Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Andreas Herrmann]

[CC-ing linux-block and linux-scsi and adding some comments]

On Mon, Feb 01, 2016 at 11:43:40PM +0100, Andreas Herrmann wrote:
> This introduces a new blk_mq hw attribute time_slice_us which allows
> to specify a time slice in usecs.
> 
> Default value is 0 and implies no modification to blk-mq behaviour.
> 
> A positive value changes blk-mq to service only one software queue
> within this time slice until it expires or the software queue is
> empty. Then the next software queue with pending requests is selected.
> 
> Signed-off-by: Andreas Herrmann <aherrmann@suse.com>
> ---
>  block/blk-mq-sysfs.c   |  27 +++++++
>  block/blk-mq.c         | 208 +++++++++++++++++++++++++++++++++++++++++--------
>  include/linux/blk-mq.h |   9 +++
>  3 files changed, 211 insertions(+), 33 deletions(-)
> 
> Hi,
> 
> This update is long overdue (sorry for the delay).
> 
> Change to v1:
> - time slice is now specified in usecs instead of msecs.
> - time slice is extended (up to 3 times the initial value) when there
>   was actually a request to be serviced for the software queue
> 
> Fio test results are sent in a separate mail to this.

See http://marc.info/?l=linux-kernel&m=145436682607949&w=2

In short it shows significant performance gains in some tests,
e.g. sequential read iops up by >40% with 8 jobs. But it's never on
par with CFQ when more than 1 job was used during the test.

> Results for fio improved to some extent with this patch. But in
> reality the picture is quite mixed. Performance is highly dependend on
> task scheduling. There is no guarantee that the requests originated
> from one CPU belong to the same process.
> 
> I think for rotary devices CFQ is by far the best choice. A simple
> illustration is:
> 
>   Copying two files (750MB in this case) in parallel on a rotary
>   device. The elapsed wall clock time (seconds) for this is
>                                mean    stdev
>    cfq, slice_idle=8           16.18   4.95
>    cfq, slice_idle=0           23.74   2.82
>    blk-mq, time_slice_usec=0   24.37   2.05
>    blk-mq, time_slice_usec=250 25.58   3.16

This illustrates that although their was performance gain with fio
tests, the patch can cause higher variance and lower performance in
comparison to unmodified blk-mq with other tests. And it underscores
superiority of CFQ for rotary disks.

Meanwhile my opinion is that it's not really worth to look further
into introduction of I/O scheduling support in blk-mq. I don't see the
need for scheduling support (deadline or something else) for fast
storage devices. And rotary devices should really avoid usage of blk-mq
and stick to CFQ.

Thus I think that introducing some coexistence of blk-mq and the
legacy block with CFQ is the best option.

Recently Johannes sent a patch to enable scsi-mq per driver, see
http://marc.info/?l=linux-scsi&m=145347009631192&w=2

Probably that is a good solution (at least in the short term) to allow
users to switch to blk-mq for some host adapters (with fast storage
attached) but to stick to legacy stuff on other host adapters with
rotary devices.

What do others think?


