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    SubjectRe: CLONE_DETACHED and exit notification (was user-vm-unlock-2.5.31-A2)

    On Fri, 16 Aug 2002, Linus Torvalds wrote:

    > So I'd like to know what the likelyhood of it being used as a futex is.
    > If 90% of all users would like a futex, that's _wonderful_. No question
    > that we should do it. But if it's dynamically fairly rare to have this
    > "synchronize with thread exit", then we may be losing more than we win.

    having looked at threading libraries i can tell you that any library
    writer who cares about performance would use a futex for exit
    notification. We've got a performance test that compares
    signal-notification and futex-notification:

    earth3:~/libc> ./p3 -s 100000 -t 1 -r 0 -T --sync-join
    Runtime: 1.208586749 seconds

    earth3:~/libc> ./p3 -s 100000 -t 1 -r 0 -T --sync-signal
    Runtime: 1.336826839 seconds

    the sync-join variant uses a futex, the sync-signal variant uses a signal
    to notify completion. Note that the test does not even use SIGCHLD because
    that is slower, it uses SIGUSR1 [in the application code] and sigwait(),
    to remove the signal handler overhead. The futex variant is still faster,
    and with futexes being used for CLONE_CLEARTID it would be faster by about
    ~0.1 seconds.

    (The kernel used is an uptodate 2.4-threading kernel with yesterday's APIs
    backported, and glibc fitted to those new interfaces.)

    sure, this solution is even less generic and thus a bit more dangerous of
    being a libpthread-specific optimization [ ;) ], but i cannot think of any
    more complex threading library - things like JVMs tend to have less
    semantical needs. And if someone wants signal-based notification then
    it's still possible.


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