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    SubjectRe: PREEMPT_RT vs I-PIPE: the numbers, part 2

    * Karim Yaghmour <> wrote:

    > Ingo Molnar wrote:
    > >>you could try the LPPTEST kernel driver and testlpp utility i
    > >>integrated into the -RT patchset. It avoids target-side latencies
    > >>almost completely. Especially since you had problems with parallel
    > >>interrupts you should give it a go and compare the results.
    > >
    > >
    > > correction: logger-side latencies are avoided.
    > Sorry, I don't see this. I've just looked at lpptest.c and it does
    > practically the same thing LRTBF is doing, have a look for yourself
    > at the code in LRTBF.

    you should take another look. The crutial difference is that AFAICS
    lrtbf is using interrupts on _both_ the logger and the target side.
    lpptest only uses interrupts on the target side (that is what we are
    measuring), but uses polling _with all interrupts disabled_ on the
    logger side. This makes things much more reliable, as it's not some
    complex mix of two worst-case latencies, but a small constant overhead
    on the logger side and the worst-case latency on the target side. This
    also means i can run whatever lpptest version on the logger side, i dont
    have to worry about its latencies because there are none that are

    > In fact lpptest.c is probably running at a higher cost on the logger
    > since it executes a copy_to_user() for every single data point
    > collected. [...]

    logger-side overhead does not matter at all, and the 8 bytes copy is not
    measured in the overhead. (it is also insignificant.)

    > [...] In the case of the LRTBF, we just buffer the results in a
    > preallocated buffer and then read them all at once after the testrun.
    > Unless I'm missing something, there is nothing done in lpptest that we
    > aren't already doing on either side, logger-side latencies included.
    > As for the interrupt problems, they were pilot error. They disappeared
    > once the APIC was enabled. That's therefore a non-issue.

    well, LPPTEST works just fine with the i8259A PIC too. (which is much
    more common in embedded setups than IO-APICs)

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