What is the cost of creating and setting a new default subscriber?

[rimutaka]

What is the cost of running these 2 lines of code?

    let sub = tracing_subscriber::fmt().without_time().finish();
    let _guard = tracing::subscriber::set_default(sub);

I looks like line 1 involved creating structs with default values, an Arc and an RW Lock and should be lightning fast.

The code behind line 2 was getting a bit too complicated for me to estimate.
Looks like it depends on the number of the callsites and the size of the registry.

Contrary to my research, I was getting about 10us duration for line 1 and 0 for line 2 in my minimal test.

I'll appreciated any insight you can share.

[davidbarsky]

I looks like line 1 involved creating structs with default values, an Arc and an RW Lock and should be lightning fast.

Yes, that is correct. It's not actually doing anything beyond

The code behind line 2 was getting a bit too complicated for me to estimate.

It is O(number_of_spans_and_events)¹, but that is also the normal work that tracing does when a subscriber first encounters spans and events. I feel like there's a question behind this question: what's your motivation in asking this?

Footnotes

1. It also skips past anything that is below the activated level. ↩

[rimutaka]

@davidbarsky , thank you for the explanation.
I was trying to remove the output of a span added by a previous service in the chain.
One solution was to use its own subscriber inside the inner service, but it may be a bad idea if the service is called in a loop.