[Rtai] Source of big latencies???

Federico Bergero fedebergero at gmail.com
Tue Sep 27 16:28:58 CEST 2011


Ok. I'll try setting only one core without HT in the bios.
That way Linux will only use one core.

Anyway I'm using LXRT so I'm not using the UP nor the SMP (nor the
MUP) scheduler,right?.

I tried taking care of SMI but when I insert the module it tells me
"No intel chip detected", so I thought that didn't need to do
anything. Is this correct? Or I have to do something anyway?

I'll reply later with the new results

Thanks

Fede

2011/9/27 Paolo Mantegazza <mantegazza at aero.polimi.it>:
> Federico Bergero wrote:
>>
>> What do you mean by uni-processor?
>> I have disabled HT but still got 6 cores (a small SMP system).
>> Anyway the test fails with only one thread, so I don't think that HT
>> or multiple cores have anything to do with it.
>> In theory RTAI shouldn't give the processor back to Linux if the RTAI
>> task is doing something (like in a busy wait), right? If so, what
>> could have HIGHER priority than RTAI task? Bios? ACPI/APM?
>>
>
> To play with the config you should first configure Linux as UP. If the
> latency remains than you have to know what hardware you bought. If it does
> not than you have a clue related to what your hardware does SMP. Have cared
> of SMI already?
> Please keep posting to the list.
>
>
> paolo
>
>> Grazie tante!
>> Fede
>>
>> 2011/9/27 Paolo Mantegazza <mantegazza at aero.polimi.it>:
>>>
>>> Federico Bergero wrote:
>>>>
>>>> I have disabled HT in the BIOS (now running with 6cores)
>>>> I'm still getting a source of high latencies.
>>>> I found out that these big latencies are PERIODIC (?)
>>>> They occur every 2.43 seg.
>>>> I'm attaching the test I have run.
>>>>
>>>> I'm using the kernel from
>>>> http://hart.sourceforge.net/rtai_installation_10_04.html
>>>>
>>>> Any ideas what could be going on?
>>>>
>>> Have you tried the latency test after making your machine as if it was
>>> UP.
>>>
>>> paolo
>>>
>>>
>>>> These is the output of the test:
>>>> [   58.336000] Error diff 2343675702
>>>> [   58.336000] Error diff 2343677010
>>>> [   59.628000] Error diff 2343704223
>>>>
>>>> 2011/9/26 Paolo Mantegazza <mantegazza at aero.polimi.it>:
>>>>>
>>>>> Federico Bergero wrote:
>>>>>>
>>>>>> Hi I'm working on a project with rtai.
>>>>>> I'm using a Intel i7 with 6 cores and HT so Linux sees 12 cores.
>>>>>> I'm running a real time LXRT-RTAI thread on each core (indicating RTAI
>>>>>> to do so with the cpus_allowed mask).
>>>>>>
>>>>>> Each thread runs a simulation of a system syncrhonized with real time,
>>>>>> and I'm using rt_sleep/rt_busy_sleep to do the waits.
>>>>>> When running the simulation SOMETIMES I get a max latencies of 900us
>>>>>> on some processor and 2us on others processors.
>>>>>> These BIG latencies occur randomly. Sometimes everything runs
>>>>>> smoothly.
>>>>>> What could be the source of these lantencies? I have turned off Freq
>>>>>> Scaling in Linux and SpeedStep in the BIOS.
>>>>>>
>>>>>> Also I don't quiet understand how the rt_get_cpu_time_ns() is
>>>>>> implemented on a SMP system, could someone explain me this?
>>>>>>
>>>>>> Thanks Fede
>>>>>>
>>>>> First of all do not use HT.
>>>>> On your machine rt_get_cpu_time_ns() simply reads and converts the APIC
>>>>> timer on each CPU. You can check if their are big difference intra CPUs
>>>>> by
>>>>> configuring RTAI to do so.
>>>>>
>>>>> paolo
>>>>>
>>>>>
>>>>> paolo
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>>
>>
>
>



-- 
I find television very educational. Every time someone switches it on,
I go into another room and read a good book.


More information about the Rtai mailing list