On Wed, Oct 03, 2018 at 03:25:54PM +0200, Jan Kara wrote:

On Wed 03-10-18 08:53:37, Linus Walleij wrote:

quoted

On Wed, Oct 3, 2018 at 8:29 AM Paolo Valente [off-list ref] wrote:

quoted

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 [off-list ref]
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 [off-list ref]
    Reviewed-by: Hannes Reinecke [off-list ref]
    Reviewed-by: Ming Lei [off-list ref]
    Acked-by: John Garry [off-list ref]
    Signed-off-by: Martin K. Petersen [off-list ref]

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
jthumshirn@suse.de                                +49 911 74053 689
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