[SRU] [G] [PATCH 1/1] bcache: consider the fragmentation when update the writeback rate
Dongdong Tao
dongdong.tao at canonical.com
Fri Mar 26 03:10:21 UTC 2021
From: dongdong tao <dongdong.tao at canonical.com>
BugLink: https://bugs.launchpad.net/bugs/1900438
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 triggered by the writeback percent,
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 average in one week's data.
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 at canonical.com>
Signed-off-by: Coly Li <colyli at suse.de>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(backported from commit 71dda2a5625f31bc3410cb69c3d31376a2b66f28)
Signed-off-by: dongdong tao <dongdong.tao at canonical.com>
---
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 80e3c4813fb0..867b7de94a71 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 0dadec5a78f6..dc9ee83326dd 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -113,10 +113,14 @@ 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);
@@ -191,6 +195,7 @@ 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,
@@ -201,6 +206,9 @@ 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) {
@@ -299,6 +307,7 @@ 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,
@@ -327,6 +336,16 @@ 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);
@@ -495,9 +514,13 @@ 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 ab101ad55459..4bb2416ec304 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->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))) {
@@ -972,6 +1010,7 @@ 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);
@@ -979,6 +1018,9 @@ 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
--
2.17.1
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