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Subject[PATCH v2] blk-throttle: simplify logic by token bucket algorithm
Date
From: Hong Zhiguo <zhiguohong@tencent.com>

Token bucket algorithm(http://en.wikipedia.org/wiki/Token_bucket)
is very simple and widely used for rate limiting and shaping.
It's well suitable for blk-throttle. And it natually works for
hierarchical cgroups. So I took it to replace the original time
_slicing_|_trimming_|_extending_ logic.

The advantage is simplicity, reliability and less fluctuation.

About 300 lines of code for time-slicing is replaced with 60 lines of
code for token bucket in blk-throttle.c.

I've tested this patch by fio with rw=randread, rw=randrw. It
behaves almost the same with original time-slicing implementation,
and with more accuracy.

For example, create 2 cgroups and set blkio.throttle.read_bps_device
to 51200(50K) and 204800(200K), start fio with rw=randread. The
Average and Standard Deviation of bandwidth report:
----------------------------------------------
Time Slicing | 50K group | 200K group
----------------------------------------------
Average: | 50.1176 | 200.4470
Standard Deviation: | 1.2977 | 3.6272
----------------------------------------------

----------------------------------------------
Token Bucket | 50K group | 200K group
----------------------------------------------
Average: | 50.1647 | 200.4705
Standard Deviation: | 0.5498 | 1.0080
----------------------------------------------

Signed-off-by: Hong Zhiguo <zhiguohong@tencent.com>
Tested-by: Hong Zhiguo <zhiguohong@tencent.com>
---
block/blk-throttle.c | 284 ++++++++-------------------------------------------
1 file changed, 43 insertions(+), 241 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8331aba..ec26555 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -18,9 +18,6 @@ static int throtl_grp_quantum = 8;
/* Total max dispatch from all groups in one round */
static int throtl_quantum = 32;

-/* Throttling is performed over 100ms slice and after that slice is renewed */
-static unsigned long throtl_slice = HZ/10; /* 100 ms */
-
static struct blkcg_policy blkcg_policy_throtl;

/* A workqueue to queue throttle related work */
@@ -91,6 +88,11 @@ struct tg_stats_cpu {
struct blkg_rwstat serviced;
};

+/* Depth of iops token bucket */
+#define THROTL_BURST_IO 8
+/* Depth of bps token bucket */
+#define THROTL_BURST_BYTES (4096 * 8)
+
struct throtl_grp {
/* must be the first member */
struct blkg_policy_data pd;
@@ -133,14 +135,13 @@ struct throtl_grp {
/* IOPS limits */
unsigned int iops[2];

- /* Number of bytes disptached in current slice */
- uint64_t bytes_disp[2];
- /* Number of bio's dispatched in current slice */
- unsigned int io_disp[2];
+ /* Token for throttling by bps */
+ uint64_t bytes_token[2];
+ /* Token for throttling by iops */
+ unsigned int io_token[2];

- /* When did we start a new slice */
- unsigned long slice_start[2];
- unsigned long slice_end[2];
+ /* Time check-point */
+ unsigned long t_c[2];

/* Per cpu stats pointer */
struct tg_stats_cpu __percpu *stats_cpu;
@@ -680,171 +681,26 @@ static bool throtl_schedule_next_dispatch(struct throtl_service_queue *sq,
return false;
}

