Re: [PATCH RFC 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O scheduler

[Paolo Valente <paolo.valente@linaro.org>]

Il giorno 15/apr/2016, alle ore 17:08, Tejun Heo <tj@kernel.org> ha scritto:

> Hello, Paolo.
> 
> On Fri, Apr 15, 2016 at 04:20:44PM +0200, Paolo Valente wrote:
>>> It's actually a lot more difficult to answer that with bandwidth
>>> scheduling.  Let's say cgroup A has 50% of disk time.  Sure, there are
>>> inaccuracies, but it should be able to get close to the ballpark -
>>> let's be lax and say between 30% and 45% of raw sequential bandwidth.
>>> It isn't ideal but now imagine bandwidth based scheduling.  Depending
>>> on what the others are doing, it may get 5% or even lower of the raw
>>> sequential bandwidth.  It isn't isolating anything.
>> 
>> Definitely. Nevertheless my point is still about the same: we have to
>> consider one system at a time. If the workload of the system is highly
>> variable and completely unpredictable, then it is hard to provide any
>> bandwidth guarantee with any solution.
> 
> I don't think that is true with time based scheduling.  If you
> allocate 50% of time, it'll get close to 50% of IO time which
> translates to bandwidth which is lower than 50% but still in the
> ballpark.

But this is the same minimal service guarantee that you get with BFQ
in any case. I'm sorry for being so confusing to not make this central
point clear :(

>  That is very different from "we can't guarantee anything if
> the other workloads are highly variable”.
> 


If you have 50% of the time, but
. you don’t know anything about your workload properties, and
. the device speed can vary by two orders of magnitude,
then you can't provide any bandwidth guarantee, with any scheduler. Of
course I'm neglecting the minimal, trivial guarantee "getting a fraction
of the minimum possible speed of the device".

If you have 50% of the time allocated for a quasi-sequential workload,
then bandwidth and latencies may vary by an uncontrollable 30 or 40%,
depending on what you and the other groups do.

With the same device, if you have 50% of the bandwidth allocated with
BFQ for a quasi-sequential workload, then you can provide bandwidth
and latencies that may vary at most by a (still uncontrollable) 3 or
4%, depending on what you and the other groups do.

This improvement is shown, e.g., in my--admittedly boring--numerical
example, and is confirmed by my experimental results so far.

> So, I get that for a lot of workload, especially interactive ones, IO
> patterns are quasi-sequential and bw based scheduling is beneficial
> and we don't care that much about fairness in general; however, it's
> problematic that it would make the behavior of proportional control
> quite surprising.

If I have somehow convinced you with what I wrote above, then I hope
we might agree that a surprising behavior of BFQ with cgroups would be
just a matter of bugs.

Thanks,
Paolo

> 
>>> As I wrote before, as fairness isn't that important for normal
>>> scheduling, if empirical data show that bandwidth based scheduling is
>>> beneficial for most common workloads, that's awesome especially given
>>> that CFQ has plenty of issues.  I don't think cgroup case is workable
>>> as currently implemented tho.
>> 
>> I was thinking about some solution to achieve both goals. An option is
>> probably to let BFQ work in a double mode: sector-based within groups
>> and time-based among groups. However, I find it a little messy and
>> confusing.
>> 
>> Other ideas/solutions? I have no better proposal at the moment :(
> 
> No idea.  I don't think isolation could work without time based
> scheduling at some level tho. :(
> 
> Thanks.
> 
> -- 
> tejun