GPIO input latency problem

Hi all!

I have Orange Pi One with Armbian Debian 5.8.9-sunxi on board.

The problem is that when I catch 2 kHz square wave with OPi gpio, I have very big delta between signals.

If I use "interrupt both endges" parameter for gpio, time between signals must be 250 microsecs. But now it floats between 200 - 290 microsecs.

And I have achive this result after some suggestions from other forum topics: using isolcpu=3, run c program on this cpu, use max priority, disable powersaving for CPU.

I try to read with wiringOP lib, and directly from sysfs, but still have jittering about 90 us.

Also I think, that I have not very bad results for cyclictest:

# cyclictest -a -t -n -p80 
# /dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.00 0.00 0.00 1/127 2335

T: 0 ( 2332) P:80 I:1000 C:  32762 Min:      8 Act:   11 Avg:   12 Max:      98
T: 1 ( 2333) P:80 I:1500 C:  21841 Min:      8 Act:   19 Avg:   11 Max:      48
T: 2 ( 2334) P:80 I:2000 C:  16381 Min:      8 Act:    9 Avg:   12 Max:     113
T: 3 ( 2335) P:80 I:2500 C:  13104 Min:     10 Act:   10 Avg:   10 Max:      16

The main quieston is - does direct programming /dev/mem gpio registers helps me to decrease floating between receiving signals?

May be other suggestions? Please, help!

Thank you!