block: recursive merge requests

In my 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. With recursive merge below, the workload throughput gets improved 20% and context switch drops 60%. Signed-off-by: Shaohua Li <shaohua.li@intel.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>

block: recursive merge requests
In my 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. With recursive merge below, the workload throughput gets improved 20% and context switch drops 60%. Signed-off-by: Shaohua Li <shaohua.li@intel.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Shaohua Li · Jens Axboe
1 parent 4a0b75c7d0
Showing 1 changed file with 12 additions and 4 deletions Side-by-side Diff
block/elevator.c
@@ -515,6 +515,7 @@
 				     struct request *rq)
 {
 	struct request *__rq;
+	bool ret;
  
 	if (blk_queue_nomerges(q))
 		return false;
  
  
@@ -528,14 +529,21 @@
 	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;
  
-	return false;
+		/* The merged request could be merged with others, try again */
+		ret = true;
+		rq = __rq;
+	}
+
+	return ret;
 }
  
 void elv_merged_request(struct request_queue *q, struct request *rq, int type)
...	...	@@ -515,6 +515,7 @@
515	515	struct request *rq)
516	516	{
517	517	struct request *__rq;
	518	+ bool ret;
518	519
519	520	if (blk_queue_nomerges(q))
520	521	return false;
521	522
522	523
...	...	@@ -528,14 +529,21 @@
528	529	if (blk_queue_noxmerges(q))
529	530	return false;
530	531
	532	+ ret = false;
531	533	/*
532	534	* See if our hash lookup can find a potential backmerge.
533	535	*/
534		- __rq = elv_rqhash_find(q, blk_rq_pos(rq));
535		- if (__rq && blk_attempt_req_merge(q, __rq, rq))
536		- return true;
	536	+ while (1) {
	537	+ __rq = elv_rqhash_find(q, blk_rq_pos(rq));
	538	+ if (!__rq \|\| !blk_attempt_req_merge(q, __rq, rq))
	539	+ break;
537	540
538		- return false;
	541	+ /* The merged request could be merged with others, try again */
	542	+ ret = true;
	543	+ rq = __rq;
	544	+ }
	545	+
	546	+ return ret;
539	547	}
540	548
541	549	void elv_merged_request(struct request_queue q, struct request rq, int type)