So there are 2 parts to cpu affinity. a) cpu assigned to ssd for handling interrupts and b) cpu assigned to fio. numactl is your friend for experimenting with with changing fio affinity.

https://access.redhat.com/documentation/en-us/red_hat_enterp... tells you how to tweak irq handlers.

You usually want to change both. pinning each fio process + each interrupt handler to specific cpus will reach highest perf.

You can even use isolcpus param to linux kernel to reduce jitter from things you don't care about to minimize latency.(wont do much for bandwidth)