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- Hi,
- When some application launches several hundreds of processes that issue
- only a few small sync I/O requests, CFQ may cause heavy latencies
- (10+ seconds at the worst case), although the request rate is low enough for
- the disk to handle it without waiting. This is because CFQ waits for
- slice_idle (default:8ms) every time before processing each request, until
- their thinktimes are evaluated.
- This scenario can be reproduced using fio with parameters below:
- fio -filename=/tmp/test -rw=randread -size=5G -runtime=15 -name=file1 \
- -bs=4k -numjobs=500 -thinktime=1000000
- In this case, 500 processes issue a random read request every second.
- This problem can be avoided by setting slice_idle to 0, but there is a
- risk to hurt throughput performance on S-ATA disks.
- This patch tries to reduce the effect of slice_idle automatically when a
- lot of busy queues are waiting in the idle window.
- It adds a counter (busy_idle_queues) of queues in idle window that have
- I/O requests to cfq_data. And if (busy_idle_queues * slice_idle) goes over
- the slice allocated to the group, it limits the idle wait time to
- (group_slice / busy_idle_queues).
- Without this patch, fio benchmark with parameters above to an ext4
- partition on a S-ATA HDD results in:
- read : io=20140KB, bw=1258.5KB/s, iops=314 , runt= 16004msec
- clat (usec): min=4 , max=6494.9K, avg=541264.54, stdev=993834.12
- With this patch:
- read : io=28040KB, bw=1750.1KB/s, iops=437 , runt= 16014msec
- clat (usec): min=4 , max=2837.2K, avg=110236.79, stdev=303351.72
- Average latency is reduced by 80%, and max is also reduced by 56%.
- Any comments are appreciated.
- Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
- ---
- block/cfq-iosched.c | 36 +++++++++++++++++++++++++++++++-----
- 1 file changed, 31 insertions(+), 5 deletions(-)
- diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
- index d5cd313..77ac27e80 100644
- --- a/block/cfq-iosched.c
- +++ b/block/cfq-iosched.c
- @@ -329,6 +329,7 @@ struct cfq_data {
- unsigned int busy_queues;
- unsigned int busy_sync_queues;
- + unsigned int busy_idle_queues; /* busy but with idle window */
- int rq_in_driver;
- int rq_in_flight[2];
- @@ -446,6 +447,20 @@ CFQ_CFQQ_FNS(deep);
- CFQ_CFQQ_FNS(wait_busy);
- #undef CFQ_CFQQ_FNS
- +static inline void cfq_set_cfqq_idle_window(struct cfq_data *cfqd,
- + struct cfq_queue *cfqq, bool idle)
- +{
- + if (idle) {
- + cfq_mark_cfqq_idle_window(cfqq);
- + if (cfq_cfqq_on_rr(cfqq))
- + cfqd->busy_idle_queues++;
- + } else {
- + cfq_clear_cfqq_idle_window(cfqq);
- + if (cfq_cfqq_on_rr(cfqq))
- + cfqd->busy_idle_queues--;
- + }
- +}
- +
- static inline struct cfq_group *pd_to_cfqg(struct blkg_policy_data *pd)
- {
- return pd ? container_of(pd, struct cfq_group, pd) : NULL;
- @@ -2164,6 +2179,8 @@ static void cfq_add_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
- cfqd->busy_queues++;
- if (cfq_cfqq_sync(cfqq))
- cfqd->busy_sync_queues++;
- + if (cfq_cfqq_idle_window(cfqq))
- + cfqd->busy_idle_queues++;
- cfq_resort_rr_list(cfqd, cfqq);
- }
- @@ -2192,6 +2209,8 @@ static void cfq_del_cfqq_rr(struct cfq_data *cfqd, struct cfq_queue *cfqq)
- cfqd->busy_queues--;
- if (cfq_cfqq_sync(cfqq))
- cfqd->busy_sync_queues--;
- + if (cfq_cfqq_idle_window(cfqq))
- + cfqd->busy_idle_queues--;
- }
- /*
- @@ -2761,6 +2780,16 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
- else
- sl = cfqd->cfq_slice_idle;
- + /*
- + * If there too many queues with idle window, slice idle can cause
- + * unacceptable latency. Then we reduce slice idle here.
- + */
- + if (cfqd->busy_idle_queues) {
- + unsigned group_slice = cfq_group_slice(cfqd, cfqq->cfqg);
- + unsigned long limit = group_slice / cfqd->busy_idle_queues;
- + sl = min(sl, limit);
- + }
- +
- mod_timer(&cfqd->idle_slice_timer, jiffies + sl);
- cfqg_stats_set_start_idle_time(cfqq->cfqg);
- cfq_log_cfqq(cfqd, cfqq, "arm_idle: %lu group_idle: %d", sl,
- @@ -3091,7 +3120,7 @@ static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
- (cfq_cfqq_slice_new(cfqq) ||
- (cfqq->slice_end - jiffies > jiffies - cfqq->slice_start))) {
- cfq_clear_cfqq_deep(cfqq);
- - cfq_clear_cfqq_idle_window(cfqq);
- + cfq_set_cfqq_idle_window(cfqd, cfqq, false);
- }
- if (cfqq->dispatched && cfq_should_idle(cfqd, cfqq)) {
- @@ -3742,10 +3771,7 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
- if (old_idle != enable_idle) {
- cfq_log_cfqq(cfqd, cfqq, "idle=%d", enable_idle);
- - if (enable_idle)
- - cfq_mark_cfqq_idle_window(cfqq);
- - else
- - cfq_clear_cfqq_idle_window(cfqq);
- + cfq_set_cfqq_idle_window(cfqd, cfqq, enable_idle);
- }
- }
- --
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