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    SubjectRe: aio is unlikely
    On Wed, 09 May 2007 18:06:58 -0400
    Jeff Garzik <> wrote:

    > > -#define in_aio() !is_sync_wait(current->io_wait)
    > > +#define in_aio() (unlikely(!is_sync_wait(current->io_wait)))
    > Please revert. Workload-dependent "likelihood" should not cause
    > programmers to add such markers.
    > This is a common misunderstanding about unlikely() and likely(). The
    > branch prediction used for each assumes 99% unlikely or 99% likely,
    > which is not true at all for workload-dependent code.
    > Even if only 1% of Linux users use AIO, for that 1%, the 'unlikely'
    > marker causes repeated branch mispredictions.
    > likely() and unlikely() should be used for cases where code is
    > likely/unlikely for EVERYBODY.

    a) disagree with the above

    b) if in_aio() ever returns true we do

    printk(KERN_ERR "%s(%s:%d) called in async context!\n",
    __FUNCTION__, __FILE__, __LINE__);

    so I sure hope it's unlikely for all workloads.

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      Last update: 2007-05-10 00:21    [W:0.020 / U:2.124 seconds]
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