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Re: [dm-devel] Questions regarding request based dm-multipath and blk-layer/elevator



On Thursday 20 May 2010 00:01:47 Vivek Goyal wrote:
> On Wed, May 19, 2010 at 05:16:38PM +0530, Nikanth Karthikesan wrote:
> > Hi
> >
> > I have couple of questions regd request based dm.
> >
> > 1. With request based multipath, we have 2 elevators in the path to the
> > device. Doesn't 2 idling I/O schedulers affect performance? Is it better
> > to use the noop elevator for the dm device? What is suggested?
> > I can send a patch to set noop as default for rq based dm, if it would be
> > better.
> 
> IIUC, we don't use the ioscheduler of unerlying device and directly insert
> the request in request queue of underlying device. So double idling should
> not be an issue.
> 
> Look at blk_insert_cloned_request(), which uses __elv_add_request() with
> option ELEVATOR_INSERT_BACK.
> 

Oh, yes. Thanks a lot for the answer.

So changing the ioscheduler of the underlying device of a dm-mulitpath device 
will have no effect on I/O via the multipath device!

Thanks
Nikanth

> Thanks
> Vivek
> 
> > 2. The blk-layer limit nr_requests is the maximum nr of requests
> > per-queue. Currently we set it to the default maximum(128) and leave it.
> >
> > Instead would it be better to set it to a higher number based on the
> > number of paths(underlying devices) and their nr_requests? In bio-based
> > dm-multipath, we were queueing upto the sum of all the underlying queue's
> > nr_requests.
> >
> > For example the attached patch would set it to the sum of nr_requests of
> > all the targets. May be it is better to do it from the user-space daemon,
> > multipathd? Or just 128 requests is enough as in the end, it is going to
> > be a single LUN? Or should we simply use the nr_request from one of the
> > underlying device? Or the maximum nr_request amoung the underlying
> > devices?
> >
> > Thanks
> > Nikanth
> >
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 9fe174d..fc33c2d 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -91,6 +91,44 @@ void blk_queue_congestion_threshold(struct
> > request_queue *q) q->nr_congestion_off = nr;
> >  }
> >
> > +unsigned long
> > +__blk_queue_set_nr_requests(struct request_queue *q, unsigned long nr)
> > +{
> > +	struct request_list *rl = &q->rq;
> > +
> > +	if (nr < BLKDEV_MIN_RQ)
> > +		nr = BLKDEV_MIN_RQ;
> > +
> > +	q->nr_requests = nr;
> > +	blk_queue_congestion_threshold(q);
> > +
> > +	if (rl->count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
> > +		blk_set_queue_congested(q, BLK_RW_SYNC);
> > +	else if (rl->count[BLK_RW_SYNC] < queue_congestion_off_threshold(q))
> > +		blk_clear_queue_congested(q, BLK_RW_SYNC);
> > +
> > +	if (rl->count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
> > +		blk_set_queue_congested(q, BLK_RW_ASYNC);
> > +	else if (rl->count[BLK_RW_ASYNC] < queue_congestion_off_threshold(q))
> > +		blk_clear_queue_congested(q, BLK_RW_ASYNC);
> > +
> > +	if (rl->count[BLK_RW_SYNC] >= q->nr_requests) {
> > +		blk_set_queue_full(q, BLK_RW_SYNC);
> > +	} else if (rl->count[BLK_RW_SYNC]+1 <= q->nr_requests) {
> > +		blk_clear_queue_full(q, BLK_RW_SYNC);
> > +		wake_up(&rl->wait[BLK_RW_SYNC]);
> > +	}
> > +
> > +	if (rl->count[BLK_RW_ASYNC] >= q->nr_requests) {
> > +		blk_set_queue_full(q, BLK_RW_ASYNC);
> > +	} else if (rl->count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
> > +		blk_clear_queue_full(q, BLK_RW_ASYNC);
> > +		wake_up(&rl->wait[BLK_RW_ASYNC]);
> > +	}
> > +	return nr;
> > +}
> > +EXPORT_SYMBOL(__blk_queue_set_nr_requests);
> > +
> >  /**
> >   * blk_get_backing_dev_info - get the address of a queue's
> > backing_dev_info * @bdev:	device
> > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > index 306759b..615198c 100644
> > --- a/block/blk-sysfs.c
> > +++ b/block/blk-sysfs.c
> > @@ -39,7 +39,6 @@ static ssize_t queue_requests_show(struct request_queue
> > *q, char *page) static ssize_t
> >  queue_requests_store(struct request_queue *q, const char *page, size_t
> > count) {
> > -	struct request_list *rl = &q->rq;
> >  	unsigned long nr;
> >  	int ret;
> >
> > @@ -47,37 +46,11 @@ queue_requests_store(struct request_queue *q, const
