For the last couple of days I've been testing the --cpu-affinity option with my dual opteron 6376 trying to get cpuminer to use just 4 threads (14/15 and 30/31) instead of everything or the first one.

For some reason the --cpu-affinity option is noticed by both programs (using --cpu-affinity 6 or --cpu-affinity 0x6 for testing) but in all of the cases the program uses all cores or using the -t 2 option core 0 and core 1. But this is not what I want.

I'm sure I'm doing something wrong, but no idea what. Should the --cpu-affinity option go with another option ?

This is my commandline :
./cpuminer -a yescrypt -o stratum+tcp://hub.miningpoolhub.com:17021 -u -p d=10

I'm using Ubuntu 16.04 LTS and this is updated useing the apt-get update -y && apt-get upgrade -y. So this shoud be just fine. Any suggestions ?