Re: Slow file transfer speeds with CFQ IO scheduler in some cases

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Nov 11 2008, Jens Axboe wrote:
> On Tue, Nov 11 2008, Jens Axboe wrote:
> > On Mon, Nov 10 2008, Jeff Moyer wrote:
> > > "Vitaly V. Bursov" <vitalyb@telenet.dn.ua> writes:
> > > 
> > > > Jens Axboe wrote:
> > > >> On Mon, Nov 10 2008, Vitaly V. Bursov wrote:
> > > >>> Jens Axboe wrote:
> > > >>>> On Mon, Nov 10 2008, Jeff Moyer wrote:
> > > >>>>> Jens Axboe <jens.axboe@oracle.com> writes:
> > > >>>>>
> > > >>>>>> http://bugzilla.kernel.org/attachment.cgi?id=18473&action=view
> > > >>>>> Funny, I was going to ask the same question.  ;)  The reason Jens wants
> > > >>>>> you to try this patch is that nfsd may be farming off the I/O requests
> > > >>>>> to different threads which are then performing interleaved I/O.  The
> > > >>>>> above patch tries to detect this and allow cooperating processes to get
> > > >>>>> disk time instead of waiting for the idle timeout.
> > > >>>> Precisely :-)
> > > >>>>
> > > >>>> The only reason I haven't merged it yet is because of worry of extra
> > > >>>> cost, but I'll throw some SSD love at it and see how it turns out.
> > > >>>>
> > > >>> Sorry, but I get "oops" same moment nfs read transfer starts.
> > > >>> I can get directory list via nfs, read files locally (not
> > > >>> carefully tested, though)
> > > >>>
> > > >>> Dumps captured via netconsole, so these may not be completely accurate
> > > >>> but hopefully will give a hint.
> > > >> 
> > > >> Interesting, strange how that hasn't triggered here. Or perhaps the
> > > >> version that Jeff posted isn't the one I tried. Anyway, search for:
> > > >> 
> > > >>         RB_CLEAR_NODE(&cfqq->rb_node);
> > > >> 
> > > >> and add a
> > > >> 
> > > >>         RB_CLEAR_NODE(&cfqq->prio_node);
> > > >> 
> > > >> just below that. It's in cfq_find_alloc_queue(). I think that should fix
> > > >> it.
> > > >> 
> > > >
> > > > Same problem.
> > > >
> > > > I did make clean; make -j3; sync; on (2 times) patched kernel and it went OK
> > > > but It won't boot anymore with cfq with same error...
> > > >
> > > > Switching cfq io scheduler at runtime (booting with "as") appears to work with
> > > > two parallel local dd reads.
> > > 
> > > Strange, I can't reproduce a failure.  I'll keep trying.  For now, these
> > > are the results I see:
> > > 
> > > [root@maiden ~]# mount megadeth:/export/cciss /mnt/megadeth/
> > > [root@maiden ~]# dd if=/mnt/megadeth/file1 of=/dev/null bs=1M
> > > 1024+0 records in
> > > 1024+0 records out
> > > 1073741824 bytes (1.1 GB) copied, 26.8128 s, 40.0 MB/s
> > > [root@maiden ~]# umount /mnt/megadeth/
> > > [root@maiden ~]# mount megadeth:/export/cciss /mnt/megadeth/
> > > [root@maiden ~]# dd if=/mnt/megadeth/file1 of=/dev/null bs=1M
> > > 1024+0 records in
> > > 1024+0 records out
> > > 1073741824 bytes (1.1 GB) copied, 23.7025 s, 45.3 MB/s
> > > [root@maiden ~]# umount /mnt/megadeth/
> > > 
> > > Here is the patch, with the suggestion from Jens to switch the cfqq to
> > > the right priority tree when the priority is changed.
> > 
> > I don't see the issue here either. Vitaly, are you using any openvz
> > kernel patches? IIRC, they patch cfq so it could just be that your cfq
> > version is incompatible with Jeff's patch.
> 
> Heh, got it to trigger about 3 seconds after sending that email! I'll
> look more into it.

