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    SubjectRe: [PATCH] Futex Asynchronous Interface
    On Wed, 12 Jun 2002, Linus Torvalds wrote:

    > Any amount of tracking would be _extremely_ expensive. Right now getting
    > an uncontended lock is about 15 CPU cycles in user space.
    >
    > Tryin to tell the kernel about gettign that lock takes about 1us on a P4
    > (system call overhead), ie we're talking 18000 cycles. 18 THOUSAND cycles
    > minimum. Compared to the current 15 cycles. That's more than three orders
    > of magnitude slower than the current code, and you just lost the whole
    > point of doing this all in user space in the first place.

    That doesn't rule out approaches like storing a cookie alongside the lock
    once it's acquired (or in a parallel space). Which can easily be done with
    a wrapper around lock acquisition. And stale lock detection needn't be
    done in kernel space either.

    --
    "Love the dolphins," she advised him. "Write by W.A.S.T.E.."

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