Re: [PATCH v3 1/3] block/diskstats: more accurate approximation of io_ticks for slow disks
From: Konstantin Khlebnikov
Date: Wed Mar 25 2020 - 02:28:18 EST
On 25/03/2020 06.40, Ming Lei wrote:
On Tue, Mar 24, 2020 at 10:06:56PM +0800, Ming Lei wrote:
On Tue, Mar 24, 2020 at 09:39:40AM +0300, Konstantin Khlebnikov wrote:
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
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@xxxxxxxxxxxxxx>
---
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(-)
diff --git a/Documentation/admin-guide/iostats.rst b/Documentation/admin-guide/iostats.rst
index df5b8345c41d..9b14b0c2c9c4 100644
--- a/Documentation/admin-guide/iostats.rst
+++ b/Documentation/admin-guide/iostats.rst
@@ -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
-------------------
diff --git a/block/bio.c b/block/bio.c
index 0985f3422556..b1053eb7af37 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1762,14 +1762,14 @@ void bio_check_pages_dirty(struct bio *bio)
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) {
@@ -1785,7 +1785,7 @@ 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));
@@ -1803,7 +1803,7 @@ 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));
diff --git a/block/blk-core.c b/block/blk-core.c
index abfdcf81a228..4401b30a1751 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -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();
}
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index d5c75df64bba..f1066f10b062 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -467,7 +467,7 @@ static inline void free_part_info(struct hd_struct *part)
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,
Looks fine:
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
BTW, there is still some gap(%65 vs. 99%) between this fix and the original
accounting(before applying Mike/Mikulas's 7 patches), and it might be
one thing to improve in future.
1) test, sda is single queue virtio-scsi, which is emulated by one HDD
image
2) fio test script:
fio --direct=1 --size=128G --bsrange=4k-4k \
--runtime=20 --numjobs=1 \
--ioengine=libaio --iodepth=16 \
--iodepth_batch_submit=16 \
--iodepth_batch_complete_min=16 \
--group_reporting=1 --filename=/dev/sda \
--name=seq-test --rw=read
3) result:
- v5.6-rc with this patch
Run status group 0 (all jobs):
READ: bw=79.4MiB/s (83.3MB/s), 79.4MiB/s-79.4MiB/s (83.3MB/s-83.3MB/s), io=155
88MiB (1665MB), run=20001-20001msec
Disk stats (read/write):
sda: ios=25039/0, merge=375596/0, ticks=18823/0, in_queue=4330, util=99.43%
- commit 112f158f66cb (which is previous commit of 5b18b5a73760)
Run status group 0 (all jobs):
READ: bw=81.4MiB/s (85.3MB/s), 81.4MiB/s-81.4MiB/s (85.3MB/s-85.3MB/s), io=166
28MiB (1707MB), run=20001-20001msec
Disk stats (read/write):
sda: ios=25749/0, merge=386236/0, ticks=17963/0, in_queue=12976, util=65.20%
Yep, my patch guarantees correct time only for queue-depth=1
it accounts jiffies between start -> end (and end -> end).
For queue-depth > 1 jiffies between start -> start are still lost.
Without exact counter for in-flight requests we cannot distinguish
cases start -> start and end -> start
All we could could is smoothing statistics even more and
account all short gaps as activity:
--- a/block/bio.c
+++ b/block/bio.c
@@ -1769,6 +1769,7 @@ void update_io_ticks(struct hd_struct *part, unsigned long now, bool end)
stamp = READ_ONCE(part->stamp);
if (unlikely(stamp != now)) {
if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+ end |= time_before(now, stamp + HZ / 100);
__part_stat_add(part, io_ticks, end ? now - stamp : 1);
}
}
Thanks,
Ming