Rescale threshold calculation#5169
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Pull request overview
This PR adjusts ExponentiallyDecayingReservoir’s rescaling interval to account for custom alpha values, aiming to prevent weight/priority overflow (leading to Double.Infinity priorities and skipped updates) after long runtimes. It also adds a regression test that reproduces the overflow scenario for high alpha.
Changes:
- Compute a per-instance
rescaleThresholdbased onalpha(falling back to the previous default threshold for typicalalphavalues). - Update rescale triggering to use the computed threshold.
- Add a test verifying updates still register after enough time has passed to otherwise overflow at higher
alpha.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| metrics-core/src/main/java/com/codahale/metrics/ExponentiallyDecayingReservoir.java | Introduces an alpha-aware rescale threshold to avoid overflow/infinite priorities. |
| metrics-core/src/test/java/com/codahale/metrics/ExponentiallyDecayingReservoirTest.java | Adds a regression test for the overflow scenario and adjusts an existing timing assertion. |
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| private long calculateRescaleThreshold(double alpha) { | ||
| long upperbound = (long) Math.floor(Math.log(Double.MAX_VALUE) / alpha); | ||
| if (upperbound == 0) { | ||
| throw new IllegalStateException("Alpha value resulted in a rescale threshold of 0 seconds"); | ||
| } | ||
| return upperbound >= DEFAULT_RESCALE_THRESHOLD ? DEFAULT_RESCALE_THRESHOLD : TimeUnit.SECONDS.toNanos(upperbound / 2); | ||
| } |
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| private long calculateRescaleThreshold(double alpha) { | ||
| long upperbound = (long) Math.floor(Math.log(Double.MAX_VALUE) / alpha); | ||
| if (upperbound == 0) { | ||
| throw new IllegalStateException("Alpha value resulted in a rescale threshold of 0 seconds"); | ||
| } | ||
| return upperbound >= DEFAULT_RESCALE_THRESHOLD ? DEFAULT_RESCALE_THRESHOLD : TimeUnit.SECONDS.toNanos(upperbound / 2); | ||
| } |
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| // wait for 10 millis and take snapshot. | ||
| // this should not trigger a rescale. Note that the number of samples will be reduced to 0 | ||
| // because scaling factor equal to zero will remove all existing entries after rescale. | ||
| clock.addSeconds(20 * 60); | ||
| clock.addSeconds(5 * 60); |
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As noted in #793, no new samples are recorded after ~13 hours as new priority returns infinity. This behavior is solved in #793 . However, it's probably an unwanted behavior that we expose
alpha, but do not automatically calculate a rescale threshold.I've added a rescale threshold calculation if the upperbound is less than the
DEFAULT_RESCALE_THRESHOLDof 1 hour; otherwise, the old threshold is used to avoid any surprising behavioral change. If you're open to this change, I'll make the corresponding modifications inLockFreeExponentiallyDecayingReservoir.cc @arteam as you looked over the original issue