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authorJan Kara <jack@suse.cz>2017-04-04 14:31:30 +0200
committerJens Axboe <axboe@fb.com>2017-04-05 08:15:08 -0600
commit142bbdfccc8b3e9f7342f2ce8422e76a3b45beae (patch)
treefaea05f2de4fe568818cc2942802c6a16ed92f52 /block
parent85003a446e9d058c9c3e6cfa5d991ace07330a68 (diff)
downloadlinux-0-day-142bbdfccc8b3e9f7342f2ce8422e76a3b45beae.tar.gz
cfq: Disable writeback throttling by default
Writeback throttling does not play well with CFQ since that also tries to throttle async writes. As a result async writeback can get starved in presence of readers. As an example take a benchmark simulating postgreSQL database running over a standard rotating SATA drive. There are 16 processes doing random reads from a huge file (2*machine memory), 1 process doing random writes to the huge file and calling fsync once per 50000 writes and 1 process doing sequential 8k writes to a relatively small file wrapping around at the end of the file and calling fsync every 5 writes. Under this load read latency easily exceeds the target latency of 75 ms (just because there are so many reads happening against a relatively slow disk) and thus writeback is throttled to a point where only 1 write request is allowed at a time. Blktrace data then looks like: 8,0 1 0 8.347751764 0 m N cfq workload slice:40000000 8,0 1 0 8.347755256 0 m N cfq293A / set_active wl_class: 0 wl_type:0 8,0 1 0 8.347784100 0 m N cfq293A / Not idling. st->count:1 8,0 1 3814 8.347763916 5839 UT N [kworker/u9:2] 1 8,0 0 0 8.347777605 0 m N cfq293A / Not idling. st->count:1 8,0 1 0 8.347784100 0 m N cfq293A / Not idling. st->count:1 8,0 3 1596 8.354364057 0 C R 156109528 + 8 (6906954) [0] 8,0 3 0 8.354383193 0 m N cfq6196SN / complete rqnoidle 0 8,0 3 0 8.354386476 0 m N cfq schedule dispatch 8,0 3 0 8.354399397 0 m N cfq293A / Not idling. st->count:1 8,0 3 0 8.354404705 0 m N cfq293A / dispatch_insert 8,0 3 0 8.354409454 0 m N cfq293A / dispatched a request 8,0 3 0 8.354412527 0 m N cfq293A / activate rq, drv=1 8,0 3 1597 8.354414692 0 D W 145961400 + 24 (6718452) [swapper/0] 8,0 3 0 8.354484184 0 m N cfq293A / Not idling. st->count:1 8,0 3 0 8.354487536 0 m N cfq293A / slice expired t=0 8,0 3 0 8.354498013 0 m N / served: vt=5888102466265088 min_vt=5888074869387264 8,0 3 0 8.354502692 0 m N cfq293A / sl_used=6737519 disp=1 charge=6737519 iops=0 sect=24 8,0 3 0 8.354505695 0 m N cfq293A / del_from_rr ... 8,0 0 1810 8.354728768 0 C W 145961400 + 24 (314076) [0] 8,0 0 0 8.354746927 0 m N cfq293A / complete rqnoidle 0 ... 8,0 1 3829 8.389886102 5839 G W 145962968 + 24 [kworker/u9:2] 8,0 1 3830 8.389888127 5839 P N [kworker/u9:2] 8,0 1 3831 8.389908102 5839 A W 145978336 + 24 <- (8,4) 44000 8,0 1 3832 8.389910477 5839 Q W 145978336 + 24 [kworker/u9:2] 8,0 1 3833 8.389914248 5839 I W 145962968 + 24 (28146) [kworker/u9:2] 8,0 1 0 8.389919137 0 m N cfq293A / insert_request 8,0 1 0 8.389924305 0 m N cfq293A / add_to_rr 8,0 1 3834 8.389933175 5839 UT N [kworker/u9:2] 1 ... 8,0 0 0 9.455290997 0 m N cfq workload slice:40000000 8,0 0 0 9.455294769 0 m N cfq293A / set_active wl_class:0 wl_type:0 8,0 0 0 9.455303499 0 m N cfq293A / fifo=ffff880003166090 8,0 0 0 9.455306851 0 m N cfq293A / dispatch_insert 8,0 0 0 9.455311251 0 m N cfq293A / dispatched a request 8,0 0 0 9.455314324 0 m N cfq293A / activate rq, drv=1 8,0 0 2043 9.455316210 6204 D W 145962968 + 24 (1065401962) [pgioperf] 8,0 0 0 9.455392407 0 m N cfq293A / Not idling. st->count:1 8,0 0 0 9.455395969 0 m N cfq293A / slice expired t=0 8,0 0 0 9.455404210 0 m N / served: vt=5888958194597888 min_vt=5888941810597888 8,0 0 0 9.455410077 0 m N cfq293A / sl_used=4000000 disp=1 charge=4000000 iops=0 sect=24 8,0 0 0 9.455416851 0 m N cfq293A / del_from_rr ... 8,0 0 2045 9.455648515 0 C W 145962968 + 24 (332305) [0] 8,0 0 0 9.455668350 0 m N cfq293A / complete rqnoidle 0 ... 