> getting /dev/rtc handling right for latency measurement is
> ... tricky.
> The method i'm using under PREEMPT_RT is:
>
> chrt -f 84 -p `pidof 'IRQ 0'`
> chrt -f 95 -p `pidof 'IRQ 8'`
> ./rtc_wakeup -f 1024 -t 100000
I tried it your way.
First impressions with realtime-preempt-2.6.12-rc1-V0.7.42-01,
rtc_wakeup and chrt:
The main difference is not between rtc_wakeup and my test script
(the timings agree very well), but comes from chrt for 'IRQ 8':
System not loaded, original 'IRQ 8' prio 49:
* rtc_wakeup at 89(99): max jitter: 378.2% (461 usec), missed irqs: 0
* rtc_wakeup at 1(11): max jitter: 97.1% (118 usec), missed irqs: 25
System not loaded, 'IRQ 8' prio at 95:
* rtc_wakeup at 89(99): max jitter: 19.1% ( 23 usec), missed irqs: 0
* rtc_wakeup at 1(11): max jitter: 190.7% (232 usec), missed irqs: 52
The following tests are made with 'IRQ 8' at 95, rtc_wakeup at 89(99):
* Heavy mmap load, no oom: max jitter: 42.1% ( 51 usec)
* Heavy mmap load, oom: max jitter: 11989.2% (14635 usec)
(but still "missed irqs: 0", so IRQ 8 was also blocked for 14 ms)
* USB reads, no error: max jitter: 707.7% ( 863 usec)
(happens rarely)
* USB reads, error: max jitter: 7247.2% ( 8846 usec)
(again, no missed irqs)
* USB stick connect: max jitter: 5776.7% ( 7051 usec)
* USB stick disconnect: max jitter: 2966.6% ( 3621 usec)
(both without error!)
Notes:
* My system does not have a 'IRQ 0' kernel thread.
It has IRQ 1, 3, 7, 8, 12 and 14.
This is strange, because /proc/interrupts says
0: 4805658 XT-PIC timer 0/5658
2: 0 XT-PIC cascade 0/0
3: 0 XT-PIC serial 0/0
7: 17538 XT-PIC uhci_hcd:usb1, eth0 0/17538
8: 838523 XT-PIC rtc 0/38523
14: 5775 XT-PIC ide0 0/5775
* The syntax of chrt has changed:
chrt -f -p <prio> <pid>
* I run rtc_wakeup with -f 8192.
This is more realistic in our case.
CF card (IDE) load does not hurt as long as the rtc test runs
with a very high rtpri: The results did not differ significantly
from no-load results.
Attempts with tracing on and results with running the test
at low rtpri will follow tomorrow...
--
Klaus Kusche
Entwicklung Software - Steuerung
Software Development - Control
KEBA AG
A-4041 Linz
Gewerbepark Urfahr
Tel +43 / 732 / 7090-3120
Fax +43 / 732 / 7090-8919
E-Mail: [email protected]
www.keba.com
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