Hi, I am running FIO script on Linux 4.15. This is generic behavior even on 3.x kernels as well. I wanted to know if my observation is correct or not. Here is FIO command - numactl -C 0-2 fio single --bs=4k --iodepth=64 --rw=randread --ioscheduler=none --group_report --numjobs=2 If driver is provides affinity_hint, kernel choose only kworker (0,1,2) (it looks like kworker binding is smartly handled by kernel because I am running FIO from cpu0,1,2) for IO submission from delayed context. 14140 root 15 -5 519296 1560 612 R 87.7 0.0 0:20.91 fio 14138 root 15 -5 519292 1556 608 R 76.1 0.0 0:21.79 fio 14142 root 15 -5 519308 1560 612 R 66.8 0.0 0:19.69 fio 14141 root 15 -5 519304 1564 616 R 54.5 0.0 0:20.51 fio 923 root 0 -20 0 0 0 S 6.3 0.0 0:09.73 kworker/1:1H 1075 root 0 -20 0 0 0 S 5.3 0.0 0:08.69 kworker/0:1H 924 root 0 -20 0 0 0 S 3.3 0.0 0:12.82 kworker/2:1H If driver is not providing affinity_hint, kernel choose *any* kworker from local numa node for IO submission from delayed context. In below snippet, you can see kworke4, kworke5 and kworke3 was participating in IO submission. 14281 root 15 -5 519308 1556 612 R 87.0 0.0 0:16.16 fio 14280 root 15 -5 519304 1560 616 R 74.1 0.0 0:14.62 fio 14279 root 15 -5 519296 1556 612 R 71.8 0.0 0:15.02 fio 14277 root 15 -5 519292 1552 608 R 66.8 0.0 0:15.06 fio 1887 root 0 -20 0 0 0 R 15.3 0.0 0:40.91 kworker/4:1H 3856 root 0 -20 0 0 0 S 13.6 0.0 0:38.90 kworker/5:1H 3646 root 0 -20 0 0 0 S 13.0 0.0 0:40.17 kworker/3:1H Which kernel component is making this decision ? Is this behavior tied to block layer/irq subsystem ? I am trying to see which behavior is most suitable for my test. I am seeing performance is not improving because it is CPU bound and If I choose not to do smp affinity hint in driver, it is helping as explained above. Thanks, Kashyap
