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    SubjectRe: High contention on the sk_buff_head.lock
    On Wed, 2009-03-18 at 18:03 -0700, David Miller wrote:
    > From: Gregory Haskins <>
    > Date: Wed, 18 Mar 2009 17:54:04 -0400
    > > Note that -rt doesnt typically context-switch under contention anymore
    > > since we introduced adaptive-locks. Also note that the contention
    > > against the lock is still contention, regardless of whether you have -rt
    > > or not. Its just that the slow-path to handle the contended case for
    > > -rt is more expensive than mainline. However, once you have the
    > > contention as stated, you have already lost.
    > First, contention is not implicitly a bad thing.

    Its a bad thing when it does not scale.

    > Second, if the -rt kernel is doing adaptive spinning I see no
    > reason why that adaptive spinning is not kicking in here to
    > make this problem just go away.

    If only the first of N contending threads gets to spin, 2..N would

    > This lock is held for mere cycles, just to unlink an SKB from
    > the networking qdisc, and then it is immediately released.

    For very short hold times, and heavy contention, as well as for
    scalability, the solution may lie in tunable spinner-count and adaptive
    spinner time-out.


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