bcache: consider the fragmentation when update the writeback rate

From: dongdong tao <[email protected]>

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 <[email protected]>

---
 drivers/md/bcache/bcache.h    |  1 +
 drivers/md/bcache/sysfs.c     |  6 ++++++
 drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
 3 files changed, 28 insertions(+)

drivers/md/bcache/bcache.h +1 -0

index 1d57f48307e6..87632f7032b6 100644
@@ -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;

drivers/md/bcache/sysfs.c +6 -0

index 554e3afc9b68..69499113aef8 100644
@@ -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,

drivers/md/bcache/writeback.c +21 -0

index 3c74996978da..34babc89fdf3 100644
@@ -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;

Quoted 23 lines · Dongdong Tao

From: dongdong tao <[email protected]>

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].

Quoted 7 lines

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 <[email protected]>

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

Quoted 96 lines

---
 drivers/md/bcache/bcache.h    |  1 +
 drivers/md/bcache/sysfs.c     |  6 ++++++
 drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
 3 files changed, 28 insertions(+)

drivers/md/bcache/bcache.h +1 -0

index 1d57f48307e6..87632f7032b6 100644
@@ -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;

drivers/md/bcache/sysfs.c +6 -0

index 554e3afc9b68..69499113aef8 100644
@@ -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,

drivers/md/bcache/writeback.c +21 -0

index 3c74996978da..34babc89fdf3 100644
@@ -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;

[Sorry again for the SPAM detection]

Thank you the reply Coly!

I agree that this patch is not a final solution for fixing the fragmentation issue, but more like a workaround to alleviate this problem.
So, part of my intention is to look for how upstream would like to fix this issue.

I've looked into the code of moving_gc part, as well as did some debug/test, unfortunately, I think it's not the solution for this issue also.
Because movnig_gc will not just move the dirty cache, but also the clean cache, so seems the purpose of moving_gc
is trying to move the data (dirty and clean) from those relatively empty buckets to some new buckets, so that to reclaim the original buckets.
For this purpose, I guess moving gc was more useful at the time when we usually don't have large nvme devices.

Let's get back to the problem I have, the problem that I'm trying to fix is that you might have lots of buckets (Say 70 percent) that are all being fully consumed,
while those buckets only contain very few dirty data (Say 10 percent ), since gc can't reclaim a bucket which contains any dirty data, so the worst situation
is that the cache_availability_percent can drop under 30 percent which will make all the write op can't perform in a writeback mode, thus kill the performance of writes.

So, unlike the moving_gc, I only want to move dirty data around (as you've suggested :)), but I don't think it's a good idea to change the behavior of moving_gc.
My current idea is to implement a compaction thread that triggers the dirty data compaction only under some certain circumstances (like when the fragmentaion and dirty buckets are both high), and this thread can be turned on/off based on an extra option, so that people can keep the original behavior if they want.

This is a rough idea now, could you please let me know if the above thought makes sense to you or any other suggestions will be appreciated!
I also understand the hardest part is making sure the general bcache performance and functionality still look sane,
so it might require much more time to do it and it's more likely a feature atm.

How to reproduce and observe this issue:
This issue is very easy to repreduce by running below fio command against a writeback mode bcache deivce:

fio --name=random-writers --filename=/dev/bcache0 --ioengine=libaio --iodepth=4 --rw=randrw --rate_iops=95,5 --bs=4k --direct=1 --numjobs=4

Note that the key option to reproduce this issue here is "rate_iops=95,5", so that you will have 95 percent read and only 5 percent write, this is to make sure
one bucket only contains very few dirty data.
Also, it's faster to reproduce this with a small cache device, I use 1GB cache, but it's same for bigger cache device, just a matter of time.

We can observe this issue by monitoring bcache stats "data_dirty" and "cache_available_percent", after the cache_available_percent dropped to 30 percent,
we can observe the write performance is hugely degraded by below bpftrace script:
---
#!/usr/bin/env bpftrace

#include <linux/bio.h>

kprobe:cached_dev_make_request

{
   @start[arg1] = nsecs;
}

kprobe:bio_endio /@start[arg0]/
{

 if(((struct bio *)arg0)->bi_opf & 1) {
    @write = hist(nsecs - @start[arg0]); delete(@start[arg0]);
 }
 else {
    @read = hist(nsecs - @start[arg0]); delete(@start[arg0]);

}
}
---

To run this script：
Save above bpftrace file to bcache_io_lat.bt, then run it with chmod +x bcache_io_lat.bt & ./bcache_io_lat.bt

By the way, we mainly hit this issue on ceph, the fio reproducer is just an easy way to reproduce it.

