Still on 6.7.4.

Started scrub.

Three threads as described above, main process & two threads (shown with H command to htop)

 PPID   PID USER         IO  IORR  IOWR IO PRI  NI  VIRT   RES   SHR S CPU% MEM%   TIME+  Command
    1 29003 root      41462 41462     0 B3  19  -1 32180   200    16 S  5.6  0.0  0:26.28 `- btrfs scrub start /N316AR6
    1 29005 root          0     0     0 B3  19  -1 32180   200    16 S  0.0  0.0  0:00.03 |  `- btrfs
    1 29004 root      41462 41462     0 id  19  -1 32180   200    16 D  5.6  0.0  0:26.26 |  `- btrfs

I'm reading that as the IO/IORR/IOWR & CPU usage is accounted for in the main process too, rather than both doing I/O & CPU.

Note active thread is idle iopriority, nice -1 (as most things spawned from readynas seem to inherit -1).

However, sort by CPU, (K command to show kernal threads), everything with CPU > 0.

 PPID   PID USER         IO  IORR  IOWR IO PRI  NI  VIRT   RES   SHR S CPU% MEM%   TIME+  Command
    2 29066 root          0     0     0 B4  20   0     0     0     0 R 39.9  0.0  3:52.37 kworker/u8:10
    2 29043 root          0     0     0 B4  20   0     0     0     0 R 38.2  0.0  4:47.15 kworker/u8:9
    2 28656 root          0     0     0 B4  20   0     0     0     0 R 37.0  0.0  4:37.22 kworker/u8:2
    2 28793 root          0     0     0 B4  20   0     0     0     0 R 37.0  0.0  4:42.18 kworker/u8:3
    2 30086 root          0     0     0 B4  20   0     0     0     0 R 35.8  0.0  1:05.82 kworker/u8:14
    2 28903 root       57.2  57.2     0 B4  20   0     0     0     0 S 33.4  0.0  4:37.18 kworker/u8:8
    2 28071 root          0     0     0 B4  20   0     0     0     0 R 33.4  0.0  4:27.62 kworker/u8:0
    2 29095 root       19.2  19.2     0 B4  20   0     0     0     0 R 31.6  0.0  1:44.06 kworker/u8:13
    2 28902 root          0     0     0 B4  20   0     0     0     0 R 31.0  0.0  4:56.74 kworker/u8:7
    2  1515 root          0     0     0 B4  17  -1     0     0     0 R 25.6  0.0 35h04:21 md127_raid6
    2 27507 root          0     0     0 B4  20   0     0     0     0 R 17.9  0.0  4:31.39 kworker/u8:1
    2 29067 root          0     0     0 B4  20   0     0     0     0 R 17.9  0.0  4:30.13 kworker/u8:11
    1 29003 root      45490 45490     0 B3  19  -1 32180   200    16 S  6.0  0.0  1:12.96 btrfs scrub start /N316AR6
    1 29004 root      45517 45517     0 id  19  -1 32180   200    16 D  6.0  0.0  1:12.87 btrfs
    2 29001 root          0     0     0 ??  39  19     0     0     0 R  3.0  0.0  2:22.40 md127_resync
28517 28522 root          0     0     0 B4  20   0 29460  3688  3004 R  3.0  0.2  0:56.68 htop
    2  1402 root          0     0     0 B0   0 -20     0     0     0 S  1.8  0.0  0:54.53 kworker/1:1H
    2  1401 root          0     0     0 B0   0 -20     0     0     0 S  1.2  0.0  0:50.00 kworker/0:1H
    2  1359 root          0     0     0 B0   0 -20     0     0     0 S  0.6  0.0  3:29.12 kworker/2:1H
    2  1389 root          0     0     0 B0   0 -20     0     0     0 S  0.6  0.0  3:34.20 kworker/3:1H
    2     7 root          0     0     0 B4  20   0     0     0     0 R  0.6  0.0  4:10.65 rcu_sched
    1  3703 nut           0     0     0 B4  20   0 17240  1296   932 S  0.6  0.1  1:31.42 /lib/nut/usbhid-ups -a UPS

Note all the CPU chewed up by kworker threads, Nice 0, doing little IO. (kworker IO increased later, similar numbers as above, but more threads doing IO)

btaroli's media processes (infering from the above iopriority B7/B6 - I don't know what they are doing) will have lesser nice values, and be CPU constrained. Those at B4 will have Nice 0, round-robin with all those kworker's, so ~1/12th CPU (?5/17th?).

How changing the iopriority of the other two threads can change this I can't fathom ATM.

Perhaps media threads with a 'interactive' workload should run nice -1 for now.

Longer term there should be a gap between current -1 processes (readynas etc) and default worker thread priority (currently 0 ), so intermidiate priority things can fit in the middle??

I'll repeat this on 6.7.5 when I get around to it, I'm currently juggling 10TB disks upgrading...nothing happens fast...

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Circle for RBR850