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    SubjectRe: [RFC, PATCH] cfq-iosched: remove redundant queuing detection code
    On Tue, Nov 10 2009, Jeff Moyer wrote:
    > Jens Axboe <> writes:
    > > On Tue, Nov 10 2009, Jeff Moyer wrote:
    > >> Jens Axboe <> writes:
    > >>
    > >> > On Tue, Nov 10 2009, Corrado Zoccolo wrote:
    > >> >> The core block layer already has code to detect presence of command
    > >> >> queuing devices. We convert cfq to use that instead of re-doing the
    > >> >> computation.
    > >> >
    > >> > There's is the major difference that the CFQ variant is dynamic and the
    > >> > block layer one is not. This change came from Aaron some time ago IIRC,
    > >> > see commit 45333d5. It's a bit of a chicken and egg problem.
    > >>
    > >> Really? blk_dequeue_request sure looks like it updates things
    > >> dynamically, but only one way (not queueing -> queueing). Would it make
    > >
    > > Yes of course the block layer one is dynamically on as well. The ideal
    > > goal would be to have every driver use the block layer tagging in which
    > > case we'd know without checking, but alas it isn't so (yet). My point is
    > > that the CFQ variant is dynamically off as well. Corrado presents his
    > > patch as a direct functional equivelant, which it definitely isn't.
    > OK. So we really want to keep track of two things:
    > 1) What queue depth does the hardware support?
    > 2) What is the command queue depth configured to?
    > That second thing can be changed by the administrator (down from or up
    > to the maximum value allowed by 1).
    > >> sense to just put CFQ's logic into the block layer so that everyone uses
    > >> the same implementation? It makes little sense to have two notions of
    > >> whether or not queueing is supported for a device.
    > >
    > > The one use in the block layer cares about the static property of the
    > > device, not the current behaviour. So I'm not sure it makes a lot of
    > > sense to unify these. It's not really a case of code duplication either,
    > > the block layer one is two checks and a bit. The cfq variant is a bit
    > > more involved in that it tracks the state continually.
    > Why don't we simply use the value configured via the queue_depth sysfs
    > file?

    First of all, that only covers SCSI. We could do that by having the tag
    on/off functions set the same flag. But even for such devices, actual
    tag depth is dependent upon what other devices are on the controller
    (since it's often a shared map) and may not even be statically
    detectable in the sense that actual depth is only really seen when the
    device returns busy on a queue attempt.

    In most cases it would work fine, but the dynamic detection is more
    reliable. The sysfs setting in reality is max setting.

    Jens Axboe

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