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    SubjectRe: [RFC git tree] Userspace RCU (urcu) for Linux (repost)
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    On Thu, Feb 5, 2009 at 11:58 PM, Mathieu Desnoyers
    <compudj@krystal.dyndns.org> wrote:
    > I figured out I needed some userspace RCU for the userspace tracing part
    > of LTTng (for quick read access to the control variables) to trace
    > userspace pthread applications. So I've done a quick-and-dirty userspace
    > RCU implementation.
    >
    > It works so far, but I have not gone through any formal verification
    > phase. It seems to work on paper, and the tests are also OK (so far),
    > but I offer no guarantee for this 300-lines-ish 1-day hack. :-) If you
    > want to comment on it, it would be welcome. It's a userland-only
    > library. It's also currently x86-only, but only a few basic definitions
    > must be adapted in urcu.h to port it.

    I have actually been fiddling with an RCU-esque design for a
    multithreaded event-driven userspace server process. Essentially all
    threads using RCU-protected data run through a central event loop
    which drives my entirely-userspace RCU state machine. I actually have
    a cooperative scheduler for groups of events to allow me to
    load-balance a large number of clients without the full overhead of a
    kernel thread per client. This does rely on
    clock_gettime(CLOCK_THREAD_CPUTIME_ID) returning a useful monotonic
    value, however.

    By building the whole internal system as an
    event-driven-state-machine, I don't need to keep a stack for blocked
    events. The events which do large amounts of work call a
    "need_resched()"-ish function every so often, and if it returns true
    they return up the stack. Relatively few threads (1 per physical CPU,
    plus a few for blocking event polling) are needed to completely
    saturate the system.

    For RCU I simply treat event-handler threads the way the kernel treats
    CPUs, I report a Quiescent State every so often in-between processing
    events.

    The event-handling mechanism is entirely agnostic to the way that
    events are generated. It has built-in mechanisms for FD, signal, and
    AIO-based events, and it's trivial to add another event-polling thread
    for GTK/Qt/etc.

    I'm still only halfway through laying out the framework for this
    library, but once it's done I'll make sure to post it somewhere for
    those who are interested.

    Cheers,
    Kyle Moffett


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