[Xenomai] i.MX6q memory write causes high latency

Federico Sbalchiero f.sbalchiero at sferapro.com
Fri Jul 6 10:41:11 CEST 2018


Il 05/07/2018 11:14, Philippe Gerum ha scritto:
> 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?
>
> TIA,
>

kernel 3.18.20-ipipe + xenomai 3.0.7

latency under load (four memwrite instances)
RTT|  00:00:01  (periodic user-mode task, 1000 us period, priority 99)
RTH|----lat min|----lat avg|----lat max|-overrun|---msw|---lat 
best|--lat worst
RTD|     24.985|     41.374|     76.351|       0| 0|     24.985|     76.351
RTD|     26.889|     41.203|     68.070|       0| 0|     24.985|     76.351
RTD|     22.828|     41.376|     67.681|       0| 0|     22.828|     76.351
RTD|     20.969|     41.043|     74.143|       0| 0|     20.969|     76.351
RTD|     27.027|     41.441|     68.037|       0| 0|     20.969|     76.351
RTD|     24.413|     41.585|     81.062|       0| 0|     20.969|     81.062
RTD|     27.234|     41.168|     76.516|       0| 0|     20.969|     81.062
RTD|     23.779|     41.141|     70.466|       0| 0|     20.969|     81.062
RTD|     24.824|     41.273|     75.322|       0| 0|     20.969|     81.062
RTD|     25.627|     41.195|     71.157|       0| 0|     20.969|     81.062
RTD|     28.874|     41.089|     66.579|       0| 0|     20.969|     81.062
RTD|     26.672|     41.638|     75.995|       0| 0|     20.969|     81.062
RTD|     25.139|     41.040|     69.543|       0| 0|     20.969|     81.062
RTD|     26.215|     41.099|     66.336|       0| 0|     20.969|     81.062
RTD|     24.192|     41.117|     76.828|       0| 0|     20.969|     81.062
RTD|     27.310|     41.942|     79.888|       0| 0|     20.969|     81.062
RTD|     24.348|     40.955|     66.484|       0| 0|     20.969|     81.062
RTD|     26.679|     41.260|     80.242|       0| 0|     20.969|     81.062
RTD|     26.820|     41.251|     74.986|       0| 0|     20.969|     81.062
RTD|     27.635|     41.301|     73.961|       0| 0|     20.969|     81.062
RTD|     26.877|     41.305|     72.789|       0| 0|     20.969|     81.062




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