On Thu, 2018-10-04 at 13:09 -0700, Bart Van Assche wrote:

On Thu, 2018-10-04 at 20:25 +0100, Alan Cox wrote:

quoted

quoted

I agree with Jens that it's best to leave it to the Linux distributors to
select a default I/O scheduler.

That assumes such a thing exists. The kernel knows what devices it is
dealing with. The kernel 'default' ought to be 'whatever is usually best
for this device'. A distro cannot just pick a correct single default
because NVME and USB sticks are both normal and rather different in needs.

Which I/O scheduler works best also depends which workload the user will run.
BFQ has significant advantages for interactive workloads like video replay
with concurrent background I/O but probably slows down kernel builds. That's
why I'm not sure whether the kernel should select the default I/O scheduler.

Whats wrong with this simple hierarchy?

1. Block core selects the default scheduler.
2. Driver can overrule it early.
3. Userspace can overrule the default later.

Everyone is happy.

Good defaults in block core are great. Those defaults + #3 may cover
99% of the population.

1% of the population can use #2. See, Linus wants "bfq" for ubiblock.
Why wouldn't we to let him work with UBI community, show that bfq is 
best for ubiblock, and just let the UBI community overrule the block
core's default.

If some day in the future there is a very good reason, we can even make
this to be a module parameter, and people could just boot with
'ubiblock.iosched=bfq'.