Commit 2b8bd423 authored by Konstantin Khlebnikov's avatar Konstantin Khlebnikov Committed by Jens Axboe
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block/diskstats: more accurate approximation of io_ticks for slow disks 



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: 5b18b5a7 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: default avatarKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 387048bf

Documentation/admin-guide/iostats.rst

+4 −1
Original line number Diff line number Diff line
@@ -100,7 +100,7 @@ Field 10 -- # of milliseconds spent doing I/Os (unsigned int) 


    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 (unsigned int)

    This field is incremented at each I/O start, I/O completion, I/O
@@ -143,6 +143,9 @@ are summed (possibly overflowing the unsigned long variable they are 
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

-------------------

block/bio.c

+4 −4
Original line number Diff line number Diff line
@@ -1768,14 +1768,14 @@ 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) {
@@ -1791,7 +1791,7 @@ void generic_start_io_acct(struct request_queue *q, int op, 


	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));
@@ -1809,7 +1809,7 @@ void generic_end_io_acct(struct request_queue *q, int req_op, 


	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));

block/blk-core.c

+2 −2
Original line number Diff line number Diff line
@@ -1337,7 +1337,7 @@ void blk_account_io_done(struct request *req, u64 now) 
		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));
@@ -1379,7 +1379,7 @@ void blk_account_io_start(struct request *rq, bool new_io) 
		rq->part = part;

	}



	update_io_ticks(part, jiffies);

	update_io_ticks(part, jiffies, false);



	part_stat_unlock();

}

include/linux/genhd.h

+1 −1
Original line number Diff line number Diff line
@@ -422,7 +422,7 @@ void part_dec_in_flight(struct request_queue *q, struct hd_struct *part, 
void part_inc_in_flight(struct request_queue *q, struct hd_struct *part,

			int rw);



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,

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