Date:   Wed, 20 Jan 2021 20:30:44 +0800
From:   Coly Li <colyli@...e.de>
To:     Dongdong Tao <tdd21151186@...il.com>
Cc:     dongdong tao <dongdong.tao@...onical.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] bcache: consider the fragmentation when update the
 writeback rate

On 1/20/21 8:01 PM, Dongdong Tao wrote:
> From: dongdong tao <dongdong.tao@...onical.com>
> 
> 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) not even
> 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.
> 
> We accelerate the rate in 3 stages with different aggressiveness,
> the first stage starts when dirty buckets percent reach above
> BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW (50), the second is
> BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID (57), the third is
> BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH (64). By default
> the first stage tries to writeback the amount of dirty data
> in one bucket (on average) in (1 / (dirty_buckets_percent - 50)) second,
> the second stage tries to writeback the amount of dirty data in one bucket
> in (1 / (dirty_buckets_percent - 57)) * 100 millisecond, the third
> stage tries to writeback the amount of dirty data in one bucket in
> (1 / (dirty_buckets_percent - 64)) millisecond.
> 
> the initial rate at each stage can be controlled by 3 configurable
> parameters writeback_rate_fp_term_{low|mid|high}, they are by default
> 1, 10, 1000, the hint of IO throughput that these values are trying
> to achieve is described by above paragraph, the reason that
> I choose those value as default is based on the testing and the
> production data, below is some details:
> 
> A. When it comes to the low stage, there is still a bit far from the 70
>    threshold, so we only want to give it a little bit push by setting the
>    term to 1, it means the initial rate will be 170 if the fragment is 6,
>    it is calculated by bucket_size/fragment, this rate is very small,
>    but still much reasonable than the minimum 8.
>    For a production bcache with unheavy workload, if the cache device
>    is bigger than 1 TB, it may take hours to consume 1% buckets,
>    so it is very possible to reclaim enough dirty buckets in this stage,
>    thus to avoid entering the next stage.
> 
> B. If the dirty buckets ratio didn’t turn around during the first stage,
>    it comes to the mid stage, then it is necessary for mid stage
>    to be more aggressive than low stage, so i choose the initial rate
>    to be 10 times more than low stage, that means 1700 as the initial
>    rate if the fragment is 6. This is some normal rate
>    we usually see for a normal workload when writeback happens
>    because of writeback_percent.
> 
> C. If the dirty buckets ratio didn't turn around during the low and mid
>    stages, it comes to the third stage, and it is the last chance that
>    we can turn around to avoid the horrible cutoff writeback sync issue,
>    then we choose 100 times more aggressive than the mid stage, that
>    means 170000 as the initial rate if the fragment is 6. This is also
>    inferred from a production bcache, I've got one week's writeback rate
>    data from a production bcache which has quite heavy workloads,
>    again, the writeback is trigged by the writeback percent,
                             ^^^^^^^^triggered


>    the highest rate area is around 100000 to 240000, so I believe this
>    kind aggressiveness at this stage is reasonable for production.
>    And it should be mostly enough because the hint is trying to reclaim
>    1000 bucket per second, and from that heavy production env,
>    it is consuming 50 bucket per second on avarage in one week's data.
                                             ^^^^^^^^average


> 
> Option writeback_consider_fragment is to control whether we want
> this feature to be on or off, it's on by default.
> 
> Lastly, below is the performance data for all the testing result,
> including the data from production env:
> https://docs.google.com/document/d/
> 1AmbIEa_2MhB9bqhC3rfga9tp7n9YX9PLn0jSUxscVW0/edit?usp=sharing
> 
> Signed-off-by: dongdong tao <dongdong.tao@...onical.com>


It looks find to me. The above two typos is fixed before I add this
patch in my for-next queue. I will have it in 5.12 bcache submission.

Thanks.


