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    SubjectRe: new IRQ scalability changes in 2.3.48
    On Mon, Mar 13, 2000 at 12:07:41PM +0100, Ingo Molnar wrote:
    >
    > On Sun, 12 Mar 2000 yodaiken@fsmlabs.com wrote:
    >
    > > > Quite regardless of how you do interrupts: it doesn't matter where youput
    > > > the ACK's, you always need to make sure that irq masking etc is correct,
    > > > and you must NOT allow a context switch while an interrupt handler is
    > > > still running.
    > >
    > > Ok. Ingo: does your "low latency" patch violate this rule?
    >
    > yep, of course. It's a grave error to schedule during IRQ contexts, and we

    I think you mean: "of course not"!

    > do have an assert in schedule() so it's plain impossible. The lowlatency
    > patch simply works by increasing the effective frequency (occurance) of
    > rescheduling (preemption) points [without actually rescheduling more
    > often].

    This is too subtle for me. I don't know how you can make true
    the first 2 things without having the third be false.

    > Having said this, i now do agree that doing a preemptible kernel (which
    > the Linux SMP kernel could become with a small amount of work) is a
    > superior solution to this, wrt. latencies.

    Well, to start, it would violate Linus' rule, an old UNIX rule, and your
    new IRQ scheme makes it more complex -- you have to make sure to not
    switch out of tasks that are handling unacked interrupts.
    I don't know how to trade throughput for latency without losing throughput.



    --
    ---------------------------------------------------------
    Victor Yodaiken
    FSMLabs: www.fsmlabs.com www.rtlinux.com
    FSMLabs is a servicemark and a service of
    VJY Associates L.L.C, New Mexico.


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