-static inline void throtl_start_new_slice_with_credit(struct throtl_grp *tg,
- bool rw, unsigned long start)
-{
- tg->bytes_disp[rw] = 0;
- tg->io_disp[rw] = 0;
-
- /*
- * Previous slice has expired. We must have trimmed it after last
- * bio dispatch. That means since start of last slice, we never used
- * that bandwidth. Do try to make use of that bandwidth while giving
- * credit.
- */
- if (time_after_eq(start, tg->slice_start[rw]))
- tg->slice_start[rw] = start;
-
- tg->slice_end[rw] = jiffies + throtl_slice;
- throtl_log(&tg->service_queue,
- "[%c] new slice with credit start=%lu end=%lu jiffies=%lu",
- rw == READ ? 'R' : 'W', tg->slice_start[rw],
- tg->slice_end[rw], jiffies);
-}
-
-static inline void throtl_start_new_slice(struct throtl_grp *tg, bool rw)
-{
- tg->bytes_disp[rw] = 0;
- tg->io_disp[rw] = 0;
- tg->slice_start[rw] = jiffies;
- tg->slice_end[rw] = jiffies + throtl_slice;
- throtl_log(&tg->service_queue,
- "[%c] new slice start=%lu end=%lu jiffies=%lu",
- rw == READ ? 'R' : 'W', tg->slice_start[rw],
- tg->slice_end[rw], jiffies);
-}
-
-static inline void throtl_set_slice_end(struct throtl_grp *tg, bool rw,
- unsigned long jiffy_end)
-{
- tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
-}
-
-static inline void throtl_extend_slice(struct throtl_grp *tg, bool rw,
- unsigned long jiffy_end)
-{
- tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
- throtl_log(&tg->service_queue,
- "[%c] extend slice start=%lu end=%lu jiffies=%lu",
- rw == READ ? 'R' : 'W', tg->slice_start[rw],
- tg->slice_end[rw], jiffies);
-}
-
-/* Determine if previously allocated or extended slice is complete or not */
-static bool throtl_slice_used(struct throtl_grp *tg, bool rw)
-{
- if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
- return 0;
-
- return 1;
-}
-
-/* Trim the used slices and adjust slice start accordingly */
-static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
-{
- unsigned long nr_slices, time_elapsed, io_trim;
- u64 bytes_trim, tmp;
-
- BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]));
-
- /*
- * If bps are unlimited (-1), then time slice don't get
- * renewed. Don't try to trim the slice if slice is used. A new
- * slice will start when appropriate.
- */
- if (throtl_slice_used(tg, rw))
- return;
-
- /*
- * A bio has been dispatched. Also adjust slice_end. It might happen
- * that initially cgroup limit was very low resulting in high
- * slice_end, but later limit was bumped up and bio was dispached
- * sooner, then we need to reduce slice_end. A high bogus slice_end
- * is bad because it does not allow new slice to start.
- */
-
- throtl_set_slice_end(tg, rw, jiffies + throtl_slice);
-
- time_elapsed = jiffies - tg->slice_start[rw];
-
- nr_slices = time_elapsed / throtl_slice;
-
- if (!nr_slices)
- return;
- tmp = tg->bps[rw] * throtl_slice * nr_slices;
- do_div(tmp, HZ);
- bytes_trim = tmp;
-
- io_trim = (tg->iops[rw] * throtl_slice * nr_slices)/HZ;
-
- if (!bytes_trim && !io_trim)
- return;
-
- if (tg->bytes_disp[rw] >= bytes_trim)
- tg->bytes_disp[rw] -= bytes_trim;
- else
- tg->bytes_disp[rw] = 0;
-
- if (tg->io_disp[rw] >= io_trim)
- tg->io_disp[rw] -= io_trim;
- else
- tg->io_disp[rw] = 0;
-
- tg->slice_start[rw] += nr_slices * throtl_slice;
-
- throtl_log(&tg->service_queue,
- "[%c] trim slice nr=%lu bytes=%llu io=%lu start=%lu end=%lu jiffies=%lu",
- rw == READ ? 'R' : 'W', nr_slices, bytes_trim, io_trim,
- tg->slice_start[rw], tg->slice_end[rw], jiffies);
-}
-
static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio,
unsigned long *wait)
{
bool rw = bio_data_dir(bio);
- unsigned int io_allowed;
- unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
- u64 tmp;
-
- jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
-
- /* Slice has just started. Consider one slice interval */
- if (!jiffy_elapsed)
- jiffy_elapsed_rnd = throtl_slice;
-
- jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);
+ u64 token;