Coly Li


> ---
>  drivers/md/bcache/bcache.h    |  4 ++++
>  drivers/md/bcache/sysfs.c     | 23 +++++++++++++++++++
>  drivers/md/bcache/writeback.c | 42 +++++++++++++++++++++++++++++++++++
>  drivers/md/bcache/writeback.h |  4 ++++
>  4 files changed, 73 insertions(+)
> 
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 1d57f48307e6..d7a84327b7f1 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -373,6 +373,7 @@ struct cached_dev {
>  	unsigned int		partial_stripes_expensive:1;
>  	unsigned int		writeback_metadata:1;
>  	unsigned int		writeback_running:1;
> +	unsigned int		writeback_consider_fragment:1;
>  	unsigned char		writeback_percent;
>  	unsigned int		writeback_delay;
>  
> @@ -385,6 +386,9 @@ struct cached_dev {
>  	unsigned int		writeback_rate_update_seconds;
>  	unsigned int		writeback_rate_i_term_inverse;
>  	unsigned int		writeback_rate_p_term_inverse;
> +	unsigned int		writeback_rate_fp_term_low;
> +	unsigned int		writeback_rate_fp_term_mid;
> +	unsigned int		writeback_rate_fp_term_high;
>  	unsigned int		writeback_rate_minimum;
>  
>  	enum stop_on_failure	stop_when_cache_set_failed;
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 00a520c03f41..eef15f8022ba 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -117,10 +117,14 @@ rw_attribute(writeback_running);
>  rw_attribute(writeback_percent);
>  rw_attribute(writeback_delay);
>  rw_attribute(writeback_rate);
> +rw_attribute(writeback_consider_fragment);
>  
>  rw_attribute(writeback_rate_update_seconds);
>  rw_attribute(writeback_rate_i_term_inverse);
>  rw_attribute(writeback_rate_p_term_inverse);
> +rw_attribute(writeback_rate_fp_term_low);
> +rw_attribute(writeback_rate_fp_term_mid);
> +rw_attribute(writeback_rate_fp_term_high);
>  rw_attribute(writeback_rate_minimum);
>  read_attribute(writeback_rate_debug);
>  
> @@ -195,6 +199,7 @@ SHOW(__bch_cached_dev)
>  	var_printf(bypass_torture_test,	"%i");
>  	var_printf(writeback_metadata,	"%i");
>  	var_printf(writeback_running,	"%i");
> +	var_printf(writeback_consider_fragment,	"%i");
>  	var_print(writeback_delay);
>  	var_print(writeback_percent);
>  	sysfs_hprint(writeback_rate,
> @@ -205,6 +210,9 @@ SHOW(__bch_cached_dev)
>  	var_print(writeback_rate_update_seconds);
>  	var_print(writeback_rate_i_term_inverse);
>  	var_print(writeback_rate_p_term_inverse);
> +	var_print(writeback_rate_fp_term_low);
> +	var_print(writeback_rate_fp_term_mid);
> +	var_print(writeback_rate_fp_term_high);
>  	var_print(writeback_rate_minimum);
>  
>  	if (attr == &sysfs_writeback_rate_debug) {
> @@ -303,6 +311,7 @@ STORE(__cached_dev)
>  	sysfs_strtoul_bool(bypass_torture_test, dc->bypass_torture_test);
>  	sysfs_strtoul_bool(writeback_metadata, dc->writeback_metadata);
>  	sysfs_strtoul_bool(writeback_running, dc->writeback_running);
> +	sysfs_strtoul_bool(writeback_consider_fragment, dc->writeback_consider_fragment);
>  	sysfs_strtoul_clamp(writeback_delay, dc->writeback_delay, 0, UINT_MAX);
>  
>  	sysfs_strtoul_clamp(writeback_percent, dc->writeback_percent,
> @@ -331,6 +340,16 @@ STORE(__cached_dev)
>  	sysfs_strtoul_clamp(writeback_rate_p_term_inverse,
>  			    dc->writeback_rate_p_term_inverse,
>  			    1, UINT_MAX);
> +	sysfs_strtoul_clamp(writeback_rate_fp_term_low,
> +			    dc->writeback_rate_fp_term_low,
> +			    1, dc->writeback_rate_fp_term_mid - 1);
> +	sysfs_strtoul_clamp(writeback_rate_fp_term_mid,
> +			    dc->writeback_rate_fp_term_mid,
> +			    dc->writeback_rate_fp_term_low + 1,
> +			    dc->writeback_rate_fp_term_high - 1);
> +	sysfs_strtoul_clamp(writeback_rate_fp_term_high,
> +			    dc->writeback_rate_fp_term_high,