Thanks,

Andreas


> Regards,
> 
> Andreas
> 
> diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> index 1cf1878..77c875c 100644
> --- a/block/blk-mq-sysfs.c
> +++ b/block/blk-mq-sysfs.c
> @@ -247,6 +247,26 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
>  	return ret;
>  }
>  
> +static ssize_t blk_mq_hw_sysfs_tslice_show(struct blk_mq_hw_ctx *hctx,
> +					  char *page)
> +{
> +	return sprintf(page, "%u\n", hctx->tslice_us);
> +}
> +
> +static ssize_t blk_mq_hw_sysfs_tslice_store(struct blk_mq_hw_ctx *hctx,
> +					const char *page, size_t length)
> +{
> +	unsigned long long store;
> +	int err;
> +
> +	err = kstrtoull(page, 10, &store);
> +	if (err)
> +		return -EINVAL;
> +
> +	hctx->tslice_us = (unsigned)store;
> +	return length;
> +}
> +
>  static struct blk_mq_ctx_sysfs_entry blk_mq_sysfs_dispatched = {
>  	.attr = {.name = "dispatched", .mode = S_IRUGO },
>  	.show = blk_mq_sysfs_dispatched_show,
> @@ -305,6 +325,12 @@ static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_poll = {
>  	.show = blk_mq_hw_sysfs_poll_show,
>  };
>  
> +static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_tslice = {
> +	.attr = {.name = "time_slice_us", .mode = S_IRUGO | S_IWUSR },
> +	.show = blk_mq_hw_sysfs_tslice_show,
> +	.store = blk_mq_hw_sysfs_tslice_store,
> +};
> +
>  static struct attribute *default_hw_ctx_attrs[] = {
>  	&blk_mq_hw_sysfs_queued.attr,
>  	&blk_mq_hw_sysfs_run.attr,
> @@ -314,6 +340,7 @@ static struct attribute *default_hw_ctx_attrs[] = {
>  	&blk_mq_hw_sysfs_cpus.attr,
>  	&blk_mq_hw_sysfs_active.attr,
>  	&blk_mq_hw_sysfs_poll.attr,
> +	&blk_mq_hw_sysfs_tslice.attr,
>  	NULL,
>  };
>  
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4c0622f..97d32f2 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -68,6 +68,7 @@ static void blk_mq_hctx_mark_pending(struct blk_mq_hw_ctx *hctx,
>  
>  	if (!test_bit(CTX_TO_BIT(hctx, ctx), &bm->word))
>  		set_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> +	cpumask_set_cpu(ctx->cpu, hctx->cpu_pending_mask);
>  }
>  
>  static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
> @@ -76,6 +77,7 @@ static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
>  	struct blk_align_bitmap *bm = get_bm(hctx, ctx);
>  
>  	clear_bit(CTX_TO_BIT(hctx, ctx), &bm->word);
> +	cpumask_clear_cpu(ctx->cpu, hctx->cpu_pending_mask);
>  }
>  
>  void blk_mq_freeze_queue_start(struct request_queue *q)
> @@ -682,15 +684,41 @@ static bool blk_mq_attempt_merge(struct request_queue *q,
>  	return false;
>  }
>  
> +static int tslice_flush_one_ctx(struct blk_mq_hw_ctx *hctx,
> +				struct list_head *list,	int cpu)
> +{
> +	struct request_queue *q = hctx->queue;
> +	struct blk_mq_ctx *ctx;
> +
> +	if (cpu == -1 || !hctx->tslice_us)
> +		return 0;
> +
> +	ctx = __blk_mq_get_ctx(q, cpu);
> +	spin_lock(&ctx->lock);
> +	if (!list_empty(&ctx->rq_list)) {
> +		list_splice_tail_init(&ctx->rq_list, list);
> +		spin_lock(&hctx->lock);
> +		hctx->tslice_inc = 1;
> +		blk_mq_hctx_clear_pending(hctx, ctx);
> +		spin_unlock(&hctx->lock);
> +	}
> +	spin_unlock(&ctx->lock);
> +	return 1;
> +}
> +
>  /*
>   * Process software queues that have been marked busy, splicing them
>   * to the for-dispatch
>   */
> -static void flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list)
> +static void flush_busy_ctxs(struct blk_mq_hw_ctx *hctx,
> +			struct list_head *list,	int cpu)
>  {
>  	struct blk_mq_ctx *ctx;
>  	int i;
>  
> +	if (tslice_flush_one_ctx(hctx, list, cpu))
> +		return;
> +
>  	for (i = 0; i < hctx->ctx_map.size; i++) {
>  		struct blk_align_bitmap *bm = &hctx->ctx_map.map[i];
>  		unsigned int off, bit;
> @@ -706,9 +734,11 @@ static void flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list)
>  				break;
>  
>  			ctx = hctx->ctxs[bit + off];
> -			clear_bit(bit, &bm->word);
>  			spin_lock(&ctx->lock);
>  			list_splice_tail_init(&ctx->rq_list, list);
> +			spin_lock(&hctx->lock);
> +			blk_mq_hctx_clear_pending(hctx, ctx);
> +			spin_unlock(&hctx->lock);
>  			spin_unlock(&ctx->lock);
>  
>  			bit++;
