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

Bart Van Assche
2017-07-12T15:39:14Z

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

Hello Ming,

That approach would result in a severe performance degradation. "active_que=
ues"
namely represents the number of queues against which I/O ever has been queu=
ed.
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 you
proposed will reduce the effective queue depth per LUN from 64 to 1.

Bart.=