Re: [PATCH] bcache: consider the fragmentation when update the writeback rate

[Coly Li]

On 2020/11/3 20:42, Dongdong Tao wrote:
> From: dongdong tao <dongdong.tao@xxxxxxxxxxxxx>
> 
> Current way to calculate the writeback rate only considered the
> dirty sectors, this usually works fine when the fragmentation
> is not high, but it will give us unreasonable small rate when
> we are under a situation that very few dirty sectors consumed
> a lot dirty buckets. In some case, the dirty bucekts can reached
> to CUTOFF_WRITEBACK_SYNC while the dirty data(sectors) noteven
> reached the writeback_percent, the writeback rate will still
> be the minimum value (4k), thus it will cause all the writes to be
> stucked in a non-writeback mode because of the slow writeback.
> 
> This patch will try to accelerate the writeback rate when the
> fragmentation is high. It calculate the propotional_scaled value
> based on below:
> (dirty_sectors / writeback_rate_p_term_inverse) * fragment
> As we can see, the higher fragmentation will result a larger
> proportional_scaled value, thus cause a larger writeback rate.
> The fragment value is calculated based on below:
> (dirty_buckets *  bucket_size) / dirty_sectors
> If you think about it, the value of fragment will be always
> inside [1, bucket_size].
> 
> This patch only considers the fragmentation when the number of
> dirty_buckets reached to a dirty threshold(configurable by
> writeback_fragment_percent, default is 50), so bcache will
> remain the original behaviour before the dirty buckets reached
> the threshold.
> 
> Signed-off-by: dongdong tao <dongdong.tao@xxxxxxxxxxxxx>

Hi Dongdong,

Change the writeback rate does not effect the real throughput indeed,
your change is just increasing the upper limit hint of the writeback
throughput, the bottle neck is spinning drive for random I/O.

A good direction should be the moving gc. If the moving gc may work
faster, the situation you mentioned above could be relaxed a lot.

I will NACK this patch unless you may have a observable and reproducible
performance number.

Thanks.

Coly Li


> ---
>  drivers/md/bcache/bcache.h    |  1 +
>  drivers/md/bcache/sysfs.c     |  6 ++++++
>  drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
>  3 files changed, 28 insertions(+)
> 
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 1d57f48307e6..87632f7032b6 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -374,6 +374,7 @@ struct cached_dev {
>  	unsigned int		writeback_metadata:1;
>  	unsigned int		writeback_running:1;
>  	unsigned char		writeback_percent;
> +	unsigned char		writeback_fragment_percent;
>  	unsigned int		writeback_delay;
>  
>  	uint64_t		writeback_rate_target;
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 554e3afc9b68..69499113aef8 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -115,6 +115,7 @@ rw_attribute(stop_when_cache_set_failed);
>  rw_attribute(writeback_metadata);
>  rw_attribute(writeback_running);
>  rw_attribute(writeback_percent);
> +rw_attribute(writeback_fragment_percent);
>  rw_attribute(writeback_delay);
>  rw_attribute(writeback_rate);
>  
> @@ -197,6 +198,7 @@ SHOW(__bch_cached_dev)
>  	var_printf(writeback_running,	"%i");
>  	var_print(writeback_delay);
>  	var_print(writeback_percent);
> +	var_print(writeback_fragment_percent);
>  	sysfs_hprint(writeback_rate,
>  		     wb ? atomic_long_read(&dc->writeback_rate.rate) << 9 : 0);
>  	sysfs_printf(io_errors,		"%i", atomic_read(&dc->io_errors));
> @@ -308,6 +310,9 @@ STORE(__cached_dev)
>  	sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent,
>  			    0, bch_cutoff_writeback);
>  
> +	sysfs_strtoul_clamp(writeback_fragment_percent, dc->writeback_fragment_percent,
> +			    0, bch_cutoff_writeback_sync);
> +
>  	if (attr == &sysfs_writeback_rate) {
>  		ssize_t ret;
>  		long int v = atomic_long_read(&dc->writeback_rate.rate);
> @@ -498,6 +503,7 @@ static struct attribute *bch_cached_dev_files[] = {
>  	&sysfs_writeback_running,
>  	&sysfs_writeback_delay,
>  	&sysfs_writeback_percent,
> +	&sysfs_writeback_fragment_percent,
>  	&sysfs_writeback_rate,
>  	&sysfs_writeback_rate_update_seconds,
>  	&sysfs_writeback_rate_i_term_inverse,
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index 3c74996978da..34babc89fdf3 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -88,6 +88,26 @@ static void __update_writeback_rate(struct cached_dev *dc)
>  	int64_t integral_scaled;
>  	uint32_t new_rate;
>  
> +	/*
> +	 * We need to consider the number of dirty buckets as well
> +	 * when calculating the proportional_scaled, Otherwise we might
> +	 * have an unreasonable small writeback rate at a highly fragmented situation
> +	 * when very few dirty sectors consumed a lot dirty buckets, the
> +	 * worst case is when dirty_data reached writeback_percent and
> +	 * dirty buckets reached to cutoff_writeback_sync, but the rate
> +	 * still will be at the minimum value, which will cause the write
> +	 * stuck at a non-writeback mode.
> +	 */
> +	struct cache_set *c = dc->disk.c;
> +
> +	if (c->gc_stats.in_use > dc->writeback_fragment_percent && dirty > 0) {
> +		int64_t dirty_buckets = (c->gc_stats.in_use * c->nbuckets) / 100;
> +		int64_t fragment = (dirty_buckets *  c->cache->sb.bucket_size) / dirty;
> +
> +		proportional_scaled =
> +		div_s64(dirty, dc->writeback_rate_p_term_inverse) * (fragment);
> +	}
> +
>  	if ((error < 0 && dc->writeback_rate_integral > 0) ||
>  	    (error > 0 && time_before64(local_clock(),
>  			 dc->writeback_rate.next + NSEC_PER_MSEC))) {
> @@ -969,6 +989,7 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
>  	dc->writeback_metadata		= true;
>  	dc->writeback_running		= false;
>  	dc->writeback_percent		= 10;
> +	dc->writeback_fragment_percent	= 50;
>  	dc->writeback_delay		= 30;
>  	atomic_long_set(&dc->writeback_rate.rate, 1024);
>  	dc->writeback_rate_minimum	= 8;
>