OK, found the issue. A few bugs there... cfq_prio_tree_lookup() doesn't
even return a hit, since it just breaks and returns NULL always. That
can cause cfq_prio_tree_add() to screw up the rbtree. The code to
correct on ioprio change wasn't correct either, I changed that as well.
New patch below, Vitaly can you give it a spin?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..b1f1b47 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -83,6 +83,12 @@ struct cfq_data {
 	 * rr list of queues with requests and the count of them
 	 */
 	struct cfq_rb_root service_tree;
+	/*
+	 * Each priority tree is sorted by next_request position.  These
+	 * trees are used when determining if two or more queues are
+	 * interleaving requests (see cfq_close_cooperator).
+	 */
+	struct rb_root prio_trees[CFQ_PRIO_LISTS];
 	unsigned int busy_queues;
 
 	int rq_in_driver;
@@ -142,6 +148,9 @@ struct cfq_queue {
 	struct rb_node rb_node;
 	/* service_tree key */
 	unsigned long rb_key;
+	/* prio tree member */
+	struct rb_node prio_node;
+	struct rb_root *prio_root;
 	/* sorted list of pending requests */
 	struct rb_root sort_list;
 	/* if fifo isn't expired, next request to serve */
@@ -415,13 +424,17 @@ static struct cfq_queue *cfq_rb_first(struct cfq_rb_root *root)
 	return NULL;
 }
 
+static void rb_erase_init(struct rb_node *n, struct rb_root *root)
+{
+	rb_erase(n, root);
+	RB_CLEAR_NODE(n);
+}
+
 static void cfq_rb_erase(struct rb_node *n, struct cfq_rb_root *root)
 {
 	if (root->left == n)
 		root->left = NULL;
-
-	rb_erase(n, &root->rb);
-	RB_CLEAR_NODE(n);
+	rb_erase_init(n, &root->rb);
 }
 
 /*
@@ -540,6 +553,67 @@ static void cfq_service_tree_add(struct cfq_data *cfqd,
 	rb_insert_color(&cfqq->rb_node, &cfqd->service_tree.rb);
 }
 
+static struct cfq_queue *cfq_prio_tree_lookup(struct cfq_data *cfqd,
+	int ioprio, sector_t sector, struct rb_node **ret_parent,
+	struct rb_node ***rb_link)
+{
+	struct rb_root *root = &cfqd->prio_trees[ioprio];
+	struct rb_node **p, *parent;
+	struct cfq_queue *cfqq = NULL;
+
+	parent = NULL;
+	p = &root->rb_node;
+	while (*p) {
+		struct rb_node **n;
+
+		parent = *p;
+		cfqq = rb_entry(parent, struct cfq_queue, prio_node);
+
+		/*
+		 * Sort strictly based on sector.  Smallest to the left,
+		 * largest to the right.
+		 */
+		if (sector > cfqq->next_rq->sector)
+			n = &(*p)->rb_right;
+		else if (sector < cfqq->next_rq->sector)
+			n = &(*p)->rb_left;
+		else
+			return cfqq;
+		p = n;
+	}
+
+	*ret_parent = parent;
+	if (rb_link)
+		*rb_link = p;
+	return NULL;
+}
+
+static void cfq_prio_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq)
+{
+	struct rb_node **p, *parent;
+	struct cfq_queue *__cfqq;
+
+	if (cfqq->prio_root) {
+		rb_erase_init(&cfqq->prio_node, cfqq->prio_root);
+		cfqq->prio_root = NULL;
+	}
+
+	if (cfq_class_idle(cfqq))
+		return;
+	if (!cfqq->next_rq)
+		return;
+
+	/*
+	 * If an alias is found, don't bother adding this one
+	 */
+	__cfqq = cfq_prio_tree_lookup(cfqd, cfqq->ioprio, cfqq->next_rq->sector, &parent, &p);
+	if (!__cfqq) {
+		cfqq->prio_root = &cfqd->prio_trees[cfqq->ioprio];
+		rb_link_node(&cfqq->prio_node, parent, p);
+		rb_insert_color(&cfqq->prio_node, cfqq->prio_root);
+	}
+}
+
 /*
  * Update cfqq's position in the service tree.
  */
@@ -548,8 +622,10 @@ static void cfq_resort_rr_list(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	/*
 	 * Resorting requires the cfqq to be on the RR list already.
 	 */
-	if (cfq_cfqq_on_rr(cfqq))
+	if (cfq_cfqq_on_rr(cfqq)) {
 		cfq_service_tree_add(cfqd, cfqq, 0);
+		cfq_prio_tree_add(cfqd, cfqq);
+	}
 }
 
 /*
@@ -578,6 +654,10 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 
 	if (!RB_EMPTY_NODE(&cfqq->rb_node))
 		cfq_rb_erase(&cfqq->rb_node, &cfqd->service_tree);
+	if (cfqq->prio_root) {
+		rb_erase_init(&cfqq->prio_node, cfqq->prio_root);
+		cfqq->prio_root = NULL;
+	}
 
