vchuravy's link gives the details, but basically, there's a microarchitecture optimization in Zen that breaks determinism of the performance counters. Fortunately, there's a chicken bit that turns it off, which is what the script does. I've been trying to convince AMD to officially document the bit such that the kernel can set it automatically, but no luck so far.

There is still one remaining annoyance, which is that AMD's NMI latency is super high, which directly tanks rr's reverse execution latency. There's probably some improvements that could be made to the replayer to be more aggressive about optimistic assumptions on NMI latency and retrying if those assumptions are off, but it'd be a fair bit of work. I don't really understand why AMD decided to use this kind of architecture. It also makes profiles much less accurate.

Thanks for the info. I was wondering what was going on in that script. It’s unfortunate that their architectural decisions had to impact rr, but I guess these days, every last bit of benchmark score really matters.

I think https://github.com/rr-debugger/rr/issues/2034#issuecomment-6... is the right synopsis.