- /*
- * jiffy_elapsed_rnd should not be a big value as minimum iops can be
- * 1 then at max jiffy elapsed should be equivalent of 1 second as we
- * will allow dispatch after 1 second and after that slice should
- * have been trimmed.
- */
-
- tmp = (u64)tg->iops[rw] * jiffy_elapsed_rnd;
- do_div(tmp, HZ);
+ token = (u64)tg->iops[rw] * (jiffies - tg->t_c[rw]);
+ do_div(token, HZ);
+ token += tg->io_token[rw];

- if (tmp > UINT_MAX)
- io_allowed = UINT_MAX;
- else
- io_allowed = tmp;
-
- if (tg->io_disp[rw] + 1 <= io_allowed) {
+ if (token) {
+ tg->io_token[rw] = token;
if (wait)
*wait = 0;
return 1;
}

/* Calc approx time to dispatch */
- jiffy_wait = ((tg->io_disp[rw] + 1) * HZ)/tg->iops[rw] + 1;
-
- if (jiffy_wait > jiffy_elapsed)
- jiffy_wait = jiffy_wait - jiffy_elapsed;
- else
- jiffy_wait = 1;
-
if (wait)
- *wait = jiffy_wait;
+ *wait = HZ / tg->iops[rw] ?: 1;
return 0;
}

@@ -852,41 +708,26 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
unsigned long *wait)
{
bool rw = bio_data_dir(bio);
- u64 bytes_allowed, extra_bytes, tmp;
- unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
-
- jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
-
- /* Slice has just started. Consider one slice interval */
- if (!jiffy_elapsed)
- jiffy_elapsed_rnd = throtl_slice;
-
- jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);
+ u64 extra_bytes, token;
+ unsigned long jiffy_wait;

- tmp = tg->bps[rw] * jiffy_elapsed_rnd;
- do_div(tmp, HZ);
- bytes_allowed = tmp;
+ token = (u64)tg->bps[rw] * (jiffies - tg->t_c[rw]);
+ do_div(token, HZ);
+ token += tg->bytes_token[rw];

- if (tg->bytes_disp[rw] + bio->bi_size <= bytes_allowed) {
+ if (bio->bi_size <= token) {
+ tg->bytes_token[rw] = token;
if (wait)
*wait = 0;
return 1;
}

/* Calc approx time to dispatch */
- extra_bytes = tg->bytes_disp[rw] + bio->bi_size - bytes_allowed;
- jiffy_wait = div64_u64(extra_bytes * HZ, tg->bps[rw]);
-
- if (!jiffy_wait)
- jiffy_wait = 1;
-
- /*
- * This wait time is without taking into consideration the rounding
- * up we did. Add that time also.
- */
- jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
- if (wait)
- *wait = jiffy_wait;
+ if (wait) {
+ extra_bytes = bio->bi_size - token;
+ jiffy_wait = div64_u64(extra_bytes * HZ, tg->bps[rw]);
+ *wait = jiffy_wait ?: 1;
+ }
return 0;
}

@@ -916,18 +757,6 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
return 1;
}

- /*
- * If previous slice expired, start a new one otherwise renew/extend
- * existing slice to make sure it is at least throtl_slice interval
- * long since now.
- */
- if (throtl_slice_used(tg, rw))
- throtl_start_new_slice(tg, rw);
- else {
- if (time_before(tg->slice_end[rw], jiffies + throtl_slice))
- throtl_extend_slice(tg, rw, jiffies + throtl_slice);
- }
-
if (tg_with_in_bps_limit(tg, bio, &bps_wait) &&
tg_with_in_iops_limit(tg, bio, &iops_wait)) {
if (wait)
@@ -940,9 +769,6 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
if (wait)
*wait = max_wait;

- if (time_before(tg->slice_end[rw], jiffies + max_wait))
- throtl_extend_slice(tg, rw, jiffies + max_wait);
-
return 0;
}

@@ -976,10 +802,16 @@ static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
{
bool rw = bio_data_dir(bio);

