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: Aaron Carroll <aaronc@gelato.unsw.edu.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 7 insertions, 14 deletions

diff --git a/block/deadline-iosched.c b/block/deadline-iosched.c
index cb94c838087a..a054eef8dff6 100644
--- a/block/deadline-iosched.c
+++ b/block/deadline-iosched.c
@@ -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;