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    SubjectRe: 2us Interrupt latency's for Linux 2.0.xx

    I see these too although I had not seen this periodic function.
    What are you using to read time? Are you reading the
    8253? In any case, the 8253/4 access seems like it can take 15us.
    I have indirectly measured a 15us delay between two inb instructions.
    But this was on an older chip set and with ISA device on the bus.
    PCI bus analyzers are needed. I'm hoping to get one in the next month.


    On Wed, Sep 02, 1998 at 03:27:15AM -0500, Manoj Apte wrote:
    >
    > > > in a handler). I created my own fast handler registered at IRQ32 (
    > > > interrupt is generated by
    > > > the APIC timer on a Ppro 200) that gives me a latency of approx 2300
    > > > nanoseconds
    > >
    > > the APIC timer interrupt is a CPU-internal IRQ, and thus has lower
    > > latencies. It should have identical latencies to a software interrupt.
    > > (like your numbers show). I measured 500 cycles _total_ profiling
    > > interrupt cost. This makes it possible to run 100 thousand IRQs a second
    > > profiling on a 100MHz P5. (i've tried it. It's noticeable but it works)
    > >
    >
    > Agreed.. but my question was about why i see periodic mega-latencies of
    > the order of 15us every 30 seconds or so. I want to track the source of
    > this, so that _if_ i can dsiable that probelm, I have a much more
    > predictable APIC interrupt latency.
    >
    > > _external_ interrupts have much higher latencies. (especially as old
    > > XT-PIC interrupts halt the CPU for a considerable amount of time ... this
    > > is not the case with IO-APIC and inter-CPU interrupts.)
    > >
    > > -- mingo
    > >
    >
    >
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    ---------------------------------
    Victor Yodaiken
    Department of Computer Science
    New Mexico Institute of Mining and Technology
    Socorro NM 87801
    Homepage http://www.cs.nmt.edu/~yodaiken
    PowerPC Linux page http://linuxppc.cs.nmt.edu
    Real-Time Page http://rtlinux.org


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