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? > > > > > > 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: > > > > > > sbitmap_weight() / hctx->tags->active_queues > > > > Hello Ming, > > > > 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 you > > proposed will reduce the effective queue depth per LUN from 64 to 1. > > 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 allows > 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 associated 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.
