alt Hacker NewsWell, if the goal is for software running on the host CPU to know the time accurately, then it does matter. The control loop for host PTP benefits from regularity. Anyway NICs that support PTP hardware timestamping may also use PCI LTR (latency tolerance reporting) to instruct the host operating system to disable high-exit-latency sleep features, and popular operating systems respect that.
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"Well, if the goal is for software running on the host CPU to know the time accurately, then it does matter. "
I'm sorry, this is just moving the goalposts.
You said "It can't achieve better-than-NTP results without disabling PCI power saving features and deep CPU sleep states."
This is flat wrong, as pointed out.
Now you are pedantically arguing that some NIC's that do PTP hardware timestamping might also use a feature that some operating systems might respect.
That's a very far cry from "It can't achieve better-than-NTP results without disabling PCI power saving features and deep CPU sleep states".
In most cases, people would just say "hey i was wrong about that but there are cases that i think matter where it falls down".
> The control loop for host PTP benefits from regularity.
How much regularity? If you sent PTP packets with 5 milliseconds of randomness in the scheduling, does that cause real problems? It's still going to have an accurate timestamp.
> instruct the host operating system to disable high-exit-latency sleep features
Why, though? You didn't explain this. As long as the packet got timestamped when it arrived, the CPU can ask the NIC how many nanoseconds ago that was, and correct for how long it was asleep. Right?