Re: [PATCH] block: Flag elevators suitable for single queue

[Ulf Hansson <ulf.hansson@xxxxxxxxxx>]

On Mon, 1 Jun 2020 at 13:58, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> On Mon, Jun 01, 2020 at 01:36:54PM +0200, Linus Walleij wrote:
> > On Mon, Jun 1, 2020 at 9:50 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> > > On Thu, May 28, 2020 at 10:10:03AM +0200, Linus Walleij wrote:
> > > > The Kyber block scheduler is not suitable for single hardware
> > > > queue devices, so add a new flag for single hardware queue
> > > > devices and add that to the deadline and BFQ schedulers
> > > > so the Kyber scheduler will not be selected for single queue
> > > > devices.
> > >
> > > The above may not be true for some single hw queue high performance HBA(
> > > such as megasas), which can get better performance from none, so it is
> > > reasonable to get better performance from kyber, see 6ce3dd6eec11 ("blk-mq:
> > > issue directly if hw queue isn't busy in case of 'none'"), and the
> > > following link:
> > >
> > > https://lore.kernel.org/linux-block/20180710010331.27479-1-ming.lei@xxxxxxxxxx/
> >
> > I see, but isn't the case rather that none is preferred and kyber gives
> > the same characteristics because it's not standing in the way
> > as much?
>
> Kyber has its own characteristic, such as fair read & write, better
> IO merge. And the decision on scheduler isn't only related with device,
> but also with workloads.
>
> >
> > It looks like if we should add a special flag for these devices with
> > very fast single queues so they can say "I prefer none", do you
> > agree?
>
> I am not sure if it is easy to add such flag, because it isn't only
> related with HBA, but also with the attached disks.
>

In general I don't mind the idea of giving hints from lower layer
block devices, about what kind of scheduling algorithm that could make
sense (as long it's on a reasonable granularity).

If I understand your point correctly, what you are saying is that it
isn't easy or even possible for some block devices HWs. However, that
should be fine, as it wouldn't be mandatory to set this kind of flags,
but
instead could help where we see it fit, right?

Kind regards
Uffe

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