Weird timestamping phenomenon on AMD Ryzen 7950X

[hchechao2]

My Enviroment: Machine A: AMD 7950X + RHEL8.3(kernel 4.18.0-240.el8) openonload7.1.2.141, sfc driver4.15.12.1008 , sfptpd3.2.6.1003) Machine B: AMD 5950X + RHEL8.3(kernel 4.18.0-240.el8) openonload7.1.2.141, sfc driver4.15.12.1008 , sfptpd3.2.6.1003)

both have stptpd running:

when running taskset -c <isolated_cpu> efsink -t <filter_spec> on machine A, I will get some discrete distribution of delta = timestamp diff (SW -HW)like this

but running the same command on machine B , i will get results like:

why timestamp diff(SW - HW) will be so strange on machine A?

btw: Can we view timestamp diff(SW - HW) as PCIE latency(From NIC to Memory)? but from the result about 1.5us, how can it be true to achieve ~2.5us latency when running eflatency if the PCIE latency is ~1.5us