Re: [PATCH]cfq-iosched: don't take requests with long distence as close

Jeff Moyer
2010-01-07T14:30:31Z

Corrado Zoccolo <czoccolo@gmail.com> writes:

> On Tue, Jan 5, 2010 at 10:16 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
>>
>> For now, I'm leaning towards asking Jens to revert this.  It may still
>> be worth making sure that we don't merge a seeky queue with a non-seeky
>> queue.  I have a patch for that if folks are interested.
> Jeff, can you send this patch to Yanmin, that is investigating a
> regression apparently caused by excessive queue merge?
>
> http://lkml.org/lkml/2010/1/4/194

You first have to back out Shaohua's patch, then apply this one.

Cheers,
Jeff

cfq-iosched: don't allow merging with seeky queues

Shaohua Li noticed that cfq currently can merge with seeky queues, which
causes unwanted merge/unmerge activity. We already know that the
cur_cfqq is not seeky, so this patch just makes sure that the non-seeky
queue is not merged with a seeky one.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 8df4fe5..3db9050 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1677,6 +1677,10 @@ static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
return cfq_dist_from_last(cfqd, rq) <= sdist;
}

+/*
+ * Search for a cfqq that is issuing non-seeky I/Os within the seek
+ * mean of the current cfqq.
+ */
static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
struct cfq_queue *cur_cfqq)
{
@@ -1701,7 +1705,14 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
* will contain the closest sector.
*/
__cfqq = rb_entry(parent, struct cfq_queue, p_node);
- if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
+ /*
+ * If the cfqq does not have enough seek samples, assume it is
+ * sequential until proven otherwise. If it is assumed that the
+ * queue is seeky first, then the close cooperator detection logic
+ * may never trigger as one queue strays further from the other(s).
+ */
+ if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq) &&
+ (!sample_valid(__cfqq->seek_samples) || !CFQQ_SEEKY(__cfqq)))
return __cfqq;

if (blk_rq_pos(__cfqq->next_rq) < sector)
@@ -1712,7 +1723,8 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
return NULL;

__cfqq = rb_entry(node, struct cfq_queue, p_node);
- if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
+ if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq) &&
+ (!sample_valid(__cfqq->seek_samples) || !CFQQ_SEEKY(__cfqq)))
return __cfqq;

return NULL;