> @@ -717,6 +747,114 @@ static void flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list)
>  }
>  
>  /*
> + * It'd be great if the workqueue API had a way to pass
> + * in a mask and had some smarts for more clever placement.
> + * For now we just round-robin here, switching for every
> + * BLK_MQ_CPU_WORK_BATCH queued items.
> + */
> +static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
> +{
> +	if (hctx->queue->nr_hw_queues == 1)
> +		return WORK_CPU_UNBOUND;
> +
> +	if (--hctx->next_cpu_batch <= 0) {
> +		int cpu = hctx->next_cpu, next_cpu;
> +
> +		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
> +		if (next_cpu >= nr_cpu_ids)
> +			next_cpu = cpumask_first(hctx->cpumask);
> +
> +		hctx->next_cpu = next_cpu;
> +		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
> +
> +		return cpu;
> +	}
> +
> +	return hctx->next_cpu;
> +}
> +
> +static int blk_mq_tslice_expired(struct blk_mq_hw_ctx *hctx)
> +{
> +	if (time_after_eq(jiffies, hctx->tslice_expiration))
> +		return 1;
> +
> +	return 0;
> +}
> +
> +static int blk_mq_tslice_next_cpu(struct blk_mq_hw_ctx *hctx)
> +{
> +	int c = hctx->tslice_cpu;
> +
> +	/* allow extension of time slice */
> +	if (hctx->tslice_inc && hctx->tslice_inc_count < 4) {
> +		hctx->tslice_inc = 0;
> +		hctx->tslice_inc_count++;
> +		goto out;
> +	}
> +	hctx->tslice_inc = 0;
> +	hctx->tslice_inc_count = 0;
> +
> +	/* clear CPU for which tslice has expired */
> +	if (c != -1)
> +		cpumask_clear_cpu(c, hctx->cpu_service_mask);
> +
> +	/* calculate mask of remaining CPUs with pending work */
> +	if (cpumask_and(hctx->cpu_next_mask, hctx->cpu_pending_mask,
> +				hctx->cpu_service_mask)) {
> +		c = cpumask_first(hctx->cpu_next_mask);
> +	} else {
> +		/*
> +		 * no remaining CPUs with pending work, reset epoch,
> +		 * start with first CPU that has requests pending
> +		 */
> +		hctx->tslice_expiration = 0;
> +		cpumask_setall(hctx->cpu_service_mask);
> +		c = cpumask_first(hctx->cpu_pending_mask);
> +	}
> +
> +	/* no CPU with pending requests */
> +	if (c >= nr_cpu_ids)
> +		return -1;
> +
> +out:
> +	hctx->tslice_expiration = jiffies + usecs_to_jiffies(hctx->tslice_us);
> +	return c;
> +}
> +
> +static int tslice_get_busy_ctx(struct blk_mq_hw_ctx *hctx)
> +{
> +	int cpu = -1;
> +
> +	if (hctx->tslice_us) {
> +		spin_lock(&hctx->lock);
> +		if (blk_mq_tslice_expired(hctx))
> +			hctx->tslice_cpu = blk_mq_tslice_next_cpu(hctx);
> +		cpu = hctx->tslice_cpu;
> +		spin_unlock(&hctx->lock);
> +	}
> +
> +	return cpu;
> +}
> +
> +/*
> + * Ensure that hardware queue is run if there are pending
> + * requests in any software queue.
> + */
> +static void tslice_schedule_pending_work(struct blk_mq_hw_ctx *hctx)
> +{
> +	long t;
> +
> +	if (!hctx->tslice_us || cpumask_empty(hctx->cpu_pending_mask))
> +		return;
> +
> +	t = hctx->tslice_expiration - jiffies;
> +	if (t < 0)
> +		t = 0;
> +	kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
> +					&hctx->run_work, (unsigned int)t);
> +}
> +
> +/*
>   * Run this hardware queue, pulling any software queues mapped to it in.
>   * Note that this function currently has various problems around ordering
>   * of IO. In particular, we'd like FIFO behaviour on handling existing
> @@ -729,7 +867,7 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>  	LIST_HEAD(rq_list);
>  	LIST_HEAD(driver_list);
>  	struct list_head *dptr;
> -	int queued;
> +	int queued, cpu;
>  
>  	WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask));
>  
> @@ -739,9 +877,11 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>  	hctx->run++;
>  
>  	/*
> -	 * Touch any software queue that has pending entries.
> +	 * Touch dedicated software queue if time slice is set or any
> +	 * software queue that has pending entries (cpu == -1).
>  	 */
> -	flush_busy_ctxs(hctx, &rq_list);
> +	cpu = tslice_get_busy_ctx(hctx);
> +	flush_busy_ctxs(hctx, &rq_list, cpu);
>  
>  	/*
>  	 * If we have previous entries on our dispatch list, grab them
> @@ -826,34 +966,8 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
