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    SubjectRe: [patch] use cheaper elv_queue_empty when unplug a device
    Arjan van de Ven wrote:
    > On Tue, 2005-03-29 at 19:19 +1000, Nick Piggin wrote:
    >
    >>- removes the relock/retry merge mechanism in __make_request if we
    >> aren't able to get the GFP_ATOMIC allocation. Just fall through
    >> and assume the chances of getting a merge will be small (is this
    >> a valid assumption? Should measure it I guess).
    >
    >
    > this may have a potential problem; if the vm gets in trouble, you
    > suddenly start to generate worse IO patterns, which means IO performance
    > goes down right when it's most needed.....
    >

    Sorry my wording was incorrect. It currently *always* retries the
    merge if it had at first failed, and after the patch, we never retry.
    So it should not result in behavioural shifts when there is a VM load
    is high.

    It seems to be a clear source of problems for Kenneth though, because
    his workload appears to have almost zero merges, so he'll always be
    invoking the merge logic twice.

    I agree there is potential for subtle interactions. But generally the
    block layer is surprisingly well behaved in my experience.

    As Jens said, the complete removal of the GFP_ATOMIC allocation probably
    has the most potential for problems in this regard, although bios are not
    using GFP_ATOMIC allocations, so I would be a little surprised if it made
    a really noticable difference.


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