I convinced myself that a viable way to measure timestamps between a request packet and its response packet can be the difference between two Intel rdtsc calls
The restrictions to valid use include:
- RTT (time difference) must be calculated on the same CORE
- fencing instructions (lfence) could be required
The time difference is OK provided,
- it delivers at least microsecond resolution (rdtsc does)
- the difference is always positive (end-start) or zero
- the details of whether the clock runs or does not run at the processor speed is not material so long as there's sufficient resolution
- DPDK gives me the frequency rte_rdtsc_cycles(); this way I can convert from a rdtsc difference to elapsed time
- The OS doesn't reset the counter or pause it for interrupts or on halts
- The TSC is not always invariant
- And of course context switches (if a thread is not pinned to a core) will invalidate any time difference
- The TSC is not incremented when the processor enters a deep sleep. I don't care about this because I'll turn off the power saving modes anyway
So I am not so sure.
Now, of course, Mellanox can report time stamps. Is it actually possible to get HW NIC timestamps reported for every packet sent and received without overburdening the NIC? Based on what I can see for my case (Connect 4 LX) resolution is nanoseconds. So I am tempted to not fool around with rdtsc and just use NIC timestamps.
What is praxis in DPDK programming when one needs RTTs?