[dm-devel] [PATCH 2/2] dm-kcopyd: Delayed unplug of queues

[Mikulas Patocka]

On Thu, 22 Jul 2010, Mikulas Patocka wrote:

> dm-kcopyd: Delayed unplug of queues
> 
> This patch improves performance aby about 20% when writing to the snapshot
> origin. We keep track of two block devices to unplug (one for read and the
> other for write) and unplug them when exiting "do_work" thread. If there are
> more devices used (in theory it could happen, in practice it is rare), we
> unplug immediatelly.

BTW. There is even bigger improvement for noop and deadline schedulers. 
This patch makes them join read and write requests.

Mikulas

> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> 
> ---
>  drivers/md/dm-kcopyd.c |   35 ++++++++++++++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.34-fast/drivers/md/dm-kcopyd.c
> ===================================================================
> --- linux-2.6.34-fast.orig/drivers/md/dm-kcopyd.c	2010-07-22 16:08:36.000000000 +0200
> +++ linux-2.6.34-fast/drivers/md/dm-kcopyd.c	2010-07-22 16:40:19.000000000 +0200
> @@ -37,6 +37,8 @@ struct dm_kcopyd_client {
>  	unsigned int nr_pages;
>  	unsigned int nr_free_pages;
>  
> +	struct block_device *unplug[2];
> +
>  	struct dm_io_client *io_client;
>  
>  	wait_queue_head_t destroyq;
> @@ -308,6 +310,23 @@ static int run_complete_job(struct kcopy
>  	return 0;
>  }
>  
> +static void unplug(struct dm_kcopyd_client *kc, int queue)
> +{
> +	if (kc->unplug[queue] != NULL) {
> +		blk_unplug(bdev_get_queue(kc->unplug[queue]));
> +		kc->unplug[queue] = NULL;
> +	}
> +}
> +
> +static void prepare_unplug(struct dm_kcopyd_client *kc, int queue,
> +			   struct block_device *bdev)
> +{
> +	if (likely(kc->unplug[queue] == bdev))
> +		return;
> +	unplug(kc, queue);
> +	kc->unplug[queue] = bdev;
> +}
> +
>  static void complete_io(unsigned long error, void *context)
>  {
>  	struct kcopyd_job *job = (struct kcopyd_job *) context;
> @@ -345,7 +364,7 @@ static int run_io_job(struct kcopyd_job 
>  {
>  	int r;
>  	struct dm_io_request io_req = {
> -		.bi_rw = job->rw | (1 << BIO_RW_UNPLUG),
> +		.bi_rw = job->rw,
>  		.mem.type = DM_IO_PAGE_LIST,
>  		.mem.ptr.pl = job->pages,
>  		.mem.offset = job->offset,
> @@ -354,10 +373,16 @@ static int run_io_job(struct kcopyd_job 
>  		.client = job->kc->io_client,
>  	};
>  
> -	if (job->rw == READ)
> +	if (job->rw == READ) {
>  		r = dm_io(&io_req, 1, &job->source, NULL);
> -	else
> +		prepare_unplug(job->kc, READ, job->source.bdev);
> +	} else {
> +		if (unlikely(job->num_dests > 1))
> +			io_req.bi_rw |= 1 << BIO_RW_UNPLUG;
>  		r = dm_io(&io_req, job->num_dests, job->dests, NULL);
> +		if (likely(!(io_req.bi_rw & (1 << BIO_RW_UNPLUG))))
> +			prepare_unplug(job->kc, WRITE, job->dests[0].bdev);
> +	}
>  
>  	return r;
>  }
> @@ -439,6 +464,8 @@ static void do_work(struct work_struct *
>  	process_jobs(&kc->complete_jobs, kc, run_complete_job);
>  	process_jobs(&kc->pages_jobs, kc, run_pages_job);
>  	process_jobs(&kc->io_jobs, kc, run_io_job);
> +	unplug(kc, READ);
> +	unplug(kc, WRITE);
>  }
>  
>  /*
> @@ -619,6 +646,8 @@ int dm_kcopyd_client_create(unsigned int
>  	INIT_LIST_HEAD(&kc->io_jobs);
>  	INIT_LIST_HEAD(&kc->pages_jobs);
>  
> +	memset(kc->unplug, 0, sizeof kc->unplug);
> +
>  	kc->job_pool = mempool_create_slab_pool(MIN_JOBS, _job_cache);
>  	if (!kc->job_pool)
>  		goto bad_slab;
> 
>