Re: [Xenomai] BeagleBone Xenomai Latency

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 01/11/2014 09:58 PM, Charles Steinkuehler wrote:
> (Starting a new thread)
> On 1/7/2014 5:13 PM, Gilles Chanteperdrix wrote:
>> On 01/07/2014 11:55 PM, Charles Steinkuehler wrote:
>>> On 1/7/2014 4:47 PM, Gilles Chanteperdrix wrote:
>>>> Is there any chance you could trigger a trace with the I-pipe 
>>>> tracer?
>>
>>> I can try...give me a while to sort through the I-pipe:Tracer wiki
>>> page (or are there better instructions?).
>>
>> Enable I-pipe tracer in kernel configuration, especially
>> IPIPE_TRACE_MCOUNT, and IPIPE_TRACE_VMALLOC.
>>
>> When the system has booted, do:
>>
>> echo 1 > /proc/ipipe/trace/enable
>> echo 1000 > /proc/ipipe/trace/back_trace_points
>>
>> Then launch latency -p 200 -f
>>
>> Run the load you usually run (for instance dohell).
>>
>> When the test is finished, do
>> cat /proc/ipipe/trace/frozen > /tmp/frozen.txt
>>
>> then post the resulting frozen.txt to this list.
>>
>> Expect the latency found by the test to be double the latency you get
>> without the I-pipe tracer.
> 
> OK, I tested a variety of kernels.  Simply enabling the ipipe trace
> options on the otherwise unchanged kernel configuration I'm using
> resulted in a system that was *EXTREMELY* slow and unresponsive, even
> before I enabled the iPipe trace.  After waiting the better part of a
> day for a GUI application to launch, I gave up on this kernel and tested
> one with the recommended performance options:
> 
> 
> Already in the BeagleBone Xenomai kernels I've been building:
>> optimize for size
>> disable CONFIG_PREEMPT
>> enable stack unwinding
>> disable root thread priority coupling (xenomai option)
> 
> Updated to desired setting in dbg_opt kernel:
>> disable unlocked context switch (xenomai option)
>> disable CONFIG_SMP
> 
> Recommended but not selected (causes linking errors apparently due to
> the kernel size being too large for some of the limited reach thumb2
> jump/branch instructions):

You mean thumb2? Did you enable the workaround for buggy gas?

> OVERALL RESULTS:
> Adding the recommended options dramatically helped with latency when
> iPipe trace was enabled, but not so much for the normal kernel.  I did
> see typical and best case latency drop significantly (by about 25%) with
> the recommended options, but worst-case latency for both kernels
> remained around 50 uS.

I also get 50us at 1kHz, I only get 40us at 10kHz.


-- 
                                                                Gilles.