>  		 * blk_mq_run_hw_queue() already checks the STOPPED bit
>  		 **/
>  		blk_mq_run_hw_queue(hctx, true);
> -	}
> -}
> -
> -/*
> - * It'd be great if the workqueue API had a way to pass
> - * in a mask and had some smarts for more clever placement.
> - * For now we just round-robin here, switching for every
> - * BLK_MQ_CPU_WORK_BATCH queued items.
> - */
> -static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
> -{
> -	if (hctx->queue->nr_hw_queues == 1)
> -		return WORK_CPU_UNBOUND;
> -
> -	if (--hctx->next_cpu_batch <= 0) {
> -		int cpu = hctx->next_cpu, next_cpu;
> -
> -		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
> -		if (next_cpu >= nr_cpu_ids)
> -			next_cpu = cpumask_first(hctx->cpumask);
> -
> -		hctx->next_cpu = next_cpu;
> -		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
> -
> -		return cpu;
> -	}
> -
> -	return hctx->next_cpu;
> +	} else
> +		tslice_schedule_pending_work(hctx);
>  }
>  
>  void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> @@ -992,7 +1106,10 @@ static void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx,
>  	struct blk_mq_ctx *ctx = rq->mq_ctx;
>  
>  	__blk_mq_insert_req_list(hctx, ctx, rq, at_head);
> +	spin_lock(&hctx->lock);
>  	blk_mq_hctx_mark_pending(hctx, ctx);
> +	spin_unlock(&hctx->lock);
> +
>  }
>  
>  void blk_mq_insert_request(struct request *rq, bool at_head, bool run_queue,
> @@ -1583,7 +1700,9 @@ static int blk_mq_hctx_cpu_offline(struct blk_mq_hw_ctx *hctx, int cpu)
>  	spin_lock(&ctx->lock);
>  	if (!list_empty(&ctx->rq_list)) {
>  		list_splice_init(&ctx->rq_list, &tmp);
> +		spin_lock(&hctx->lock);
>  		blk_mq_hctx_clear_pending(hctx, ctx);
> +		spin_unlock(&hctx->lock);
>  	}
>  	spin_unlock(&ctx->lock);
>  
> @@ -1602,7 +1721,9 @@ static int blk_mq_hctx_cpu_offline(struct blk_mq_hw_ctx *hctx, int cpu)
>  	}
>  
>  	hctx = q->mq_ops->map_queue(q, ctx->cpu);
> +	spin_lock(&hctx->lock);
>  	blk_mq_hctx_mark_pending(hctx, ctx);
> +	spin_unlock(&hctx->lock);
>  
>  	spin_unlock(&ctx->lock);
>  
> @@ -1691,6 +1812,12 @@ static int blk_mq_init_hctx(struct request_queue *q,
>  	hctx->queue_num = hctx_idx;
>  	hctx->flags = set->flags & ~BLK_MQ_F_TAG_SHARED;
>  
> +	hctx->tslice_us = 0;
> +	hctx->tslice_expiration = 0;
> +	hctx->tslice_cpu = -1;
> +	hctx->tslice_inc = 0;
> +	hctx->tslice_inc_count = 0;
> +
>  	blk_mq_init_cpu_notifier(&hctx->cpu_notifier,
>  					blk_mq_hctx_notify, hctx);
>  	blk_mq_register_cpu_notifier(&hctx->cpu_notifier);
> @@ -2006,6 +2133,18 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>  						node))
>  			goto err_hctxs;
>  
> +		if (!zalloc_cpumask_var_node(&hctxs[i]->cpu_pending_mask,
> +						GFP_KERNEL, node))
> +			goto err_hctxs;
> +
> +		if (!zalloc_cpumask_var_node(&hctxs[i]->cpu_service_mask,
> +						GFP_KERNEL, node))
> +			goto err_hctxs;
> +
> +		if (!zalloc_cpumask_var_node(&hctxs[i]->cpu_next_mask,
> +						GFP_KERNEL, node))
> +			goto err_hctxs;
> +
>  		atomic_set(&hctxs[i]->nr_active, 0);
>  		hctxs[i]->numa_node = node;
>  		hctxs[i]->queue_num = i;
> @@ -2069,6 +2208,9 @@ err_hctxs:
>  		if (!hctxs[i])
>  			break;
>  		free_cpumask_var(hctxs[i]->cpumask);
> +		free_cpumask_var(hctxs[i]->cpu_pending_mask);
> +		free_cpumask_var(hctxs[i]->cpu_service_mask);
> +		free_cpumask_var(hctxs[i]->cpu_next_mask);
>  		kfree(hctxs[i]);
>  	}
>  err_map:
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 7fc9296..a8ca685 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -57,6 +57,15 @@ struct blk_mq_hw_ctx {
>  
>  	atomic_t		nr_active;
>  
> +	cpumask_var_t		cpu_service_mask; /* CPUs not yet serviced */
> +	cpumask_var_t		cpu_pending_mask; /* CPUs with pending work */
> +	cpumask_var_t		cpu_next_mask;	/* CPUs to be serviced */
> +	int			tslice_us;	/* time slice in µs */
> +	int			tslice_inc;	/* extend time slice */
> +	int			tslice_inc_count;
> +	unsigned long		tslice_expiration; /* in jiffies */
> +	int			tslice_cpu;	/* cpu of time slice */
> +
>  	struct blk_mq_cpu_notifier	cpu_notifier;
>  	struct kobject		kobj;
>  
> -- 
> 1.9.1
> 

Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Markus Trippelsdorf]

On 2016.02.09 at 18:12 +0100, Andreas Herrmann wrote:
> [CC-ing linux-block and linux-scsi and adding some comments]
> 
> On Mon, Feb 01, 2016 at 11:43:40PM +0100, Andreas Herrmann wrote:
> > This introduces a new blk_mq hw attribute time_slice_us which allows
> > to specify a time slice in usecs.
> > 
> > Fio test results are sent in a separate mail to this.
> 
> See http://marc.info/?l=linux-kernel&m=145436682607949&w=2
> 
> In short it shows significant performance gains in some tests,
> e.g. sequential read iops up by >40% with 8 jobs. But it's never on
> par with CFQ when more than 1 job was used during the test.
> 
> > Results for fio improved to some extent with this patch. But in
> > reality the picture is quite mixed. Performance is highly dependend on
> > task scheduling. There is no guarantee that the requests originated
> > from one CPU belong to the same process.
> > 
> > I think for rotary devices CFQ is by far the best choice. A simple
> > illustration is:
> > 
> >   Copying two files (750MB in this case) in parallel on a rotary
> >   device. The elapsed wall clock time (seconds) for this is
> >                                mean    stdev
> >    cfq, slice_idle=8           16.18   4.95
> >    cfq, slice_idle=0           23.74   2.82
> >    blk-mq, time_slice_usec=0   24.37   2.05
> >    blk-mq, time_slice_usec=250 25.58   3.16
> 
> This illustrates that although their was performance gain with fio
> tests, the patch can cause higher variance and lower performance in
> comparison to unmodified blk-mq with other tests. And it underscores
> superiority of CFQ for rotary disks.
> 
> Meanwhile my opinion is that it's not really worth to look further
> into introduction of I/O scheduling support in blk-mq. I don't see the
> need for scheduling support (deadline or something else) for fast
> storage devices. And rotary devices should really avoid usage of blk-mq
> and stick to CFQ.
> 
> Thus I think that introducing some coexistence of blk-mq and the
> legacy block with CFQ is the best option.
> 
> Recently Johannes sent a patch to enable scsi-mq per driver, see
> http://marc.info/?l=linux-scsi&m=145347009631192&w=2
> 
> Probably that is a good solution (at least in the short term) to allow
> users to switch to blk-mq for some host adapters (with fast storage
> attached) but to stick to legacy stuff on other host adapters with
> rotary devices.

