On Tue, Nov 06, 2012 at 12:39:13PM +0100, Jens Axboe wrote:
> On 2012-11-06 12:34, Shaohua Li wrote:
> > request is queued in cfqq->fifo list. Looks it's possible we are
> > moving a request from one cfqq to another in request merge case. In
> > such case, adjusting the fifo list order doesn't make sense and is
> > impossible if we don't iterate the whole fifo list.
> >
> > My test does hit one case the two cfqq are different, but didn't cause
> > kernel crash, maybe it's because fifo list isn't used frequently.
> > Anyway, from the code logic, this is buggy.
>
> Good find!! Usually we never merge between cfqq's as our lookup basis is
> the cfqq. And yes, the fifo generally isn't used a lot, it's only a
> fallback measure to prevent inter-cfqq unfairness.
>
> Applied to for-3.8/core.
>
> And lets re-enable the recursive merging, please do send a patch for
> that too.
Here it is.
Subject: block: recursive merge requests
In a workload, thread 1 accesses a, a+2, ..., thread 2 accesses a+1, a+3,....
When the requests are flushed to queue, a and a+1 are merged to (a, a+1), a+2
and a+3 too to (a+2, a+3), but (a, a+1) and (a+2, a+3) aren't merged.
If we do recursive merge for such interleave access, some workloads throughput
get improvement. A recent worload I'm checking on is swap, below change
boostes the throughput around 5% ~ 10%.
Signed-off-by: Shaohua Li <s...@fusionio.com>
---
block/elevator.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
Index: linux/block/elevator.c
===================================================================
--- linux.orig/block/elevator.c 2012-10-15 10:01:52.763544641 +0800
+++ linux/block/elevator.c 2012-11-06 15:16:57.987363075 +0800
@@ -458,6 +458,7 @@ static bool elv_attempt_insert_merge(str
struct request *rq)
{
struct request *__rq;
+ bool ret;
if (blk_queue_nomerges(q))
return false;
@@ -471,14 +472,21 @@ static bool elv_attempt_insert_merge(str
if (blk_queue_noxmerges(q))
return false;
+ ret = false;
/*
* See if our hash lookup can find a potential backmerge.
*/
- __rq = elv_rqhash_find(q, blk_rq_pos(rq));
- if (__rq && blk_attempt_req_merge(q, __rq, rq))
- return true;
+ while (1) {
+ __rq = elv_rqhash_find(q, blk_rq_pos(rq));
+ if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
+ break;
+
+ /* The merged request could be merged with others, try again */
+ ret = true;
+ rq = __rq;
+ }
- return false;
+ return ret;
}
void elv_merged_request(struct request_queue *q, struct request *rq, int type)
--
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