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

[Johannes Thumshirn <jthumshirn@suse.de>]

On Wed, Oct 03, 2018 at 03:25:54PM +0200, Jan Kara wrote:
> On Wed 03-10-18 08:53:37, Linus Walleij wrote:
> > On Wed, Oct 3, 2018 at 8:29 AM Paolo Valente <paolo.valente@linaro.org> wrote:
> > 
> > > 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?
> > 
> > I understand if we need to ease things in as well, I don't intend this
> > change for the current merge window or anything, since v4.19
> > will notably have this patch:
> > 
> > commit d5038a13eca72fb216c07eb717169092e92284f1
> > Author: Johannes Thumshirn <jthumshirn@suse.de>
> > Date:   Wed Jul 4 10:53:56 2018 +0200
> > 
> >     scsi: core: switch to scsi-mq by default
> > 
> >     It has been more than one year since we tried to change the default from
> >     legacy to multi queue in SCSI with commit c279bd9e406 ("scsi: default to
> >     scsi-mq"). But due to issues with suspend/resume and performance problems
> >     it had been reverted again with commit cbe7dfa26eee ("Revert "scsi: default
> >     to scsi-mq"").
> > 
> >     In the meantime there have been a substantial amount of performance
> >     improvements and suspend/resume got fixed as well, thus we can re-enable
> >     scsi-mq without a significant performance penalty.
> > 
> >     Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
> >     Reviewed-by: Hannes Reinecke <hare@suse.com>
> >     Reviewed-by: Ming Lei <ming.lei@redhat.com>
> >     Acked-by: John Garry <john.garry@huawei.com>
> >     Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
> > 
> > I guess that patch can be a bit scary by itself. But IIUC it all went
> > fine this time!
> > 
> > But hey, if that works, that means $SUBJECT patch will enable BFQ on all
> > libata devices and any SCSI that is single queue as well, not just
> > "obscure" stuff like MMC/SD and UBI, and that is
> > indeed a massive crowd of legacy devices. But we're talking
> > v4.21 here.
> > 
> > Johannes, you might be interested in $SUBJECT patch.
> > It'd be nice to hear what SUSE people have to add, since they
> > are pretty proactive in this area.
> 
> So we do have a udev rules in our distro which sets the IO scheduler based
> on device parameters (rotational at least, with blk-mq we might start
> considering number of queues as well, plus we have some exceptions like
> virtio, loop, etc.). So the kernel default doesn't concern us too much as a
> distro.
> 
> I personally would consider bfq a safer default for single-queue devices
> (loop probably needs exception) but I don't feel too strongly about it.

[Full quote for context]

What about resurrecting CONFIG_DEFAULT_IOSCHED for MQ as well and
leave it default to mq-deadline but give bfq, kyber and none as a
choice as well?

The question is shall we only do it for single queue devices or for
native MQ devices as well if we go down that road?

I understand the embedded floks will want a different interface than
udev, but from the non-embedded point of view I'm with Jens and Jan
here, let udev do the job.

      Johannes
-- 
Johannes Thumshirn                                          Storage
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