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
@@ -623,6 +703,9 @@ static void cfq_add_rq_rb(struct request *rq)
 	 * check if this request is a better next-serve candidate
 	 */
 	cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
+	/* this changes the position in the priority tree */
+	cfq_prio_tree_add(cfqd, cfqq);
+
 	BUG_ON(!cfqq->next_rq);
 }
 
@@ -831,11 +914,11 @@ static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
 /*
  * Get and set a new active queue for service.
  */
-static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd)
+static struct cfq_queue *cfq_set_active_queue(struct cfq_data *cfqd,
+					      struct cfq_queue *cfqq)
 {
-	struct cfq_queue *cfqq;
-
-	cfqq = cfq_get_next_queue(cfqd);
+	if (!cfqq)
+		cfqq = cfq_get_next_queue(cfqd);
 	__cfq_set_active_queue(cfqd, cfqq);
 	return cfqq;
 }
@@ -859,15 +942,78 @@ static inline int cfq_rq_close(struct cfq_data *cfqd, struct request *rq)
 	return cfq_dist_from_last(cfqd, rq) <= cic->seek_mean;
 }
 
-static int cfq_close_cooperator(struct cfq_data *cfq_data,
-				struct cfq_queue *cfqq)
+static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
+				    struct cfq_queue *cur_cfqq)
+{
+	struct rb_root *root = &cfqd->prio_trees[cur_cfqq->ioprio];
+	struct rb_node *parent, *node;
+	struct cfq_queue *__cfqq;
+	sector_t sector = cfqd->last_position;
+
+	if (RB_EMPTY_ROOT(root))
+		return NULL;
+
+	/*
+	 * First, if we find a request starting at the end of the last
+	 * request, choose it.
+	 */
+	__cfqq = cfq_prio_tree_lookup(cfqd, cur_cfqq->ioprio,
+				      sector, &parent, NULL);
+	if (__cfqq)
+		return __cfqq;
+
+	/*
+	 * If the exact sector wasn't found, the parent of the NULL leaf
+	 * will contain the closest sector.
+	 */
+	__cfqq = rb_entry(parent, struct cfq_queue, prio_node);
+	if (cfq_rq_close(cfqd, __cfqq->next_rq))
+		return __cfqq;
+	if (__cfqq->next_rq->sector < sector)
+		node = rb_next(&__cfqq->prio_node);
+	else
+		node = rb_prev(&__cfqq->prio_node);
+	if (!node)
+		return NULL;
+	__cfqq = rb_entry(node, struct cfq_queue, prio_node);
+	if (cfq_rq_close(cfqd, __cfqq->next_rq))
+		return __cfqq;
+
+	return NULL;
+}
+
+/*
+ * cfqd - obvious
+ * cur_cfqq - passed in so that we don't decide that the current queue is
+ * 	      closely cooperating with itself.
+ *
+ * So, basically we're assuming that that cur_cfqq has dispatched at least
+ * one request, and that cfqd->last_position reflects a position on the disk
+ * associated with the I/O issued by cur_cfqq.  I'm not sure this is a valid
+ * assumption.
+ */
+static struct cfq_queue *cfq_close_cooperator(struct cfq_data *cfqd,
+					      struct cfq_queue *cur_cfqq)
 {
+	struct cfq_queue *cfqq;
+
+	/*
+	 * A valid cfq_io_context is necessary to compare requests against
+	 * the seek_mean of the current cfqq.
+	 */
+	if (!cfqd->active_cic)
+		return NULL;
+
 	/*
 	 * We should notice if some of the queues are cooperating, eg
 	 * working closely on the same area of the disk. In that case,
 	 * we can group them together and don't waste time idling.
 	 */
-	return 0;
+	cfqq = cfqq_close(cfqd, cur_cfqq);
+	if (cfqq)
+		return cfqq;
+
+	return NULL;
 }
 
 #define CIC_SEEKY(cic) ((cic)->seek_mean > (8 * 1024))
@@ -908,13 +1054,6 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
 	if (!cic || !atomic_read(&cic->ioc->nr_tasks))
 		return;
 
-	/*
-	 * See if this prio level has a good candidate
-	 */
-	if (cfq_close_cooperator(cfqd, cfqq) &&
-	    (sample_valid(cic->ttime_samples) && cic->ttime_mean > 2))
-		return;
-
 	cfq_mark_cfqq_must_dispatch(cfqq);
 	cfq_mark_cfqq_wait_request(cfqq);
 
