[NO-TICKET] Profiling: Assume wall-time can't go backwards

[ivoanjo]

What does this PR do?

This PR removes the max_of(elapsed_time_ns, 0) we had for wall-time, replacing it with an exception similar to the one we have for cpu-time.

In practice, it means that rather than "paving over"/"ignoring" weird behavior in the clock, it now becomes an explicit error state.

Update: This has been open for a while but should be good for review now. It's probably easier to review commit-by-commit.

Motivation:

In the past, we actually had this "if wall-time goes back it's an exception logic", and we changed it in
#2336 because we saw that CLOCK_MONOTONIC did go backwards on macOS.

Recently we stopped using CLOCK_MONOTONIC on macOS, replacing it with CLOCK_MONOTONIC_RAW (that's
#5994) and @eregon did a bunch of experiments in #2336 to show that this clock doesn't go backwards on macOS.

So we can tighten our logic. Our research shows that CLOCK_MONOTONIC does not have this issue on Linux, so there we expect to be fine.

Change log entry

None.

Additional Notes:

This PR is stacked atop #6016 as that PR also touches some of the same helpers, but it's otherwise conceptually independent.

#6016 has been merged so this is good to go.

This PR is stacked atop #6302 to avoid conflicts with tweaks done in that PR or the earlier #6364. They are otherwise independent.

All prior PRs merged, ready to go again!

How to test the change?

I've added test coverage for this.

[datadog-datadog-prod-us1]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 90.36% (-0.02%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 53651d8 | Docs | View more details | Give us feedback!}

[chatgpt-codex-connector]

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[ivoanjo]

Hmm the assertion seems to be triggering sometimes. Investigating...

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-07-13 15:08:14

Comparing candidate commit c0add03 in PR branch ivoanjo/assume-time-doesnt-go-backwards with baseline commit c30c2eb in branch master.

📊 Benchmarking dashboard

Found 1 performance improvements and 0 performance regressions! Performance is the same for 47 metrics, 1 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

scenario:profiling - intern mixed existing and new

• 🟩 throughput [+1.829op/s; +2.409op/s] or [+6.165%; +8.119%]

Unstable benchmarks

These benchmarks have a confidence interval too wide to call a change; treat them as noise rather than signal.

scenario:tracing - trace.to_digest - Continue

• unstable throughput [-1476.559op/s; +1649.393op/s] or [-5.154%; +5.757%]