Question about IO schedulers

[Etal]

Linux 4.12 includes two new IO schedulers. Curiously, "make oldconfig" asked me if I wanted to enable them but didn't ask me what I wanted as the default, so I decided to double-check with menuconfig:

[Ant P.]

The menu choice for default is only for legacy single-queue schedulers (the ones above it), the ones at the bottom are multiqueue ones. You can only be using one of the two lists at any time depending on if you have CONFIG_SCSI_MQ_DEFAULT or the corresponding bootparam set. The unused ones are dead code and you may as well remove them from your kernel.

There isn't a menu option for default mq scheduler, it picks a per-device default in block/elevator.c based on whether the hardware reports native multiqueue support or not. If you want to override it, the only way right now is to remove mq-deadline or use an /etc/local.d/ script.

[mbar]

Also https://www.phoronix.com/scan.php?page=article&item=linux412-hddssd-io&num=1

[Jaglover]

Phoronix test does not mention what filesystem was used. XFS wiki.
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[CaptainBlood]

[Jaglover]

[BlueFusion]

I just upgraded to kernel 4.12 today and wanted to take advantage of in-kernel BFQ IO scheduler. Perhaps, I misunderstand it's usefulness, however.

I disabled all IO schedulers except BFQ, which is compiled in-kernel. I added the following to my kernel command line:

[thumper]

Blk-mq-based device drivers bypass the previous Linux I/O scheduler.

This should get you started in the proper direction.
https://www.thomas-krenn.com/en/wiki/Linux_Multi-Queue_Block_IO_Queueing_Mechanism_(blk-mq)

George

[Moderator edit: fixed [url] tag; auto-linking does not work with embedded parentheses. -Hu]

[BlueFusion]

I was under the impression that there are 3 distinct blk-mq schedulers - Kyber, Deadline-mq, and BFQ (maybe a noop-style one, too?).

If I remove the scsi_mod.use_blk_mq=y dm_mod.use_blk_mq=y from the kernel command line, and just specify elevator=bfq, the system does not boot - it stops just after loading modules. The fact that I can also manually activate the bfq scheduler via the command referenced in my previous post also gives the impression that you can specify specific blk-mq schedulers but it is not possible to activate non-blk-mq schedulers (i.e. CFQ). BFQ is what used to be called bfq-mq, no?

Thanks for the link, George. I stumbled upon that earlier in my Google searching. It seems to be somewhat dated, however. I've read elsewhere that the reason it bypassed all other IO schedulers is simply because they needed to be rewritten to work together or something of the sort. Perhaps that is why we can now switch between them?
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[mbar]

[Etal]

Thanks for the pointers, everyone!

From the source file Ant P. pointed out, it looks like that it completely ignores any parameters with MQ schedulers and just selects "mq-deadline" if available and the the device has 1 hardware queue, or "none" otherwise. So the only way to change it is through an init script.

By the way, is there a way to check how many hardware queues a device has?
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[Juippisi]

The mainline kernel didnt update their Kconfig when they added new schedulers, theres a patch that allows you to select BFQ as default in menuconfig.
https://patchwork.kernel.org/patch/9825423/

I think you also have enable few things in kernel config in order for the system to boot with BFQ.