Low USB polling frequency with RT-PREEMPT

Low USB polling frequency with RT-PREEMPT

[Simon Thelen]

Hello,

We have some Dynamixel robitic actuators hooked up via USB to an Ubuntu
14.04 system running the 4.4.79-rt92 kernel. When using a kernel built
without the realtime patches (CONFIG_PREEMPT not set) we can query the
bus at 1000Hz (2 devices at 500Hz each) whereas with the realtime kernel
(CONFIG_PREEMPT=y, CONFIG_PREEMPT_RT_FULL=y) we only get 62.5Hz per
device. The USB device requests Ivl=0ms in both cases and the issue
isn't related to load etc. The polling rate is always 62.5Hz (It takes
~15ms for every read()).


Is this a known limitation of USB with the CONFIG_PREEMPT_RT patches and
is there any method to improve the polling rate?

-- 
Simon Thelen

Re: Low USB polling frequency with RT-PREEMPT

[Niels Kolthoff]

Hi Simon,

Not sure if this helps; are you polling the actuators using usb-serial io?

I've had performance issues reading from the raspberry pi serial port.
see: http://www.spinics.net/lists/linux-rt-users/msg16880.html


On Tue, Sep 5, 2017 at 5:05 PM, Simon Thelen <urday@student.kit.edu> wrote:
> Hello,
>
> We have some Dynamixel robitic actuators hooked up via USB to an Ubuntu
> 14.04 system running the 4.4.79-rt92 kernel. When using a kernel built
> without the realtime patches (CONFIG_PREEMPT not set) we can query the
> bus at 1000Hz (2 devices at 500Hz each) whereas with the realtime kernel
> (CONFIG_PREEMPT=y, CONFIG_PREEMPT_RT_FULL=y) we only get 62.5Hz per
> device. The USB device requests Ivl=0ms in both cases and the issue
> isn't related to load etc. The polling rate is always 62.5Hz (It takes
> ~15ms for every read()).
>
>
> Is this a known limitation of USB with the CONFIG_PREEMPT_RT patches and
> is there any method to improve the polling rate?
>
> --
> Simon Thelen
> --
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