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    SubjectRe: [PATCH 04/10] block: initial patch for on-stack per-task plugging
    On Thu, Mar 10, 2011 at 08:32:05PM +0100, Jens Axboe wrote:
    > On 2011-03-10 17:54, Vivek Goyal wrote:
    > > On Sat, Jan 22, 2011 at 01:17:23AM +0000, Jens Axboe wrote:
    > >
    > > [..]
    > >> -/*
    > >> - * Only disabling plugging for non-rotational devices if it does tagging
    > >> - * as well, otherwise we do need the proper merging
    > >> - */
    > >> -static inline bool queue_should_plug(struct request_queue *q)
    > >> -{
    > >> - return !(blk_queue_nonrot(q) && blk_queue_tagged(q));
    > >> -}
    > >> -
    > >
    > > Jens,
    > >
    > > While discussing stack plug with Mike Snitzer, this occurred to us that in
    > > new code we seem to be plugging even if underlying device is SSD with NCQ.
    > > Should we continue to maintain the old behavior of not plugging for NCQ SSD?
    >
    > The main reason plugging was turned off for SSD's previously was because
    > it ended up hammering on the queue lock a lot. So it was turned off to
    > speed them up.
    >
    > The new plugging scheme is faster than hitting the queue directly, so
    > now it would be a good idea to do the plugging in fact. Plus even for
    > high performance SSD's, things like merging are still a good idea.
    >
    > So yes, it's on now and on purpose.

    Ok. Thanks for the explanation. That helps.

    Vivek


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