Re: [Xenomai-help] Re: Latencies due to RT-Socket-CAN register accesses

[Wolfgang Grandegger]

Roland Tollenaar wrote:
Hi Wolfgang,
Typically without load the average latecny is 30 us and 20 us min 
latency. Yes I am using the latency program of the test suit.

Where do I check to find the latency "killer" settings? The 
powermanagement and all that s switched of. As is the PC speaker. What 
else can I look at?

Here is what I get on a much slower PC (Athlon 1.1 GHz):
[EMAIL PROTECTED] bin]# ./latency -p500
   RTT|  00:00:01  (periodic user-mode task, 500 us period, priority 99)
   RTH|-----lat min|-----lat avg|-----lat max|-overrun|
   ---|------------|------------|------------|--------|
   RTS|       1.309|       1.988|      10.723|       0|

Could you show us your .config file as Stephane ask for and the output 
of /proc/cpuinfo. Unfortunately, I have little experience with PC 
hardware and it's bad impact on the latency.

Wolfgang.


Wolfgang Grandegger wrote:
Roland Tollenaar wrote:
Hi Wolfgang,

I think that this kind of latency should be fine for controller 
intervals of 1ms.

Another question:
The PC has a 3Ghz processor and an 800Mhz FSB. That should not class 
as "slow" in the conventional sense. What hardware is responsible for 
the latency i.e. makes the PC "slow" from the RT perspective? In 
other words what must one look at when purchasing a machine that 
should give minimal latency?

I ment "slow" in the sense of little CPU power, but that's obviously 
not the case. How have you measured these figures? With the "latency" 
program from the test suite? What is the min latency _without_ load? 
And also check Xenomai's TROUBLESHOOTING file. Already changing the 
kernel configuration to avoid latency "killers" might help.

Wolfgang.

Regards,

Roland.


Wolfgang Grandegger wrote:
roland Tollenaar wrote:
Hi Wolfgang,

Just to answer an open question still

> BTW, what are the latencies you measure on your system under load
> (without RT-Socket-CAN).

lat min 20.952
lat ave 31.009
lat max 60.504
lat best 19.276

this was with the loading as you described in this thread and running
for 12 hours.

Anything special about the above figures? Not very good I presume but
I am not running with tsc enabled.

The latencies depend a lot on the hardware. The minimal latency of 
20 us indicates, that you have a rather slow system. Then a maximum 
of 60 us is reasonable. Note that worst case latencies of better 
than 20us are difficult to achieve, even with high end PCs.

Wolfgang.

Kind regards,

Roland




Roland


>
> Wolfgang.
>
>> Thanks,
>>
>> Roland
>>
>>
>>
>> Wolfgang Grandegger wrote:
>>> Hallo,
>>>
>>> in the meantime I have measured the latencies introduced through
>>> messages sent and received by RT-Socket-CAN. The SJA1000 register
>>> access times on my rather old PC with an Athlon 1100 Mhz are:
>>>
>>> PEAK-Dongle: read  access: 11807 ns
>>> PEAK-Dongle: write access: 11677 ns
>>>
>>> IXXAT-PCI  : read  access:   729 ns
>>> IXXAT-PCI  : write access:   305 ns
>>>
>>> I measured an increase of the latency of approx. 170us with the
>>> PEAK-Dongle and approx 13us with the IXXAT-PCI card for the 
reception
>>> of a full CAN message (with 8 bytes payload). Sending messages 
is a
>>> bit less disturbing. I have attached a small patch to measure the
>>> SJA1000 register access times when the driver is initialized. 
You are
>>> welcome to apply it on your setup and report the results. I'm
>>> especially interested in numbers for the ISA bus (or PC-104).
>>>
>>> Wolfgang.
>>>
>>>
>>>
>>> 
------------------------------------------------------------------------ 

>>>
>>> + diff -u xenomai/ksrc/drivers/can/rtcan_dev.c.IOTEST
>>> xenomai/ksrc/drivers/can/rtcan_dev.c
>>> + diff -u xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c.IOTEST
>>> xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c
>>> --- xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c.IOTEST
>>> 2007-02-26 09:17:27.000000000 +0100
>>> +++ xenomai/ksrc/drivers/can/sja1000/rtcan_sja1000.c    
2007-03-13
>>> 10:01:47.000000000 +0100
>>> @@ -728,6 +728,30 @@
>>>      if (chip == NULL)
>>>      return -EINVAL;
>>>
>>> +#if 1
>>> +    {
>>> +    nanosecs_abs_t begin, diff;
>>> +    volatile u8 reg;
>>> +    int i, count = 100000;
>>> +    begin = rtdm_clock_read();
>>> +    for (i = 0; i < count; i++) {
>>> +        reg = chip->read_reg(dev, 0);
>>> +    }
>>> +    diff = rtdm_clock_read() - begin;
>>> +    printk("%s: register read time for %d accessed: %ld (%ld per
>>> access)\n",
>>> +           dev->board_name, count,
>>> +           (unsigned long)diff, (unsigned long)diff / count);
>>> +    begin = rtdm_clock_read();
>>> +    for (i = 0; i < count; i++) {
>>> +        chip->write_reg(dev, 0, reg);
>>> +    }
>>> +    diff = rtdm_clock_read() - begin;
>>> +    printk("%s: register write time for %d accessed: %ld (%ld 
per
>>> access)\n",
>>> +           dev->board_name, count,
>>> +           (unsigned long)diff, (unsigned long)diff / count);
>>> +    }
>>> +#endif
>>> +
>>>      /* Set dummy state for following call */
>>>      dev->state = CAN_STATE_ACTIVE;
>>>      /* Enter reset mode */
>>
>>
>
>

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