I don't think that Johannes' patch is a good solution.

The best solution for the user would be if blk-mq could be toggled per
drive (or even automatically enabled if queue/rotational == 0). Is there
a fundamental reason why this is not feasible?

Your solution is better than nothing, but it requires that the user
finds out the drive <=> host mapping by hand and then runs something
like: 
echo "250" > /sys/devices/pci0000:00/0000:00:11.0/ata2/host1/target1:0:0/1:0:0:0/block/sdb/mq/0/time_slice_us
during boot for spinning rust drives...

-- 
Markus

Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Andreas Herrmann]

On Tue, Feb 09, 2016 at 06:41:56PM +0100, Markus Trippelsdorf wrote:
> On 2016.02.09 at 18:12 +0100, Andreas Herrmann wrote:
> > [CC-ing linux-block and linux-scsi and adding some comments]
> > 
> > On Mon, Feb 01, 2016 at 11:43:40PM +0100, Andreas Herrmann wrote:
> > > This introduces a new blk_mq hw attribute time_slice_us which allows
> > > to specify a time slice in usecs.
> > > 
> > > Fio test results are sent in a separate mail to this.
> > 
> > See http://marc.info/?l=linux-kernel&m=145436682607949&w=2
> > 
> > In short it shows significant performance gains in some tests,
> > e.g. sequential read iops up by >40% with 8 jobs. But it's never on
> > par with CFQ when more than 1 job was used during the test.
> > 
> > > Results for fio improved to some extent with this patch. But in
> > > reality the picture is quite mixed. Performance is highly dependend on
> > > task scheduling. There is no guarantee that the requests originated
> > > from one CPU belong to the same process.
> > > 
> > > I think for rotary devices CFQ is by far the best choice. A simple
> > > illustration is:
> > > 
> > >   Copying two files (750MB in this case) in parallel on a rotary
> > >   device. The elapsed wall clock time (seconds) for this is
> > >                                mean    stdev
> > >    cfq, slice_idle=8           16.18   4.95
> > >    cfq, slice_idle=0           23.74   2.82
> > >    blk-mq, time_slice_usec=0   24.37   2.05
> > >    blk-mq, time_slice_usec=250 25.58   3.16
> > 
> > This illustrates that although their was performance gain with fio
> > tests, the patch can cause higher variance and lower performance in
> > comparison to unmodified blk-mq with other tests. And it underscores
> > superiority of CFQ for rotary disks.
> > 
> > Meanwhile my opinion is that it's not really worth to look further
> > into introduction of I/O scheduling support in blk-mq. I don't see the
> > need for scheduling support (deadline or something else) for fast
> > storage devices. And rotary devices should really avoid usage of blk-mq
> > and stick to CFQ.
> > 
> > Thus I think that introducing some coexistence of blk-mq and the
> > legacy block with CFQ is the best option.
> > 
> > Recently Johannes sent a patch to enable scsi-mq per driver, see
> > http://marc.info/?l=linux-scsi&m=145347009631192&w=2
> > 
> > Probably that is a good solution (at least in the short term) to allow
> > users to switch to blk-mq for some host adapters (with fast storage
> > attached) but to stick to legacy stuff on other host adapters with
> > rotary devices.
> 
> I don't think that Johannes' patch is a good solution.

