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

[Wolfgang Grandegger <wg@domain.hid>]

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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> 
>