Better implementations of System.{currentTimeMillis,nanoTime}. #5228
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gzm0
approved these changes
Aug 31, 2025
| */ | ||
| val start = System.nanoTime() | ||
| val elapsed = System.nanoTime() - start | ||
| assertTrue(elapsed.toString(), elapsed >= 0L && elapsed < 100000000L) |
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I'm not sure we should assume monotonicity here. In general, it is not a valid assumption to make for clocks. The JDK doc doesn't give an explicit guarantee about it.
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Hum, nanoTime is definitely supposed to be monotonic. However I did find this bug report, which says that sometimes it incorrectly goes back in time. 😕
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We encountered this bug on a Linux box with Jvm, which then cause the Akka stream of keepAlive operator failed.
All browsers have been supporting the official `performance.now()` for more than 10 years, at this point.
`js.Date.now()` was already present in ECMAScript 5.1. We were using it in the constructor of `ju.Date`, but not in the two methods of `System`. In this commit, we switch to `js.Date.now()` in `System`. We also change the constructor of `ju.Date` to delegate to `currentTimeMillis`, in order to have a single source of truth.
Instead of storing a lambda returning the high precision time, we store the global object whose `now()` method returns the high precision time. This is possible because the only two alternatives, `performance.now()` and `Date.now()`, happen to share their method name and the scale of their results. This implementation removes a JS function call dispatch, which makes the call faster both in JS and in Wasm.
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