Re: [GIT PULL] Block pull request for- 4.11-rc1

[Jens Axboe]

On 02/22/2017 02:50 PM, Markus Trippelsdorf wrote:
> On 2017.02.22 at 11:44 -0700, Jens Axboe wrote:
>> On 02/22/2017 11:42 AM, Linus Torvalds wrote:
>>> On Wed, Feb 22, 2017 at 10:26 AM, Linus Torvalds
>>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> And dammit, IF YOU DON'T EVEN KNOW, WHY THE HELL ARE YOU ASKING THE POOR USER?
>>>
>>> Basically, I'm pushing back on config options that I can't personally
>>> even sanely answer.
>>
>> I got that much, and I don't disagree on that part.
>>
>>> If it's a config option about "do I have a particular piece of
>>> hardware", it makes sense. But these new ones were just complete
>>> garbage.
>>>
>>> The whole "default IO scheduler" thing is a disease. We should stop
>>> making up these shit schedulers and then say "we don't know which one
>>> works best for you".
>>>
>>> All it does is encourage developers to make shortcuts and create crap
>>> that isn't generically useful, and then blame the user and say "well,
>>> you should have picked a different scheduler" when they say "this does
>>> not work well for me".
>>>
>>> We have had too many of those kinds of broken choices.  And when the
>>> new Kconfig options get so confusing and so esoteric that I go "Hmm, I
>>> have no idea if my hardware does a single queue or not", I put my foot
>>> down.
>>>
>>> When the IO scheduler questions were about a generic IO scheduler for
>>> everything, I can kind of understand them. I think it was still a
>>> mistake (for the reasons outline above), but at least it was a
>>> comprehensible question to ask.
>>>
>>> But when it gets to "what should I do about a single-queue version of
>>> a MQ scheduler", the question is no longer even remotely sensible. The
>>> question should simply NOT EXIST. There is no possible valid reason to
>>> ask that kind of crap.
>>
>> OK, so here's what I'll do:
>>
>> 1) We'll kill the default scheduler choices. sq blk-mq will default to
>>    mq-deadline, mq blk-mq will default to "none" (at least for now, until
>>    the new scheduler is done).
> 
> But what about e.g. SATA SSDs? Wouldn't they be better off without any
> scheduler? 

Marginal. If they are single queue, using a basic scheduler like
deadline isn't going to be a significant amount of overhead. In some
cases they are going to be better off, due to better merging. In the
worst case, overhead is slightly higher. Net result is positive, I'd
say.

> So perhaps setting "none" for queue/rotational==0 and mq-deadline for
> spinning drives automatically in the sq blk-mq case?

You can do that through a udev rule. The kernel doesn't know if the
device is rotational or not when we set up the scheduler. So we'd either
have to add code to do that, or simply just do it with a udev rule. I'd
prefer the latter.

-- 
Jens Axboe