System time is running very fast

I’m using QNX 6.4.1 on ARM board. qconn shows 100% cpu usage. When I use “date” command I see that time running about 4-10 seconds per real second and isn’t regular.

When I slay some applications and cpu usage lowers I see that time become normal again.
How it can be and how system manages CLOCK_REALTIME ?

CLOCK_REALTIME is set by a timer interrupt. It sounds like you have other interrupt handlers that are hogging the cpu. I don’t know about ARM but on X86 you can pick the interrupt that you want to have the highest priority. Of course interrupt handlers and programs can turn off interrupts which will also cause this problem.