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    SubjectRe: Fall back io scheduler for 2.6.15?
    On Thu, Jan 19 2006, Nate Diller wrote:
    > On 1/16/06, Jens Axboe <axboe@suse.de> wrote:
    > > On Fri, Jan 13 2006, Andrew Morton wrote:
    > > > Mingming Cao <cmm@us.ibm.com> wrote:
    > > > >
    > > > > On 2.6.14, the
    > > > > fall back io scheduler (if the chosen io scheduler is not found) is set
    > > > > to the default io scheduler (anticipatory, in this case), but since
    > > > > 2.6.15-rc1, this semanistic is changed to fall back to noop.
    > > >
    > > > OK. And I assume that AS wasn't compiled, so that's why it fell back?
    > > >
    > > > I actually thought that elevator= got removed, now we have
    > > > /sys/block/sda/queue/scheduler. But I guess that's not very useful with
    > > > CONFIG_SYSFS=n.
    > > >
    > > > > Is there any reason to fall back to noop instead of as? It seems
    > > > > anticipatory is much better than noop for ext3 with large sequential
    > > > > write tests (i.e, 1G dd test) ...
    > > >
    > > > I suspect that was an accident. Jens?
    > >
    > > It is, it makes more sense to fallback to the default of course.
    >
    > Not an accident at all, actually, because the original patch i
    > submitted allowed you to select a scheduler as 'default' even if it
    > were compiled as a module in kconfig. Since noop is guaranteed to be
    > present in any system, it is the obvious choice if the chosen or
    > default scheduler is not loaded.

    Yes and that was a bug in that patch. The default scheduler must be
    builtin, that's a given. The Kconfig rules should make a default
    selection as a module illegal. And they do, they have since been fixed.

    > If you change it to fall back to the default, it will oops if the
    > default is not available.

    It must be.

    --
    Jens Axboe

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