> > char *page, size_t count) return -EINVAL;
> >
> >  	ret = queue_var_store(&nr, page, count);
> > -	if (nr < BLKDEV_MIN_RQ)
> > -		nr = BLKDEV_MIN_RQ;
> >
> >  	spin_lock_irq(q->queue_lock);
> > -	q->nr_requests = nr;
> > -	blk_queue_congestion_threshold(q);
> > -
> > -	if (rl->count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
> > -		blk_set_queue_congested(q, BLK_RW_SYNC);
> > -	else if (rl->count[BLK_RW_SYNC] < queue_congestion_off_threshold(q))
> > -		blk_clear_queue_congested(q, BLK_RW_SYNC);
> > -
> > -	if (rl->count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
> > -		blk_set_queue_congested(q, BLK_RW_ASYNC);
> > -	else if (rl->count[BLK_RW_ASYNC] < queue_congestion_off_threshold(q))
> > -		blk_clear_queue_congested(q, BLK_RW_ASYNC);
> > -
> > -	if (rl->count[BLK_RW_SYNC] >= q->nr_requests) {
> > -		blk_set_queue_full(q, BLK_RW_SYNC);
> > -	} else if (rl->count[BLK_RW_SYNC]+1 <= q->nr_requests) {
> > -		blk_clear_queue_full(q, BLK_RW_SYNC);
> > -		wake_up(&rl->wait[BLK_RW_SYNC]);
> > -	}
> > -
> > -	if (rl->count[BLK_RW_ASYNC] >= q->nr_requests) {
> > -		blk_set_queue_full(q, BLK_RW_ASYNC);
> > -	} else if (rl->count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
> > -		blk_clear_queue_full(q, BLK_RW_ASYNC);
> > -		wake_up(&rl->wait[BLK_RW_ASYNC]);
> > -	}
> > +	__blk_queue_set_nr_requests(q, nr);
> >  	spin_unlock_irq(q->queue_lock);
> > +
> >  	return ret;
> >  }
> >
> > diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> > index 9924ea2..c6deff9 100644
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -526,6 +526,18 @@ int dm_set_device_limits(struct dm_target *ti,
> > struct dm_dev *dev, }
> >  EXPORT_SYMBOL_GPL(dm_set_device_limits);
> >
> > +int dm_set_device_nr_requests(struct dm_target *ti, struct dm_dev *dev,
> > +			 sector_t start, sector_t len, void *data)
> > +{
> > +	unsigned long *nr_requests = data;
> > +	struct block_device *bdev = dev->bdev;
> > +	struct request_queue *q = bdev_get_queue(bdev);
> > +
> > +	*nr_requests += q->nr_requests;
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(dm_set_device_nr_requests);
> > +
> >  int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
> >  		  struct dm_dev **result)
> >  {
> > @@ -983,12 +995,13 @@ struct dm_target *dm_table_find_target(struct
> > dm_table *t, sector_t sector) * Establish the new table's queue_limits
> > and validate them.
> >   */
> >  int dm_calculate_queue_limits(struct dm_table *table,
> > -			      struct queue_limits *limits)
> > +	      struct queue_limits *limits, unsigned long *nr_requests)
> >  {
> >  	struct dm_target *uninitialized_var(ti);
> >  	struct queue_limits ti_limits;
> >  	unsigned i = 0;
> >
> > +	*nr_requests = 0;
> >  	blk_set_default_limits(limits);
> >
> >  	while (i < dm_table_get_num_targets(table)) {
> > @@ -1005,6 +1018,13 @@ int dm_calculate_queue_limits(struct dm_table
> > *table, ti->type->iterate_devices(ti, dm_set_device_limits,
> >  					  &ti_limits);
> >
> > +		/*
> > +		 * Combine queue nr_requests limit of all the devices this
> > +		 * target uses.
> > +		 */
> > +		ti->type->iterate_devices(ti, dm_set_device_nr_requests,
> > +					  nr_requests);
> > +
> >  		/* Set I/O hints portion of queue limits */
> >  		if (ti->type->io_hints)
> >  			ti->type->io_hints(ti, &ti_limits);
> > @@ -1074,7 +1094,7 @@ no_integrity:
> >  }
> >
> >  void dm_table_set_restrictions(struct dm_table *t, struct request_queue
> > *q, -			       struct queue_limits *limits)
> > +	       struct queue_limits *limits, unsigned long nr_requests)
> >  {
> >  	/*
> >  	 * Copy table's limits to the DM device's request_queue
> > @@ -1098,8 +1118,10 @@ void dm_table_set_restrictions(struct dm_table *t,
> > struct request_queue *q, * Those bios are passed to request-based dm at
> > the resume time. */
> >  	smp_mb();
> > -	if (dm_table_request_based(t))
> > +	if (dm_table_request_based(t)) {
> >  		queue_flag_set_unlocked(QUEUE_FLAG_STACKABLE, q);
> > +		__blk_queue_set_nr_requests(q, nr_requests);
> > +	}
> >  }
> >
> >  unsigned int dm_table_get_num_targets(struct dm_table *t)
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index d21e128..2d45689 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -2052,7 +2052,7 @@ static void __set_size(struct mapped_device *md,
