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| author | Konstantin Khlebnikov <khlebnikov@yandex-team.ru> | 2020-03-25 16:07:04 +0300 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2020-10-07 08:01:29 +0200 |
| commit | 2334b2d5a2bd1a4e9877623ef532b114d83bdf94 (patch) | |
| tree | 98f66508469fb81ee82b0482587d8a401adc0172 /Documentation | |
| parent | 1d13c3a5000b50de04e1611f7f8f04d3fd350d82 (diff) | |
block/diskstats: more accurate approximation of io_ticks for slow disks
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>
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/admin-guide/iostats.rst | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/Documentation/admin-guide/iostats.rst b/Documentation/admin-guide/iostats.rst index 5d63b18bd6d1..60c45c916f7d 100644 --- a/Documentation/admin-guide/iostats.rst +++ b/Documentation/admin-guide/iostats.rst @@ -99,7 +99,7 @@ Field 10 -- # of milliseconds spent doing I/Os 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 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 ------------------- |