performance degradation after spin wait

[VariantF]

Hi, I'm tunning my program for low-latency.

I have a tight calculation function calc(); which is using SIMD floating point instructions heavily.

I had test the performance of calc(); using perf command. it shows that this calc function is using ~10k instructions and ~5k cpu cycles in average.

However, when I put this calc function after a spin-wait like

while(true) {
  if (!flag.load(std::memory_order_acquire)) {
      continue;
  }

  calc();
}

the calc part is using about 10k cycles. and other perf counters like `l1d-cache-misses`, `llc-misses`, `branch-misses` and `instructions` remain the same.

Can anyone help me to explain how this happened and what should I do to avoid this? I mean to keep the calc function as fast as possible.

Also, I have 2 interesting findings:

1. If I got the flag variable set in a very short period(less than 1ms). I cannot notice any performance degradation for function calc.

2. if I add some garbage simd floating point calcution in the middle of spin-wait. I can achieve the expected performance.

[VariantF]

My CPU is 13900K. I also tested at 12900K and Ice Lake CPUs like Xeon 8368. looks they have the same behaviour.

I noticed from `Optimization Reference Manual` that there's something called `Thread Director` which can automatically detect the thread classes in runtime and there's a special class called` Pause (spin-wait) dominated code`. I don't know if this is related but looks like after some time period, the CPU detected that the thread is in a spin-wait loop and then reduced the resource that is allocated to this thread ?