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    SubjectRe: [patch] SMP alternatives
    On 11/23/05, Linus Torvalds <> wrote:
    > It _should_ be fairly easy to do something like that - just a simple
    > global flag that gets set and makes CPL3 ignore lock prefixes.

    This is a goof rist step. But the effectiveness will descrease
    significantly in the near future. It can only work for
    single-threaded processes without writable shared memory.

    With the growing number of cores/threads the need to use parallelism
    rises. With techniques like OpenMP the threshold to do this is
    lowered significantly. The process-model, so much preferred on this
    list over the thread model, requires shared memory, therefore also
    eliminating the effectiveness of this functionality.

    A real solution needs to be more fine grained. It is often known in
    the userland code whether the specific word which is accessed using
    atomic ops really can be shared. The POSIX interfaces, for instance,
    require that all mutexes etc which are placed in shared memory are
    attributed as such. Combine this with the knowledge about the number
    of threads in use and the result is that even with writable shared
    memory segments the lock prefix can be avoided. There are a whole
    bunch of cases where we already do conditional locking. It's just
    plain ugly and not as efficient as we would like i t.

    So, implementing control with per-userlevel context will se rapidly
    diminishing success and I'm wondering whether it is better to go for
    something with a bit finer level of control.

    ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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