thu, 2017-07-13 at 18:43 +0800, Ming Lei wrote:
> On Wed, Jul 12, 2017 at 03:39:14PM +0000, Bart Van Assche wrote:
> > On Wed, 2017-07-12 at 10:30 +0800, Ming Lei wrote:
> > > On Tue, Jul 11, 2017 at 12:25:16PM -0600, Jens Axboe wrote:
> > > > What happens with fluid congestion boundaries, with shared tags?
> > >=20
> > > The approach in this patch should work, but the threshold may not
> > > be accurate in this way, one simple method is to use the average
> > > tag weight in EWMA, like this:
> > >=20
> > > sbitmap_weight() / hctx->tags->active_queues
> >=20
> > Hello Ming,
> >=20
> > That approach would result in a severe performance degradation. "active=
_queues"
> > namely represents the number of queues against which I/O ever has been =
queued.
> > If e.g. 64 LUNs would be associated with a single SCSI host and all 64 =
LUNs are
> > responding and if the queue depth would also be 64 then the approach yo=
u
> > proposed will reduce the effective queue depth per LUN from 64 to 1.
>=20
> No, this approach does _not_ reduce the effective queue depth, it only
> stops the queue for a while when the queue is busy enough.
>
> In this case, there may not have congestion because for blk-mq at most al=
lows
> to assign queue_depth/active_queues tags to each LUN, please see hctx_may=
_queue().

Hello Ming,

hctx_may_queue() severely limits the queue depth if many LUNs are associate=
d
with the same SCSI host. I think that this is a performance regression
compared to scsi-sq and that this performance regression should be fixed.

Bart.=