8,0 1 4371 9.455710115 5839 G W 145978336 + 24 [kworker/u9:2] 8,0 1 4372 9.455712350 5839 P N [kworker/u9:2] 8,0 1 4373 9.455730159 5839 A W 145986616 + 24 <- (8,4) 52280 8,0 1 4374 9.455732674 5839 Q W 145986616 + 24 [kworker/u9:2] 8,0 1 4375 9.455737563 5839 I W 145978336 + 24 (27448) [kworker/u9:2] 8,0 1 0 9.455742871 0 m N cfq293A / insert_request 8,0 1 0 9.455747550 0 m N cfq293A / add_to_rr 8,0 1 4376 9.455756629 5839 UT N [kworker/u9:2] 1 So we can see a Q event for a write request, then IO is blocked by writeback throttling and G and I events for the request happen only once other writeback IO is completed. Thus CFQ always sees only one write request. When it sees it, it queues the async queue behind all the read queues and the async queue gets scheduled after about one second. When it is scheduled, that one request gets dispatched and async queue is expired as it has no more requests to submit. Overall we submit about one write request per second. Although this scheduling is beneficial for read latency, writes are heavily starved and this causes large delays all over the system (due to processes blocking on page lock, transaction starts, etc.). When writeback throttling is disabled, write throughput is about one fifth of a read throughput which roughly matches readers/writers ratio and overall the system stalls are much shorter. Mixing writeback throttling logic with CFQ throttling logic is always a recipe for surprises as CFQ assumes it sees the big part of the picture which is not necessarily true when writeback throttling is blocking requests. So disable writeback throttling logic by default when CFQ is used as an IO scheduler. Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'block')
-rw-r--r--block/cfq-iosched.c17
1 files changed, 5 insertions, 12 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 440b95e..da69b07 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3761,16 +3761,14 @@ static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
}
#ifdef CONFIG_CFQ_GROUP_IOSCHED
-static bool check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
+static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
{
struct cfq_data *cfqd = cic_to_cfqd(cic);
struct cfq_queue *cfqq;
uint64_t serial_nr;
- bool nonroot_cg;
rcu_read_lock();
serial_nr = bio_blkcg(bio)->css.serial_nr;
- nonroot_cg = bio_blkcg(bio) != &blkcg_root;
rcu_read_unlock();
/*
@@ -3778,7 +3776,7 @@ static bool check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
* spuriously on a newly created cic but there's no harm.
*/
if (unlikely(!cfqd) || likely(cic->blkcg_serial_nr == serial_nr))
- return nonroot_cg;
+ return;
/*
* Drop reference to queues. New queues will be assigned in new
@@ -3799,12 +3797,10 @@ static bool check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
}
cic->blkcg_serial_nr = serial_nr;
- return nonroot_cg;
}
#else
-static inline bool check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
+static inline void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
{
- return false;
}
#endif /* CONFIG_CFQ_GROUP_IOSCHED */
@@ -4449,12 +4445,11 @@ cfq_set_request(struct request_queue *q, struct request *rq, struct bio *bio,
const int rw = rq_data_dir(rq);
const bool is_sync = rq_is_sync(rq);
struct cfq_queue *cfqq;
- bool disable_wbt;
spin_lock_irq(q->queue_lock);
check_ioprio_changed(cic, bio);
- disable_wbt = check_blkcg_changed(cic, bio);
+ check_blkcg_changed(cic, bio);
new_queue:
cfqq = cic_to_cfqq(cic, is_sync);
if (!cfqq || cfqq == &cfqd->oom_cfqq) {
@@ -4491,9 +4486,6 @@ new_queue:
rq->elv.priv[1] = cfqq->cfqg;
spin_unlock_irq(q->queue_lock);
- if (disable_wbt)
- wbt_disable_default(q);
-
return 0;
}
@@ -4706,6 +4698,7 @@ static void cfq_registered_queue(struct request_queue *q)
*/
if (blk_queue_nonrot(q))
cfqd->cfq_slice_idle = 0;
+ wbt_disable_default(q);
}
/*