Messages in this thread | | | Date | Wed, 22 Jun 2005 20:14:49 +0200 | From | Ingo Molnar <> | Subject | Re: PREEMPT_RT vs I-PIPE: the numbers, part 2 |
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* Karim Yaghmour <karim@opersys.com> 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 variable.
> 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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