Why? Because it's not per device?

> The best solution for the user would be if blk-mq could be toggled
> per drive (or even automatically enabled if queue/rotational == 0).

Yes, I aggree, but ...

> Is there a fundamental reason why this is not feasible?

... it's not possible (*) with the current implementation.

Tag handling/command allocation differs. Respective functions are set
per host.

(*) Or maybe it's possible but just hard to achieve and I didn't look
long enough into relevant code to get an idea how to do it.

> Your solution is better than nothing, but it requires that the user
> finds out the drive <=> host mapping by hand and then runs something
> like: 
> echo "250" > /sys/devices/pci0000:00/0000:00:11.0/ata2/host1/target1:0:0/1:0:0:0/block/sdb/mq/0/time_slice_us
> during boot for spinning rust drives...

Or it could automatically be set in case of rotational device.
(Once we know for sure that it doesn't cause performance degradation.)

> -- 
> Markus

Andreas

Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Markus Trippelsdorf]

On 2016.02.10 at 20:34 +0100, Andreas Herrmann wrote:
> On Tue, Feb 09, 2016 at 06:41:56PM +0100, Markus Trippelsdorf wrote:
> > > Recently Johannes sent a patch to enable scsi-mq per driver, see
> > > http://marc.info/?l=linux-scsi&m=145347009631192&w=2
> > > 
> > > Probably that is a good solution (at least in the short term) to allow
> > > users to switch to blk-mq for some host adapters (with fast storage
> > > attached) but to stick to legacy stuff on other host adapters with
> > > rotary devices.
> > 
> > I don't think that Johannes' patch is a good solution.
> 
> Why? Because it's not per device?

Yes. Like Christoph said in his reply to the patch: »The host is simply
the wrong place to decide these things.«

> > The best solution for the user would be if blk-mq could be toggled
> > per drive (or even automatically enabled if queue/rotational == 0).
> 
> Yes, I aggree, but ...
> 
> > Is there a fundamental reason why this is not feasible?
> 
> ... it's not possible (*) with the current implementation.
> 
> Tag handling/command allocation differs. Respective functions are set
> per host.
> 
> (*) Or maybe it's possible but just hard to achieve and I didn't look
> long enough into relevant code to get an idea how to do it.
> 
> > Your solution is better than nothing, but it requires that the user
> > finds out the drive <=> host mapping by hand and then runs something
> > like: 
> > echo "250" > /sys/devices/pci0000:00/0000:00:11.0/ata2/host1/target1:0:0/1:0:0:0/block/sdb/mq/0/time_slice_us
> > during boot for spinning rust drives...
> 
> Or it could automatically be set in case of rotational device.
> (Once we know for sure that it doesn't cause performance degradation.)

Yes, this sound like a good idea.

