Re: [PATCH] block: BFQ default for single queue devices

[Paolo Valente <paolo.valente@xxxxxxxxxx>]

> Il giorno 03 ott 2018, alle ore 13:49, Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> ha scritto:
> 
> Hi.
> 
> On 03.10.2018 08:29, Paolo Valente wrote:
>> As also Linus Torvalds complained [1], people feel lost among
>> I/O-scheduler options.  Actual differences across I/O schedulers are
>> basically obscure to non experts.  In this respect, Linux-kernel
>> 'users' are way more than a few top-level distros that can afford a
>> strong performance team, and that, basing on the input of such a team,
>> might venture light-heartedly to change a critical component like an
>> I/O scheduler.  Plus, as Linus Walleij pointed out, some users simply
>> are not distros that use udev.
> 
> I feel a contradiction in this counter-argument. On one hand, there are lots of, let's call them, home users, that use major distributions with udev, so the distribution maintainers can reasonably decide which scheduler to use for which type of device based on the udev rule and common sense provided via Documentation/ by linux-block devs. Moreover, most likely, those rules should be similar or the same across all the major distros and available via some (systemd?) upstream.
> 

Let me basically repeat Mark's answer here, with my words.

Unfortunately, facts mismatch with your optimistic view: after so many
years and concordant test results, only very few distributions
switched to bfq, no major distribution did (AFAIK).  As I already
wrote, the reason is the one pointed out by Torvalds [1].  Do you want
a simple example?  Take the last sentence in Jan's email in this
thread: "I *personally would* consider bfq a safer default ...  but *I
don't feel too strongly* about it." And he is definitely a storage
expert.

The problem, in particular, is that bfq is a complex beast, fighting
against a jungle of I/O issues.  You have to be really into bfq, even
to just know all of its features!

> On another hand, the users of embedded devices, mentioned by Linus, should already know what scheduler to choose because dealing with embedded world assumes the person can decide this on their own, or with the help of abovementioned udev scripts and/or Documentation/ as a reference point.
> 

Same situation for embedded devices, if not even worse.  Again for the
same reasons above.  In the end, it is hard even for a kernel expert
to be an in-depth expert of every possible complex component.

> So I see no obstacles here, and the choice to rely on udev by default sounds reasonable.
> 
> The question that remain is whether it is really important to mount a root partition while already using some specific scheduler? Why it cannot be done with "none", for instance?
> 
>> So, probably 99% of Linux-kernel users will just stick to the default
>> I/O scheduler, mq-deadline, assuming that the algorithm by which that
>> scheduler was chosen was not "pick the scheduler with the longest
>> name", but "pick the best scheduler for most cases".  The problem is
>> that, for single-queue devices with a speed below 400/500 KIOPS, the
>> default scheduler is apparently incomparably worse than bfq in terms
>> of responsiveness and latency for time-sensitive applications [2], and
>> in terms of throughput reached while controlling I/O [3].  And, in all
>> other tests ran so far, by any entity or group I'm aware of, bfq
>> results basically on par with or better than mq-deadline.
> 
> And that's why major distributions are likely to default to BFQ via udev. No one argues with BFQ superiority here ☺.
> 
>> So, I do understand your need for conservativeness, but, after so much
>> evidence on single-queue devices, and so many years! :), what's the
>> point in keeping Linux worse for virtually everybody, by default?
> 
> From my point of view this is not a conservative approach at all. On contrary, offloading decisions to userspace aligns pretty well with recent trends like pressure metrics/userspace OOM killer, eBPF etc. The less unnecessary logic the kernel handles, the more flexibility it affords.
> 

To not answer too seriously here, let me answer with a quote that is
still missing a clear paternity: "Everything should be made as simple
as possible, but not simpler." :)

Thanks,
Paolo

> -- 
>  Oleksandr Natalenko (post-factum)


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