[jnordwick]

that's what the vdso version of clock_gettime does. If you use one of the *_COARSE clocks it will only update periodically and be much faster, but that means like 15 milliseconds of log messages will all have the same timestamp.

The fastest for nanosecond precision (bonus is this is even sub nanosecond) is just to store the return value of RDTSC and let the background thread figure it all out. You don't even need to precalcuate the freq or epoch offset. Just write a couple logging messages of the rdtsc value and CLOCK_REALTIME and let the post processing figure it out.

To cut down on I/O each log message's timestamp can just be an offset from the last even.

If you are willing to push a lot of work to the background thread and even more to the post processsing step, you really don't need to do very much.

[Thaxll]

> hat's what the vdso version of clock_gettime does. If you use one of the *_COARSE clocks it will only update periodically and be much faster, but that means like 15 milliseconds of log messages will all have the same timestamp.

Not sure it matters a lot of to have multiple messages with the same timestamp, since they were added in order you still know which one is older, the problem might arise when you send those logs to a remote place and the order of insert is discarded and the timestamp is used instead.

I assume that when you use a single thread with a queue / ring buffer the order of insertion is kept.

[gpderetta]

FWIW, I have relied on the accuracy of log timestamps well into the low microseconds.

If you have an event loop, it might be acceptable to sample the counter once per iteration, but less than that it becomes too lossy.

[pas]

probably in this case it's important to use some kind of synthetic timestamping to preserve the ordering (for example, for a 5.123 [ms] timestamp one can add ~1000000 ns timestamps, so let's say are a thousand entries that need to be ordered, one can then represent them as 5.123000[n] ... and the "000" part is just a silly in-band way to give a hint to someone who will later examine the logs)

[jnordwick]

since you aren't going to be writing a message per nanosecond, you can always just do `last nanos = max(last nanos + 1, cur nanos)` and then use last nanos for the timestamp. you can even do it in rdtsc ticks and get 1/3 of nano values. Obv the clock isn't nearly that accurate, but it lets you use those fractional nanos to ensure a strictly increasing ordering.