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

[Tejun Heo]

Hello, Paolo.

On Fri, Apr 15, 2016 at 06:17:55PM +0200, Paolo Valente wrote:
> > 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 :(

lol sorry about being dumb.

> >  That is very different from "we can't guarantee anything if
> > the other workloads are highly variableâ.
> 
> If you have 50% of the time, but
> . you donât 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".

Oh, the guarantee is about "getting close to half of the available IO
resource", what that translates to depends on the underlying hardware
and the workload of course.

> 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.

Yes, may be but it won't dive to 5% depending on what others are
doing.

> 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.

I don't think the above is true.  Are you saying that the following
two cases would lead to the same outcome for cgroup A?

		cgroup A (50)		cgroup B (50)
 case 1		sequential		sequential
 case 2		sequential		random (to a certain degree)

The aggregate bandwidths for case 1 and 2 would be wildly different
depending on the randomness of the second workload.  What cgroup A
would be able to get would fluctuate accordingly, no?

> > 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.

I think I might still need more help.  What am I missing?

Thanks.

-- 
tejun