Quoted 48 lines · Dongdong Tao

Quoted 23 lines

From: dongdong tao <[email protected]>

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].

Quoted 7 lines

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 <[email protected]>

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

From: dongdong tao <[email protected]>

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 <[email protected]>

---
  drivers/md/bcache/bcache.h    |  1 +
  drivers/md/bcache/sysfs.c     |  6 ++++++
  drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
  3 files changed, 28 insertions(+)

drivers/md/bcache/bcache.h +1 -0

index 1d57f48307e6..87632f7032b6 100644
@@ -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;

drivers/md/bcache/sysfs.c +0 -0

index 554e3afc9b68..69499113aef8 100644

drivers/md/bcache/sysfs.c +5 -0

  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,

drivers/md/bcache/writeback.c +21 -0

index 3c74996978da..34babc89fdf3 100644
@@ -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;

Quoted 53 lines · Dongdong Tao

From: dongdong tao <[email protected]>

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 <[email protected]>

---
  drivers/md/bcache/bcache.h    |  1 +
  drivers/md/bcache/sysfs.c     |  6 ++++++
  drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
  3 files changed, 28 insertions(+)

drivers/md/bcache/bcache.h +1 -0

index 1d57f48307e6..87632f7032b6 100644
@@ -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;

drivers/md/bcache/sysfs.c +0 -0

index 554e3afc9b68..69499113aef8 100644

drivers/md/bcache/sysfs.c rw_attribute(stop_when_cache_set_failed); · line 118

Quoted 4 lines

drivers/md/bcache/sysfs.c +1 -0

@@ -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);

Hi Dongdong and Coly,

What is the status about this patch? In my opinion, it is a problem
we need to solve,

but can we just reuse the parameter of writeback_percent, rather than introduce a new writeback_fragment_percent?

That means the semantic of writeback_percent will act on dirty data percent and dirty bucket percent.

When we found there are dirty buckets more than (c->nbuckets * writeback_percent), start the writeback.

Thanx

Yang

Quoted 68 lines

drivers/md/bcache/sysfs.c +5 -0

  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,

drivers/md/bcache/writeback.c +21 -0

index 3c74996978da..34babc89fdf3 100644
@@ -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;

Hi Dongsheng,

I'm working on it, next step I'm gathering some testing data and upload (very sorry for the delay...)
Thanks for the comment.
One of the main concerns to alleviate this issue with the writeback process is that we need to minimize the impact on the client IO performance.
writeback_percent by default is 10, start writeback when dirty buckets reached 10 percent might be a bit too aggressive, as the writeback_cutoff_sync is 70 percent.
So i chose to start the writeback when dirty buckets reached 50 percent so that this patch will only take effect after dirty buckets percent is above that

Thanks,
Dongdong

Quoted 147 lines · Dongsheng Yang

Quoted 53 lines · Dongdong Tao

From: dongdong tao <[email protected]>

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 <[email protected]>

---
   drivers/md/bcache/bcache.h    |  1 +
   drivers/md/bcache/sysfs.c     |  6 ++++++
   drivers/md/bcache/writeback.c | 21 +++++++++++++++++++++
   3 files changed, 28 insertions(+)

drivers/md/bcache/bcache.h +1 -0

index 1d57f48307e6..87632f7032b6 100644
@@ -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;

drivers/md/bcache/sysfs.c +0 -0

index 554e3afc9b68..69499113aef8 100644

drivers/md/bcache/sysfs.c rw_attribute(stop_when_cache_set_failed); · line 118

Quoted 4 lines

drivers/md/bcache/sysfs.c +1 -0

@@ -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);

Hi Dongdong and Coly,

What is the status about this patch? In my opinion, it is a problem
we need to solve,

but can we just reuse the parameter of writeback_percent, rather than introduce a new writeback_fragment_percent?

That means the semantic of writeback_percent will act on dirty data percent and dirty bucket percent.

When we found there are dirty buckets more than (c->nbuckets * writeback_percent), start the writeback.

Thanx

Yang

Quoted 68 lines

drivers/md/bcache/sysfs.c +5 -0

   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,

drivers/md/bcache/writeback.c +21 -0

index 3c74996978da..34babc89fdf3 100644
@@ -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;

Hi Coly and Dongsheng,

I've get the testing result and confirmed that this testing result is reproducible by repeating it many times.
I ran fio to get the write latency log and parsed the log and then generated below latency graphs with some visualization tool

So, my code will accelerate the writeback process when the dirty buckets exceeds 50%( can be tuned), as we can see
the cache_available_percent does increase once it hit 50, so we won't hit writeback cutoff issue.

