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

[Ming Lei <ming.lei@redhat.com>]

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@redhat.com> 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@redhat.com/
> 
> 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.


Thanks,
Ming