@@ -992,7 +1131,7 @@ cfq_prio_to_maxrq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
  */
 static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
 {
-	struct cfq_queue *cfqq;
+	struct cfq_queue *cfqq, *new_cfqq = NULL;
 
 	cfqq = cfqd->active_queue;
 	if (!cfqq)
@@ -1012,6 +1151,15 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
 		goto keep_queue;
 
 	/*
+ 	 * If another queue has a request waiting within our mean seek
+ 	 * distance, let it run.  The expire code will check for close
+ 	 * cooperators and put the close queue at the front of the service
+ 	 * tree.
+ 	 */
+	if ((new_cfqq = cfq_close_cooperator(cfqd, cfqq)))
+		goto expire;
+
+	/*
 	 * No requests pending. If the active queue still has requests in
 	 * flight or is idling for a new request, allow either of these
 	 * conditions to happen (or time out) before selecting a new queue.
@@ -1025,7 +1173,7 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
 expire:
 	cfq_slice_expired(cfqd, 0);
 new_queue:
-	cfqq = cfq_set_active_queue(cfqd);
+	cfqq = cfq_set_active_queue(cfqd, new_cfqq);
 keep_queue:
 	return cfqq;
 }
@@ -1174,6 +1322,7 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
 
 	cfq_log_cfqq(cfqd, cfqq, "put_queue");
 	BUG_ON(rb_first(&cfqq->sort_list));
+	BUG_ON(!RB_EMPTY_NODE(&cfqq->prio_node));
 	BUG_ON(cfqq->allocated[READ] + cfqq->allocated[WRITE]);
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
 
@@ -1386,6 +1535,15 @@ static void cfq_init_prio_data(struct cfq_queue *cfqq, struct io_context *ioc)
 	}
 
 	/*
+	 * Reposition in prio_tree array, if we are already there
+	 */
+	if (cfqq->prio_root) {
+		rb_erase_init(&cfqq->prio_node, cfqq->prio_root);
+		cfqq->prio_root = NULL;
+		cfq_prio_tree_add(cfqq->cfqd, cfqq);
+	}
+
+	/*
 	 * keep track of original prio settings in case we have to temporarily
 	 * elevate the priority of this queue
 	 */
@@ -1466,6 +1624,7 @@ retry:
 		}
 
 		RB_CLEAR_NODE(&cfqq->rb_node);
+		RB_CLEAR_NODE(&cfqq->prio_node);
 		INIT_LIST_HEAD(&cfqq->fifo);
 
 		atomic_set(&cfqq->ref, 0);
@@ -1949,7 +2108,18 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
 			cfq_set_prio_slice(cfqd, cfqq);
 			cfq_clear_cfqq_slice_new(cfqq);
 		}
-		if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
+		/*
+		 * If there are no requests waiting in this queue, and
+		 * there are other queues ready to issue requests, AND
+		 * those other queues are issuing requests within our
+		 * mean seek distance, give them a chance to run instead
+		 * of idling.
+		 */
+		if (RB_EMPTY_ROOT(&cfqq->sort_list) &&
+		    !RB_EMPTY_ROOT(&cfqd->service_tree.rb)) {
+			if (cfq_close_cooperator(cfqd, cfqq))
+				cfq_slice_expired(cfqd, 1);
+		} else if (cfq_slice_used(cfqq) || cfq_class_idle(cfqq))
 			cfq_slice_expired(cfqd, 1);
 		else if (sync && RB_EMPTY_ROOT(&cfqq->sort_list))
 			cfq_arm_slice_timer(cfqd);
@@ -2210,12 +2380,17 @@ static void cfq_exit_queue(elevator_t *e)
 static void *cfq_init_queue(struct request_queue *q)
 {
 	struct cfq_data *cfqd;
+	int i;
 
 	cfqd = kmalloc_node(sizeof(*cfqd), GFP_KERNEL | __GFP_ZERO, q->node);
 	if (!cfqd)
 		return NULL;
 
 	cfqd->service_tree = CFQ_RB_ROOT;
+
+	for (i = 0; i < CFQ_PRIO_LISTS; i++)
+		cfqd->prio_trees[i] = RB_ROOT;
+
 	INIT_LIST_HEAD(&cfqd->cic_list);
 
 	cfqd->queue = q;

-- 
Jens Axboe