- /* Charge the bio to the group */
- tg->bytes_disp[rw] += bio->bi_size;
- tg->io_disp[rw]++;
+ /* Charge the bio to the group and trim the bucket */
+ tg->bytes_token[rw] -= bio->bi_size;
+ if (tg->bytes_token[rw] > THROTL_BURST_BYTES)
+ tg->bytes_token[rw] = THROTL_BURST_BYTES;

+ tg->io_token[rw]--;
+ if (tg->io_token[rw] > THROTL_BURST_IO)
+ tg->io_token[rw] = THROTL_BURST_IO;
+
+ tg->t_c[rw] = jiffies;
/*
* REQ_THROTTLED is used to prevent the same bio to be throttled
* more than once as a throttled bio will go through blk-throtl the
@@ -1055,16 +887,6 @@ static void tg_update_disptime(struct throtl_grp *tg)
tg->flags &= ~THROTL_TG_WAS_EMPTY;
}

-static void start_parent_slice_with_credit(struct throtl_grp *child_tg,
- struct throtl_grp *parent_tg, bool rw)
-{
- if (throtl_slice_used(parent_tg, rw)) {
- throtl_start_new_slice_with_credit(parent_tg, rw,
- child_tg->slice_start[rw]);
- }
-
-}
-
static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
{
struct throtl_service_queue *sq = &tg->service_queue;
@@ -1093,7 +915,6 @@ static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
*/
if (parent_tg) {
throtl_add_bio_tg(bio, &tg->qnode_on_parent[rw], parent_tg);
- start_parent_slice_with_credit(tg, parent_tg, rw);
} else {
throtl_qnode_add_bio(bio, &tg->qnode_on_parent[rw],
&parent_sq->queued[rw]);
@@ -1101,8 +922,6 @@ static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
tg->td->nr_queued[rw]--;
}

- throtl_trim_slice(tg, rw);
-
if (tg_to_put)
blkg_put(tg_to_blkg(tg_to_put));
}
@@ -1386,12 +1205,8 @@ static int tg_set_conf(struct cgroup_subsys_state *css, struct cftype *cft,
* We're already holding queue_lock and know @tg is valid. Let's
* apply the new config directly.
*
- * Restart the slices for both READ and WRITES. It might happen
- * that a group's limit are dropped suddenly and we don't want to
- * account recently dispatched IO with new low rate.
+ * Update dispatch time.
*/
- throtl_start_new_slice(tg, 0);
- throtl_start_new_slice(tg, 1);

if (tg->flags & THROTL_TG_PENDING) {
tg_update_disptime(tg);
@@ -1527,19 +1342,6 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
throtl_charge_bio(tg, bio);

/*
- * We need to trim slice even when bios are not being queued
- * otherwise it might happen that a bio is not queued for
- * a long time and slice keeps on extending and trim is not
- * called for a long time. Now if limits are reduced suddenly
- * we take into account all the IO dispatched so far at new
- * low rate and * newly queued IO gets a really long dispatch
- * time.
- *
- * So keep on trimming slice even if bio is not queued.
- */
- throtl_trim_slice(tg, rw);
-
- /*
* @bio passed through this layer without being throttled.
* Climb up the ladder. If we''re already at the top, it
* can be executed directly.
@@ -1552,10 +1354,10 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
}

/* out-of-limit, queue to @tg */
- throtl_log(sq, "[%c] bio. bdisp=%llu sz=%u bps=%llu iodisp=%u iops=%u queued=%d/%d",
+ throtl_log(sq, "[%c] bio. btk=%llu sz=%u bps=%llu iotk=%u iops=%u queued=%d/%d",
rw == READ ? 'R' : 'W',
- tg->bytes_disp[rw], bio->bi_size, tg->bps[rw],
- tg->io_disp[rw], tg->iops[rw],
+ tg->bytes_token[rw], bio->bi_size, tg->bps[rw],
+ tg->io_token[rw], tg->iops[rw],
sq->nr_queued[READ], sq->nr_queued[WRITE]);

bio_associate_current(bio);
--
1.8.1.2


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