dump1090-mutability & gain - effect on CPU consumption


#1

I have been running with gain=“agc” in Dump1090-mutability 1.15-dev. Earlier today I switched to gain=“max”, and I also tried gain="-10" and gain=“0”. Switching from agc cut the cpu consumed by the demodulator almost in half. It’s hard to tell without running it longer, but I don’t think it had a positive effect on the number of aircraft I was seeing. In the end I went back to agc, but I thought I’d post the following charts in case anyone else is interested. This is running on a Pi2, and clearly it isn’t heavily stressed so it’s just an interesting observation at this point.
https://dl.dropboxusercontent.com/u/42479319/Untitled-1.jpg


#2

Yeah, at very high gains there are a lot of false positives in the demodulator’s preamble detector, so it spends more CPU running the next stage of demodulation which is relatively expensive.


#3

Makes sense. Thank you.


#4

I tried those changes on a pi B+ running dump1090-mutability down in a hole with not particularly good coverage.
Baseline with gain=“max” shows consistent 30% CPU.
Test #1 with gain="-10" shows 35% CPU no observed change in aircraft seen.
Test #2 with gain=“0” shows 25% CPU no observed aircraft.
Looks like Gain=0 best for keeping CPU down with ‘minor’ side effect of not reporting anything. Will revert to gain=“max” YMMV