> +			    dc->writeback_rate_fp_term_mid + 1, UINT_MAX);
>  	sysfs_strtoul_clamp(writeback_rate_minimum,
>  			    dc->writeback_rate_minimum,
>  			    1, UINT_MAX);
> @@ -499,9 +518,13 @@ static struct attribute *bch_cached_dev_files[] = {
>  	&sysfs_writeback_delay,
>  	&sysfs_writeback_percent,
>  	&sysfs_writeback_rate,
> +	&sysfs_writeback_consider_fragment,
>  	&sysfs_writeback_rate_update_seconds,
>  	&sysfs_writeback_rate_i_term_inverse,
>  	&sysfs_writeback_rate_p_term_inverse,
> +	&sysfs_writeback_rate_fp_term_low,
> +	&sysfs_writeback_rate_fp_term_mid,
> +	&sysfs_writeback_rate_fp_term_high,
>  	&sysfs_writeback_rate_minimum,
>  	&sysfs_writeback_rate_debug,
>  	&sysfs_io_errors,
> diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
> index a129e4d2707c..82d4e0880a99 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -88,6 +88,44 @@ 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 buckets reached cutoff_writeback_sync and
> +	 * dirty data is still not even reached to writeback percent, so 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;
> +
> +	int64_t dirty_buckets = c->nbuckets - c->avail_nbuckets;
> +
> +	if (dc->writeback_consider_fragment &&
> +		c->gc_stats.in_use > BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW && dirty > 0) {
> +		int64_t fragment =
> +			div_s64((dirty_buckets *  c->cache->sb.bucket_size), dirty);
> +		int64_t fp_term;
> +		int64_t fps;
> +
> +		if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID) {
> +			fp_term = dc->writeback_rate_fp_term_low *
> +			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW);
> +		} else if (c->gc_stats.in_use <= BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH) {
> +			fp_term = dc->writeback_rate_fp_term_mid *
> +			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID);
> +		} else {
> +			fp_term = dc->writeback_rate_fp_term_high *
> +			(c->gc_stats.in_use - BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH);
> +		}
> +		fps = div_s64(dirty, dirty_buckets) * fp_term;
> +		if (fragment > 3 && fps > proportional_scaled) {
> +			/* Only overrite the p when fragment > 3 */
> +			proportional_scaled = fps;
> +		}
> +	}
> +
>  	if ((error < 0 && dc->writeback_rate_integral > 0) ||
>  	    (error > 0 && time_before64(local_clock(),
>  			 dc->writeback_rate.next + NSEC_PER_MSEC))) {
> @@ -977,6 +1015,7 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
>  
>  	dc->writeback_metadata		= true;
>  	dc->writeback_running		= false;
> +	dc->writeback_consider_fragment = true;
>  	dc->writeback_percent		= 10;
>  	dc->writeback_delay		= 30;
>  	atomic_long_set(&dc->writeback_rate.rate, 1024);
> @@ -984,6 +1023,9 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
>  
>  	dc->writeback_rate_update_seconds = WRITEBACK_RATE_UPDATE_SECS_DEFAULT;
>  	dc->writeback_rate_p_term_inverse = 40;
> +	dc->writeback_rate_fp_term_low = 1;
> +	dc->writeback_rate_fp_term_mid = 10;
> +	dc->writeback_rate_fp_term_high = 1000;
>  	dc->writeback_rate_i_term_inverse = 10000;
>  
>  	WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags));
> diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
> index 3f1230e22de0..02b2f9df73f6 100644
> --- a/drivers/md/bcache/writeback.h
> +++ b/drivers/md/bcache/writeback.h
> @@ -16,6 +16,10 @@
>  
>  #define BCH_AUTO_GC_DIRTY_THRESHOLD	50
>  
> +#define BCH_WRITEBACK_FRAGMENT_THRESHOLD_LOW 50
> +#define BCH_WRITEBACK_FRAGMENT_THRESHOLD_MID 57
> +#define BCH_WRITEBACK_FRAGMENT_THRESHOLD_HIGH 64
> +
>  #define BCH_DIRTY_INIT_THRD_MAX	64
>  /*
>   * 14 (16384ths) is chosen here as something that each backing device
> 

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