Hi all, During our test of the CFQ group schedule, we found a performance related problem. Rate-capped fio jobs in a CFQ group will degrade the performance of fio jobs in another CFQ group. Both of the CFQ groups have the same blkio.weight.
We launch two fios in difference terminals. The following is the content of these two job files: # cat a.job [global] bs=4k ioengine=psync iodepth=1 direct=1 rw=randwrite time_based runtime=30 cgroup_nodelete=1 group_reporting=1 [test1] filename=test1.dat size=2G cgroup_weight=500 cgroup=testA thread=1 numjobs=2 [test3] stonewall filename=test3.dat size=2G rate=4m cgroup_weight=500 cgroup=testA thread=1 numjobs=2 # cat b.job [global] bs=4k ioengine=psync iodepth=1 direct=1 rw=randwrite time_based runtime=60 cgroup_nodelete=1 group_reporting=1 [test2] filename=test2.dat size=2G cgroup_weight=500 cgroup=testB thread=1 numjobs=2 In the first 30 seconds, the iops of both "test1" and "test3" is ~5000 and the rate is about 20MBps. In the last 30 seconds, we start "test3" and the rate of "test3" is limited to 8MBps (4MBps * 2 jobs). The cap works fine, but the iops of "test2" also degraded from ~5000 to ~2040 or lower. This may seems reasonable for CFQ group schedule because it needs to dispatch disk time equally to these two CFQ group, but it doesn't fully utilize the disk throughput. It can be reproduced on 4.4 and 4.10.0-rc7, the version of fio is 2.2.4. The configuration options of CFQ are untouched. I am not sure whether this situation is a bug or not. Are there some configuration options we can used to alleviate the performance degradation problem ? And how CFQ group schedule handles the trace-off between fairness and performance ? Any suggestion will be appreciated. Regards, Tao
