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    SubjectRe: [ANNOUNCE] Native POSIX Thread Library 0.1
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    On Mon, Sep 23, 2002 at 08:30:04AM -0700, Larry McVoy wrote:
    > > No matter how fast you do context switch in and out of kernel and a sched
    > > to see what runs next, it can't be done as fast as it can be avoided.
    >
    > You are arguing about how many angels can dance on the head of a pin.
    > Sure, there are lotso benchmarks which show how fast user level threads
    > can context switch amongst each other and it is always faster than going
    > into the kernel. So what? What do you think causes a context switch in
    > a threaded program? What? Could it be blocking on I/O? Like 99.999%

    That's just for traditional Unix applications, which is only one category.
    You exclude CPU intensive applications in that criticism, media related
    and otherwise. What about cases where you need to balance a large data
    structure across large number of threads or something like that ?

    > of the time? And doesn't that mean you already went into the kernel to
    > see if the I/O was ready? And doesn't that mean that in all the real
    > world applications they are already doing all the work you are arguing
    > to avoid?

    IO isn't the only thing that's event driven. What about event driven
    systems that depend on a fast condition-variable ? That's very cheap in
    a UTS (userspace thread system), 2 context switches, a call to thread-kernel
    to dequeue a waiter and releasing/aquiring some very light weight userspace
    locks. And difficult to beat if you think about it.

    So that level of confidence in 1:1 is a intuitively presumptuous for those
    reasons.

    But if you're architecture is broken or exotic...then it gets more complicated ;)

    bill

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