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    SubjectRe: [RFC, PATCH] cfq-iosched: remove redundant queuing detection code
    On Tue, Nov 10 2009, Corrado Zoccolo wrote:
    > On Tue, Nov 10, 2009 at 4:14 PM, Jens Axboe <> wrote:
    > > 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.
    > The comment by Aaron:
    > CFQ's detection of queueing devices assumes a non-queuing device and detects
    > if the queue depth reaches a certain threshold. Under some workloads (e.g.
    > synchronous reads), CFQ effectively forces a unit queue depth,
    > thus defeating
    > the detection logic. This leads to poor performance on queuing hardware,
    > since the idle window remains enabled.
    > makes me think that the dynamic-off detection in cfq may really be
    > buggy (BTW this could explain the bad results on SSD Jeff observed
    > before my patch set).
    > The problem is, that once the hw_tag is 0, it is difficult for it to
    > become 1 again, as explained by Aaron, since cfq will hardly send more
    > than 1 request at a time. My patch set fixes this for SSDs (the seeky
    > readers will still be sent without idling, and if they are enough, the
    > logic will see a large enough depth to reconsider the initial
    > decision).
    > So the only sound way to do the detection is to start in an
    > indeterminate state, in which CFQ behaves as if hw_tag = 1, and then,
    > if for a long observation period we never saw large depth, we switch
    > to hw_tag = 0, otherwise we stick to hw_tag = 1, without reconsidering
    > it.

    That is probably the better way to do it, as I said earlier it is indeed
    a chicken and egg problem. Care to patch something like that up?

    > I think the correct logic could be pushed to the blk-core, by
    > introducing also an indeterminate bit.

    And I still don't think that is a good idea. The block layer case cares
    more about the capability side ("is this a good ssd?") where as the CFQ
    case incorporates process behaviour as well. I'll gladly take patches to
    improve the CFQ logic.

    Jens Axboe

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      Last update: 2009-11-12 13:19    [W:0.022 / U:13.460 seconds]
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