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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>
This commit is contained in:
Aaron Carroll 2007-10-30 10:40:13 +01:00 committed by Jens Axboe
parent dfb3d72a9a
commit 6f5d8aa638
1 changed files with 7 additions and 14 deletions
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21
block/deadline-iosched.c
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@ -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;