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(). Then get_driver_tag() can only allow to return one pending tag at most to the request_queue(LUN). The algorithm in this patch only starts to work when congestion happens, that said it is only run when BLK_STS_RESOURCE is returned from .queue_rq(). This approach is for avoiding to dispatch requests to one busy queue unnecessarily, so that we don't need to heat CPU unnecessarily, and merge gets improved meantime. -- Ming
