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authorJeff Moyer <jmoyer@redhat.com>2011-06-02 15:19:05 -0400
committerJens Axboe <jaxboe@fusionio.com>2011-06-02 15:19:05 -0400
commit796d5116c407690b14fd5bda136aa67a39e7061a (patch)
treebb0522aacb0a923e6636c82aff143df95ea1d730 /block/cfq-iosched.c
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/cfq-iosched.c')
-rw-r--r--block/cfq-iosched.c9
1 files changed, 2 insertions, 7 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 7c52d6888924..a2fb14bb1402 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1501,16 +1501,11 @@ static void cfq_add_rq_rb(struct request *rq)
1501{ 1501{
1502 struct cfq_queue *cfqq = RQ_CFQQ(rq); 1502 struct cfq_queue *cfqq = RQ_CFQQ(rq);
1503 struct cfq_data *cfqd = cfqq->cfqd; 1503 struct cfq_data *cfqd = cfqq->cfqd;
1504 struct request *__alias, *prev; 1504 struct request *prev;
1505 1505
1506 cfqq->queued[rq_is_sync(rq)]++; 1506 cfqq->queued[rq_is_sync(rq)]++;
1507 1507
1508 /* 1508 elv_rb_add(&cfqq->sort_list, rq);
1509 * looks a little odd, but the first insert might return an alias.
1510 * if that happens, put the alias on the dispatch list
1511 */
1512 while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
1513 cfq_dispatch_insert(cfqd->queue, __alias);
1514 1509
1515 if (!cfq_cfqq_on_rr(cfqq)) 1510 if (!cfq_cfqq_on_rr(cfqq))
1516 cfq_add_cfqq_rr(cfqd, cfqq); 1511 cfq_add_cfqq_rr(cfqd, cfqq);