IO Performance loss after kernel upgrades from 5.15 to 6.x

[devsk]

Has anybody tracked the IO performance across kernel upgrades?

I seem to have run into a situation where the performance is halved in the worst case. I am comparing three LTS kernels:

5.15.145
6.1.68
6.6.8 (purported next LTS)

I have a RAIDZ1 of 4 NVME SSDs. Same ZFS version 2.2.2. The only thing I change is the kernel version, rest everything is same (same machine, same ZFS version, same pool, same userspace). The kernel config has been brought forward with 'make oldconfig'. So, in almost all aspects, it should be identical for common config elements between 5.15, 6.1 and 6.6.

Zpool scrub times:

5.15.145 - 10m
6.1.68 - 12m
6.6.8 - 20m

Attached kdiskmark shots from 5.15.145 vs 6.6.8 (I did not run for 6.1.68 ). I ran it on top of the same ZFS dataset where compression is disabled. All numbers are lower for 6.6.8.

5.15.145 https://www.phoronix.com/forums/filedata/fetch?id=1431236
6.6.8 https://www.phoronix.com/forums/filedata/fetch?id=1431237

I was expecting 6.6 to be higher numbers because of all the IO enhancements work that has gone in since 5.15.

Is there something obvious that I missed and I need to tune specifically to double up the sequential IO speed in 6.6.8?

[devsk]

BTW, its not a ZFS regression because the raw IO speed with 'dd' in non-cached mode has fallen as well.

[devsk]

Is it possibly because of this: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3ca9a836ff53

Looks like we changed the CPU scheduler implementation completely in 6.6. I wonder how this affects the disk IO. Is there some sort of throttling going on because of this?

[pietinger]

Moderator note: I have split some posts and moved them to Discussion about CUDA -- pietinger
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[pietinger]

With kernel version 6.6 we have a new task scheduler: EEVDF
( https://kernelnewbies.org/LinuxChanges#Linux_6.6.New_task_scheduler:_EEVDF )
... but its hard to believe this can be the cause for your IO problems ... ? ... maybe ZFS needs some adjustments to this ?
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[gentoo_ram]

I believe the default I/O schedulers have changed in the various kernel versions. That might have something to do with it. If you can boot your different kernels then maybe compare the I/O scheduler settings. Try:

[devsk]

[devsk]

This regression is present in 6.7.3 as well. The system boots in 45seconds while 5.15 boots in about 25s. The zpool scrub over a pool of NVME drives is running at half the speed compared to 5.15. So, definitely some major issues with kernels beyond 5.15.

I am surprised that no one else is noticing any slowdowns over 2 LTS kernels (6.1 and 6.6). Maybe folks don't test performance that often.

[logrusx]

I recently switched to 6.6 and is seems like there's some degradation in responsiveness of the desktop which could be attributed to the CPU scheduler. I remember at one time I used to use Con Kolivas patches to overcome that, then it was not necessary anymore.

Best Regards,
Georgi

[devsk]

The worst part here is that once the IO starts to build up because of slowness in IO, the desktop starts to suffer: mouse stutters, menus don't open, applications don't fire when you click on them. Its a complete shutshow.

I can not reproduce any of that with 5.15.

I think they made that CPU scheduler change in a hurry and did not add an option to stay with older code via a config option. If they did that, we could at least confirm that that indeed is the issue.

[Saundersx]

Just for giggles try this, I had a regression back in 5.19.6 and this made it tolerable.

[devsk]