path: root/block
diff options
authorJeff Moyer <jmoyer@redhat.com>2011-06-02 21:19:05 +0200
committerJens Axboe <jaxboe@fusionio.com>2011-06-02 21:19:05 +0200
commit796d5116c407690b14fd5bda136aa67a39e7061a (patch)
treebb0522aacb0a923e6636c82aff143df95ea1d730 /block
parent1fa7b6a29c61358cc2ca6f64cef4aa0e1a7ca74c (diff)
iosched: prevent aliased requests from starving other I/O
Hi, Jens, If you recall, I posted an RFC patch for this back in July of last year: http://lkml.org/lkml/2010/7/13/279 The basic problem is that a process can issue a never-ending stream of async direct I/Os to the same sector on a device, thus starving out other I/O in the system (due to the way the alias handling works in both cfq and deadline). The solution I proposed back then was to start dispatching from the fifo after a certain number of aliases had been dispatched. Vivek asked why we had to treat aliases differently at all, and I never had a good answer. So, I put together a simple patch which allows aliases to be added to the rb tree (it adds them to the right, though that doesn't matter as the order isn't guaranteed anyway). I think this is the preferred solution, as it doesn't break up time slices in CFQ or batches in deadline. I've tested it, and it does solve the starvation issue. Let me know what you think. Cheers, Jeff Signed-off-by: Jeff Moyer <jmoyer@redhat.com> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Diffstat (limited to 'block')
3 files changed, 5 insertions, 15 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 7c52d688892..a2fb14bb140 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1501,16 +1501,11 @@ static void cfq_add_rq_rb(struct request *rq)
struct cfq_queue *cfqq = RQ_CFQQ(rq);
struct cfq_data *cfqd = cfqq->cfqd;
- struct request *__alias, *prev;
+ struct request *prev;
- /*
- * looks a little odd, but the first insert might return an alias.
- * if that happens, put the alias on the dispatch list
- */
- while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
- cfq_dispatch_insert(cfqd->queue, __alias);
+ elv_rb_add(&cfqq->sort_list, rq);
if (!cfq_cfqq_on_rr(cfqq))
cfq_add_cfqq_rr(cfqd, cfqq);
diff --git a/block/deadline-iosched.c b/block/deadline-iosched.c
index 5139c0ea186..c644137d9cd 100644
--- a/block/deadline-iosched.c
+++ b/block/deadline-iosched.c
@@ -77,10 +77,8 @@ static void
deadline_add_rq_rb(struct deadline_data *dd, struct request *rq)
struct rb_root *root = deadline_rb_root(dd, rq);
- struct request *__alias;
- while (unlikely(__alias = elv_rb_add(root, rq)))
- deadline_move_request(dd, __alias);
+ elv_rb_add(root, rq);
static inline void
diff --git a/block/elevator.c b/block/elevator.c
index b0b38ce0dcb..a3b64bc71d8 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -353,7 +353,7 @@ static struct request *elv_rqhash_find(struct request_queue *q, sector_t offset)
* RB-tree support functions for inserting/lookup/removal of requests
* in a sorted RB tree.
-struct request *elv_rb_add(struct rb_root *root, struct request *rq)
+void elv_rb_add(struct rb_root *root, struct request *rq)
struct rb_node **p = &root->rb_node;
struct rb_node *parent = NULL;
@@ -365,15 +365,12 @@ struct request *elv_rb_add(struct rb_root *root, struct request *rq)
if (blk_rq_pos(rq) < blk_rq_pos(__rq))
p = &(*p)->rb_left;
- else if (blk_rq_pos(rq) > blk_rq_pos(__rq))
+ else if (blk_rq_pos(rq) >= blk_rq_pos(__rq))
p = &(*p)->rb_right;
- else
- return __rq;
rb_link_node(&rq->rb_node, parent, p);
rb_insert_color(&rq->rb_node, root);
- return NULL;