> > sector_t size) * Returns old map, which caller must destroy.
> >   */
> >  static struct dm_table *__bind(struct mapped_device *md, struct dm_table
> > *t, -			       struct queue_limits *limits)
> > +		       struct queue_limits *limits, unsigned long nr_requests)
> >  {
> >  	struct dm_table *old_map;
> >  	struct request_queue *q = md->queue;
> > @@ -2083,11 +2083,13 @@ static struct dm_table *__bind(struct
> > mapped_device *md, struct dm_table *t,
> >
> >  	__bind_mempools(md, t);
> >
> > -	write_lock_irqsave(&md->map_lock, flags);
> > +	spin_lock_irqsave(q->queue_lock, flags);
> > +	write_lock(&md->map_lock);
> >  	old_map = md->map;
> >  	md->map = t;
> > -	dm_table_set_restrictions(t, q, limits);
> > -	write_unlock_irqrestore(&md->map_lock, flags);
> > +	dm_table_set_restrictions(t, q, limits, nr_requests);
> > +	write_unlock(&md->map_lock);
> > +	spin_unlock_irqrestore(q->queue_lock, flags);
> >
> >  	return old_map;
> >  }
> > @@ -2390,6 +2392,7 @@ struct dm_table *dm_swap_table(struct mapped_device
> > *md, struct dm_table *table) struct dm_table *map = ERR_PTR(-EINVAL);
> >  	struct queue_limits limits;
> >  	int r;
> > +	unsigned long nr_requests;
> >
> >  	mutex_lock(&md->suspend_lock);
> >
> > @@ -2397,7 +2400,7 @@ struct dm_table *dm_swap_table(struct mapped_device
> > *md, struct dm_table *table) if (!dm_suspended_md(md))
> >  		goto out;
> >
> > -	r = dm_calculate_queue_limits(table, &limits);
> > +	r = dm_calculate_queue_limits(table, &limits, &nr_requests);
> >  	if (r) {
> >  		map = ERR_PTR(r);
> >  		goto out;
> > @@ -2410,7 +2413,7 @@ struct dm_table *dm_swap_table(struct mapped_device
> > *md, struct dm_table *table) goto out;
> >  	}
> >
> > -	map = __bind(md, table, &limits);
> > +	map = __bind(md, table, &limits, nr_requests);
> >
> >  out:
> >  	mutex_unlock(&md->suspend_lock);
> > diff --git a/drivers/md/dm.h b/drivers/md/dm.h
> > index bad1724..9e25c48 100644
> > --- a/drivers/md/dm.h
> > +++ b/drivers/md/dm.h
> > @@ -50,9 +50,9 @@ void dm_table_event_callback(struct dm_table *t,
> >  struct dm_target *dm_table_get_target(struct dm_table *t, unsigned int
> > index); struct dm_target *dm_table_find_target(struct dm_table *t,
> > sector_t sector); int dm_calculate_queue_limits(struct dm_table *table,
> > -			      struct queue_limits *limits);
> > +		      struct queue_limits *limits, unsigned long *nr_requests);
> >  void dm_table_set_restrictions(struct dm_table *t, struct request_queue
> > *q, -			       struct queue_limits *limits);
> > +		       struct queue_limits *limits, unsigned long nr_requests);
> >  struct list_head *dm_table_get_devices(struct dm_table *t);
> >  void dm_table_presuspend_targets(struct dm_table *t);
> >  void dm_table_postsuspend_targets(struct dm_table *t);
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 6690e8b..366bba5 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -934,6 +934,9 @@ extern void blk_limits_io_min(struct queue_limits
> > *limits, unsigned int min); extern void blk_queue_io_min(struct
> > request_queue *q, unsigned int min); extern void blk_limits_io_opt(struct
> > queue_limits *limits, unsigned int opt); extern void
> > blk_queue_io_opt(struct request_queue *q, unsigned int opt); +extern
> > unsigned long __blk_queue_set_nr_requests(struct request_queue *q,
> > +							unsigned long nr);
> > +
> >  extern void blk_set_default_limits(struct queue_limits *lim);
> >  extern int blk_stack_limits(struct queue_limits *t, struct queue_limits
> > *b, sector_t offset);
> > diff --git a/include/linux/device-mapper.h
> > b/include/linux/device-mapper.h index 1381cd9..b0f9443 100644
> > --- a/include/linux/device-mapper.h
> > +++ b/include/linux/device-mapper.h
> > @@ -108,6 +108,11 @@ void dm_error(const char *message);
> >   */
> >  int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
> >  			 sector_t start, sector_t len, void *data);
> > +/*
> > + * Combine device nr_requests.
> > + */
> > +int dm_set_device_nr_requests(struct dm_target *ti, struct dm_dev *dev,
> > +			 sector_t start, sector_t len, void *data);
> >
> >  struct dm_dev {
> >  	struct block_device *bdev;
> 


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