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    SubjectRe: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once
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    On Thu, Aug 6, 2009 at 4:02 AM, Peter Zijlstra<a.p.zijlstra@chello.nl> wrote:
    >
    > Taking that many locks in general, some apps (JVM based usually) tend to
    > be thread heavy and can easily have hundreds of them, even on relatively

    Oh, I'm well aware that apps can be heavily multi-threaded - we have
    much worse cases at Google.

    >
    > Now that's not real nice is it ;-)

    Not particularly - but who exactly is going to be moving processes
    with thousands of threads between cgroups on a lockdep-enabled debug
    kernel?

    >
    >> But given that AFAICS we can eliminate the overhead associated with a
    >> single lock by piggy-backing on the cache line containing
    >> sighand->count, hopefully this won't be an issue any more.
    >
    > Right, so this is a write rarely, read frequently thing, which suggests
    > an RCU like approach where the readers pay a minimum synchronization
    > penalty.

    The documentation for SRCU mentions:

    Therefore, SRCU should be used in preference to rw_semaphore
    only in extremely read-intensive situations, or in situations
    requiring SRCU's read-side deadlock immunity or low read-side
    realtime latency.

    What benefits does the additional complexity of SRCU give, over the
    simple solution of putting an rwsem in the same cache line as
    sighand->count ?

    Paul


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