rdtsc accuracy across CPU cores

Question

I am sending network packets from one thread and receiving replies on a 2nd thread that runs on a different CPU core. My process measures the time between send & receiv

Answer 1

On recent processors you can do it between separate cores of the same package (i.e. a system with just one core iX processor), you just can't do it in separate packages (processors), because they won't share the rtc. You can get away with it via cpu affinity (locking relevant threads to specific cores), but then again it would depend on the way your application behaves.

On linux you can check constant_tsc on /proc/cpuinfo in order to see if the processor has a single tsc valid for the entire package. The raw register is in CPUID.80000007H:EDX[8]

What I read around, but have not yet confirmed programatically, is that AMD cpus from revision 11h onwards have the same meaning for this cpuid bit.