[Xenomai] i.MX6q memory write causes high latency

Philippe Gerum rpm at xenomai.org
Thu Jul 5 11:14:11 CEST 2018

On 07/04/2018 07:06 PM, Federico Sbalchiero wrote:
> Hi,
> first I want to say thanks to everyone involved in Xenomai for their job.
> I'm testing Xenomai 3.0.7 and ipipe-arm/4.14 on Freescale/NXP i.MX6q
> sabresd board using Yocto. System boots fine and is stable, but latency
> under load (xeno-test) is higher than in my reference system (Xenomai
> 2.6.5 on Freescale kernel 3.10.17 + ipipe 3.10.18).
> This is after disabling power management, frequency scaling, CMA,
> graphics, tracing, debug.
> I have found that a simple non-realtime user space process writing a
> buffer in memory (memwrite) is able to trigger such high latencies.
> Latency worsen a lot running a copy of the process on each core.
> There is a correlation between buffer size and cache size suggesting
> an L2 cache issue, like the L2 write allocate discussed in the mailing
> list, but I can confirm L2 WA is disabled (see log).
> I'm looking for comments or suggestions.

A basic dd if=/dev/zero of=/dev/null loop in the background is enough to
raise the latency actually. Could you try the Xenomai 3 + 3.18 combo on
your hw and let us know whether you see the same regression?



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