[PATCH 3/3] Deadline iosched: Fix batching fairness

Aaron Carroll
2007-10-30T04:13:25Z

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>
---
block/deadline-iosched.c | 21 +++++++--------------
1 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/block/deadline-iosched.c b/block/deadline-iosched.c
index cb94c83..a054eef 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;