Re: [Xenomai-core] latencys drifting into negative (Xenomai 2.4.2/2.4.3)

[Jan Kiszka <jan.kiszka@domain.hid>]

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Cornelius Köpp wrote:
> Hello,
> I run the latency test from testsuite on several hard and software configurations. Running on Xenomai 2.4.2, Linux 2.6.24 the results shows a "strange" behavior: In Kernel mode (-t1) the latencys constantly linear decrease. See attached plot 'drifting_latencys_in_kernelmode.png' of latency test running 48h on Pentium3 700. This effect could be reproduced, even on other hardware (Pentium-M 1400).

As our P3 boards did not support APIC-based timing (IIRC), your kernel 
has correctly disabled the related kernel support. But the Pentium M 
should be fine. So could you check if we are seeing some TSC clocks vs. 
PIT timer rounding issue by enabling the local APIC on the Pentium M?

> The usermode (-t0) did not show a drifting, but is influenced by a test ran in kernelmode before.

What do you mean with "is influenced"?

> 
> I talked with Sebastian Smolorz about this and he builds his own independent kernel-config to check. He got the same drifting-effect with Xenomai 2.4.2 and Xenomai 2.4.3 running latency over several hours. His kernel-config ist attached as 'config-2.6.24-xenomai-2.4.3__ssm'.
> 
> Our kernel-configs are both based on a config used with Xenomai 2.3.4 and Linux 2.6.20.15 without any drifting effects.

2.3.x did not incorporate the new TSC-to-ns conversion. Maybe it is not 
a PIC vs. APIC thing, but rather a rounding problem of larger TSC values 
(that naturally show up when the system runs for a longer time).

Jan


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