On 11/2/22 16:34, Tom Cameron wrote:
> All,
>
> I recently filed https://github.com/axboe/fio/issues/1483 on github
> describing my usage of multiple concurrent jobs to split random read
> and random write so I can have a different number of reader and writer
> threads. I was informed that what I actually want is to use "flow=" to
> specify the ratio of activity between jobs.
>
> My issue while testing the use of flow, is that it seems to be off by
> a factor of 10x. For example, the following has two jobs, one for
> readers and one for writers. The writer, as can be seen, has "numjobs"
> set to 8, while the reader has "numjobs" set to 64. Flow is set to
> "100" for reader, and "1" for writer. But from the output below we can
> see the actual split is more like 786 to 1.
>
> Jobs: 72 (f=72):
> [w(8),r(64)][26.6%][r=1254MiB/s,w=1632KiB/s][r=321k,w=408 IOPS][eta
> 44m:11s]
>
> Should I be dividing the flow value by the ratio of "numjobs" as well?
> The command running obviously has an 8:1 ratio of readers to writers,
> and I've defined a 100:1 ratio in the value of "flow", so am I
> actually seeing an 800:1 here?
>
> I suspect I'm missing a key piece of information, but the
> documentation doesn't explicitly call this out. The line that makes me
> think I may need to adjust for "numjobs" within each job is "The flow
> parameter stands for how much should be added or subtracted to the
> flow counter on each iteration of the main I/O loop.". This makes me
> suspect that each of the threads spawned under a job definition is
> spawned with it's own 100:1 ratio, and since there's an 8:1 numjobs
> ratio, I end up seeing the above.
>
> The command, in total, is below.
>
> fio
--output=fio-logs/m2test-01/20221102/manual-flow-test.write-1.qd-32/fio.manual-flow-test.log
> \
> --exitall_on_error --bs=4Ki --direct=1 --fadvise_hint=random
> --filename=/mnt/testing/fio \
> --group_reporting=1 --iodepth=32 --ioengine=libaio
> --ioscheduler=mq-deadline --log_avg_msec=1000 \
> --log_compression=1m --log_store_compressed=1 --ramp_time=10
> --randrepeat=0 --runtime=1h --thread \
> --time_based
--write_bw_log=fio-logs/m2test-01/20221102/manual-flow-test.write-1.qd-32/fio-stress
> \
>
--write_iops_log=fio-logs/m2test-01/20221102/manual-flow-test.write-1.qd-32/fio-stress
> \
>
--write_lat_log=fio-logs/m2test-01/20221102/manual-flow-test.write-1.qd-32/fio-stress
> \
> --name=writer --new_group --stonewall --numjobs=8 --rw=randwrite
> --flow=1 --exitall \
> --name=reader --new_group --numjobs=64 --rw=randread --flow=100
>
>
> Thanks for any advice,
> Tom
numjobs=X directs fio to behave as if you had specified the same job
configuration X times. So your options create 8 writer jobs with flow=1
and 64 reader jobs with flow=100.
If you want a 100:1 r/w ratio you will need to use rw=randrw and rwmixread.
The flow feature does not guarantee a specified r/w mix because your
device may have different read and write performance. What flow does is
to direct jobs to skip opportunities to issue IO using the specified
weights. The resulting r/w mix depends on this of course but also
depends on the relative r/w performance of your device.
Vincent
