Commit 6f5d8aa6 authored by Aaron Carroll's avatar Aaron Carroll Committed by Jens Axboe
Browse files

Deadline iosched: Fix batching fairness 



After switching data directions, deadline always starts the next batch
from the lowest-sector request.  This gives excessive deadline expiries
and large latency and throughput disparity between high- and low-sector
requests; an order of magnitude in some tests.

This patch changes the batching behaviour so new batches start from the
request whose expiry is earliest.

Signed-off-by: default avatarAaron Carroll <aaronc@gelato.unsw.edu.au>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent dfb3d72a

block/deadline-iosched.c

+7 −14
Original line number Diff line number Diff line
@@ -306,27 +306,20 @@ dispatch_writes: 
dispatch_find_request:

	/*

	 * we are not running a batch, find best request for selected data_dir

	 * and start a new batch

	 */

	if (deadline_check_fifo(dd, data_dir)) {

		/* An expired request exists - satisfy it */

	if (deadline_check_fifo(dd, data_dir) || !dd->next_rq[data_dir]) {

		/*

		 * A deadline has expired, the last request was in the other

		 * direction, or we have run out of higher-sectored requests.

		 * Start again from the request with the earliest expiry time.

		 */

		rq = rq_entry_fifo(dd->fifo_list[data_dir].next);

	} else if (dd->next_rq[data_dir]) {

	} else {

		/*

		 * The last req was the same dir and we have a next request in

		 * sort order. No expired requests so continue on from here.

		 */

		rq = dd->next_rq[data_dir];

	} else {

		struct rb_node *node;

		/*

		 * The last req was the other direction or we have run out of

		 * higher-sectored requests. Go back to the lowest sectored

		 * request (1 way elevator) and start a new batch.

		 */

		node = rb_first(&dd->sort_list[data_dir]);

		if (node)

			rq = rb_entry_rq(node);

	}



	dd->batching = 0;

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