But, if I understand things correctly, your patch is only an interim
solution until proper I/O scheduler support gets implemented for blk-mq, no?

-- 
Markus

Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Andreas Herrmann]

On Wed, Feb 10, 2016 at 08:47:15PM +0100, Markus Trippelsdorf wrote:
> On 2016.02.10 at 20:34 +0100, Andreas Herrmann wrote:
> > On Tue, Feb 09, 2016 at 06:41:56PM +0100, Markus Trippelsdorf wrote:
> > > > Recently Johannes sent a patch to enable scsi-mq per driver, see
> > > > http://marc.info/?l=linux-scsi&m=145347009631192&w=2
> > > > 
> > > > Probably that is a good solution (at least in the short term) to allow
> > > > users to switch to blk-mq for some host adapters (with fast storage
> > > > attached) but to stick to legacy stuff on other host adapters with
> > > > rotary devices.
> > > 
> > > I don't think that Johannes' patch is a good solution.
> > 
> > Why? Because it's not per device?
> 
> Yes. Like Christoph said in his reply to the patch: »The host is simply
> the wrong place to decide these things.«
> 
> > > The best solution for the user would be if blk-mq could be toggled
> > > per drive (or even automatically enabled if queue/rotational == 0).
> > 
> > Yes, I aggree, but ...
> > 
> > > Is there a fundamental reason why this is not feasible?
> > 
> > ... it's not possible (*) with the current implementation.
> > 
> > Tag handling/command allocation differs. Respective functions are set
> > per host.
> > 
> > (*) Or maybe it's possible but just hard to achieve and I didn't look
> > long enough into relevant code to get an idea how to do it.
> > 
> > > Your solution is better than nothing, but it requires that the user
> > > finds out the drive <=> host mapping by hand and then runs something
> > > like: 
> > > echo "250" > /sys/devices/pci0000:00/0000:00:11.0/ata2/host1/target1:0:0/1:0:0:0/block/sdb/mq/0/time_slice_us
> > > during boot for spinning rust drives...
> > 
> > Or it could automatically be set in case of rotational device.
> > (Once we know for sure that it doesn't cause performance degradation.)
> 
> Yes, this sound like a good idea.
> 
> But, if I understand things correctly, your patch is only an interim
> solution until proper I/O scheduler support gets implemented for blk-mq, no?

That's to be discussed. (Hence the RFC)

My (potentially wrong) claims are

- I don't think that fast storage (e.g. SSDs) requires I/O scheduler
  support with blk-mq. blk-mq is very good at pushing a large number
  of requests from per CPU sw queues to hw queue(s). Why then
  introduce any overhead for I/O scheduler support?

- Slow storage (e.g. spinning drives) is fine with the old code which
  provides scheduler support and I doubt that there is any benefit for
  those devices when switching to blk-mq.

- The big hammer (scsi_mod.use_blk_mq) for the entire scsi stack to
  decide what to use is suboptimal. You can't have optimal performance
  when you have both slow and fast storage devices in your system.

I doubt that it is possible to add I/O scheduling support to blk-mq
which can be on par with what CFQ is able to achieve for slow devices
at the moment.

Requests are scattered among per-CPU software queues (and almost
instantly passed to hardware queue(s)). Due to CPU scheduling,
requests initiated from one process might come down via different
software queues. What is an efficient way to sort/merge requests from
all the software queues in such a way that the result is comparable to
what CFQ does (assuming that CFQ provides optimal performance)? So far
I didn't find a solution to this problem. (I just have this patch
which adds not too much overhead and improves the situation a little
bit.)

Maybe the solution is to avoid per-CPU queues for slow storage and
fall back to a set of queues comparable to what CFQ uses.

One way to do this is by falling back to non-blk-mq code and direct
use of CFQ.

Code that allows to select blk-mq per host would help to some
extent. But when you have both device types connected to the same host
adapter it doesn't help either.


Andreas