From: Konstantin Khlebnikov khlebnikov@yandex-team.ru
commit 2b8bd423614c595540eaadcfbc702afe8e155e50 upstream.
Currently io_ticks is approximated by adding one at each start and end of requests if jiffies counter has changed. This works perfectly for requests shorter than a jiffy or if one of requests starts/ends at each jiffy.
If disk executes just one request at a time and they are longer than two jiffies then only first and last jiffies will be accounted.
Fix is simple: at the end of request add up into io_ticks jiffies passed since last update rather than just one jiffy.
Example: common HDD executes random read 4k requests around 12ms.
fio --name=test --filename=/dev/sdb --rw=randread --direct=1 --runtime=30 & iostat -x 10 sdb
Note changes of iostat's "%util" 8,43% -> 99,99% before/after patch:
Before:
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util sdb 0,00 0,00 82,60 0,00 330,40 0,00 8,00 0,96 12,09 12,09 0,00 1,02 8,43
After:
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util sdb 0,00 0,00 82,50 0,00 330,00 0,00 8,00 1,00 12,10 12,10 0,00 12,12 99,99
Now io_ticks does not loose time between start and end of requests, but for queue-depth > 1 some I/O time between adjacent starts might be lost.
For load estimation "%util" is not as useful as average queue length, but it clearly shows how often disk queue is completely empty.
Fixes: 5b18b5a73760 ("block: delete part_round_stats and switch to less precise counting") Signed-off-by: Konstantin Khlebnikov khlebnikov@yandex-team.ru Reviewed-by: Ming Lei ming.lei@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk From: "Banerjee, Debabrata" dbanerje@akamai.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- Documentation/admin-guide/iostats.rst | 5 ++++- block/bio.c | 8 ++++---- block/blk-core.c | 4 ++-- include/linux/genhd.h | 2 +- 4 files changed, 11 insertions(+), 8 deletions(-)
--- a/Documentation/admin-guide/iostats.rst +++ b/Documentation/admin-guide/iostats.rst @@ -99,7 +99,7 @@ Field 10 -- # of milliseconds spent doin
Since 5.0 this field counts jiffies when at least one request was started or completed. If request runs more than 2 jiffies then some - I/O time will not be accounted unless there are other requests. + I/O time might be not accounted in case of concurrent requests.
Field 11 -- weighted # of milliseconds spent doing I/Os This field is incremented at each I/O start, I/O completion, I/O @@ -133,6 +133,9 @@ are summed (possibly overflowing the uns summed to) and the result given to the user. There is no convenient user interface for accessing the per-CPU counters themselves.
+Since 4.19 request times are measured with nanoseconds precision and +truncated to milliseconds before showing in this interface. + Disks vs Partitions -------------------
--- a/block/bio.c +++ b/block/bio.c @@ -1754,14 +1754,14 @@ defer: schedule_work(&bio_dirty_work); }
-void update_io_ticks(struct hd_struct *part, unsigned long now) +void update_io_ticks(struct hd_struct *part, unsigned long now, bool end) { unsigned long stamp; again: stamp = READ_ONCE(part->stamp); if (unlikely(stamp != now)) { if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) { - __part_stat_add(part, io_ticks, 1); + __part_stat_add(part, io_ticks, end ? now - stamp : 1); } } if (part->partno) { @@ -1777,7 +1777,7 @@ void generic_start_io_acct(struct reques
part_stat_lock();
- update_io_ticks(part, jiffies); + update_io_ticks(part, jiffies, false); part_stat_inc(part, ios[sgrp]); part_stat_add(part, sectors[sgrp], sectors); part_inc_in_flight(q, part, op_is_write(op)); @@ -1795,7 +1795,7 @@ void generic_end_io_acct(struct request_
part_stat_lock();
- update_io_ticks(part, now); + update_io_ticks(part, now, true); part_stat_add(part, nsecs[sgrp], jiffies_to_nsecs(duration)); part_stat_add(part, time_in_queue, duration); part_dec_in_flight(q, part, op_is_write(req_op)); --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1334,7 +1334,7 @@ void blk_account_io_done(struct request part_stat_lock(); part = req->part;
- update_io_ticks(part, jiffies); + update_io_ticks(part, jiffies, true); part_stat_inc(part, ios[sgrp]); part_stat_add(part, nsecs[sgrp], now - req->start_time_ns); part_stat_add(part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns)); @@ -1376,7 +1376,7 @@ void blk_account_io_start(struct request rq->part = part; }
- update_io_ticks(part, jiffies); + update_io_ticks(part, jiffies, false);
part_stat_unlock(); } --- a/include/linux/genhd.h +++ b/include/linux/genhd.h @@ -419,7 +419,7 @@ static inline void free_part_info(struct kfree(part->info); }
-void update_io_ticks(struct hd_struct *part, unsigned long now); +void update_io_ticks(struct hd_struct *part, unsigned long now, bool end);
/* block/genhd.c */ extern void device_add_disk(struct device *parent, struct gendisk *disk,