Below are the steps that I used to do the experiment:
1. make-bcache -B <hdd> -C <nvme> --writeback -- I prepared the nvme size to 1G, so it can be reproduced faster

2. sudo fio --name=random-writers --filename=/dev/bcache0 --ioengine=libaio --iodepth=1 --rw=randrw --bs=16k --direct=1 --rate_iops=90,10 --numjobs=1 --write_lat_log=16k

3. For 1 G nvme, running for about 20 minutes is enough get the data.

Using randrw with rate_iops=90,10 is just one way to reproduce this easily, this can be reproduced as long as we can create a fragmented situation that quite few dirty data consumed a lot dirty buckets thus killing the write performance.

This bug nowadays is becoming very critical, as ceph is hitting it, ceph mostly submitting random small IO.
Please let me know what you need in order to move forward in this direction, I'm sure this patch can be improved also.

Hi Coly,

Thank you so much for your prompt reply!

So, I've performed the same fio testing based on 1TB NVME and 10 TB HDD disk as the backing device.
I've run them both for about 4 hours, since it's 1TB nvme device, it will roughly take about 10 days to consume 50 percent dirty buckets I did increase the iops to 500,100, but the dirty buckets only increased to about 30 after 2 days run, and because the read is much limited by the backing hdd deivce, so the actual maximum read iops I can constantly get is only about 200.

Though the 4 hours run on 1TB nvme bcache didn't make us hit the 50 percent dirty bucket threshold, but I think it's still valuable to prove that
bcache with my patch behaves the same way as expected in terms of the latency when the dirty bucket is under 50 percent. I guess this might be what you wanted to confirm with this run.

Here is the result:
Master:

fio-master.png

Master + My patch:
fio-patch.png
As we can see, the latency distributions for the outliers or those majorities are the same between these two runs。
Let's combine those two together, and they are more clear:
fio-full.png
The test steps are exactly the same for those two runs:

1. make-bcache -B <hdd> -C <nvme> --writeback

2. sudo fio --name=random-writers --filename=/dev/bcache0 --ioengine=libaio --iodepth=1 --rw=randrw --bs=16k --direct=1 --rate_iops=90,10 --numjobs=1 --write_lat_log=16k --runtime=14000

Thank you so much!
Regards,
Dongdong

On Tue, Dec 15, 2020 at 1:07 AM Coly Li <[email protected] <mailto:[email protected]>> wrote:

On 12/14/20 11:30 PM, Dongdong Tao wrote:
> Hi Coly and Dongsheng,
>
> I've get the testing result and confirmed that this testing result is > reproducible by repeating it many times.
> I ran fio to get the write latency log and parsed the log and then > generated below latency graphs with some visualization tool >

Hi Dongdong,

Thank you so much for the performance number!

[snipped]
> So, my code will accelerate the writeback process when the dirty buckets
> exceeds 50%( can be tuned), as we can see
> the cache_available_percent does increase once it hit 50, so we won't > hit writeback cutoff issue.
>
> Below are the steps that I used to do the experiment:
> 1. make-bcache -B <hdd> -C <nvme> --writeback -- I prepared the nvme > size to 1G, so it can be reproduced faster
>
> 2. sudo fio --name=random-writers --filename=/dev/bcache0 > --ioengine=libaio --iodepth=1 --rw=randrw --bs=16k --direct=1 > --rate_iops=90,10 --numjobs=1 --write_lat_log=16k
>
> 3. For 1 G nvme, running for about 20 minutes is enough get the data.

1GB cache and 20 minutes is quite limited for the performance valuation. Could you please to do similar testing with 1TB SSD and 1 hours for each run of the benchmark ?

>
> Using randrw with rate_iops=90,10 is just one way to reproduce this > easily, this can be reproduced as long as we can create a fragmented > situation that quite few dirty data consumed a lot dirty buckets thus > killing the write performance.
>

Yes this is a good method to generate dirty data segments.

> This bug nowadays is becoming very critical, as ceph is hitting it, ceph
> mostly submitting random small IO.
> Please let me know what you need in order to move forward in this > direction, I'm sure this patch can be improved also.

The performance number is quite convinced and the idea in your patch is promising.

I will provide my comments on your patch after we see the performance number for larger cache device and longer run time.

Thanks again for the detailed performance number, which is really desired for performance optimization changes :-)

Coly Li