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2b8bd42361
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:
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|---|---|---|
| .. | ||
| acpi | ||
| asm-generic | ||
| clocksource | ||
| crypto | ||
| drm | ||
| dt-bindings | ||
| keys | ||
| kunit | ||
| kvm | ||
| linux | ||
| math-emu | ||
| media | ||
| misc | ||
| net | ||
| pcmcia | ||
| ras | ||
| rdma | ||
| scsi | ||
| soc | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| vdso | ||
| video | ||
| xen | ||