Mmuzaf commented on code in PR #3102:
URL: https://github.com/apache/cassandra/pull/3102#discussion_r1714133974
##########
src/java/org/apache/cassandra/metrics/DecayingEstimatedHistogramReservoir.java:
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@@ -371,42 +363,50 @@ private void rescaleIfNeeded(long now)
{
try
{
- rescale(now);
+ decayingBuckets = rescale(now);
}
finally
{
- decayLandmark = now;
rescaling.set(false);
}
}
}
}
- private void rescale(long now)
+ /**
+ * rescale races with update;
+ * If rescale is called at the same time as an update, and an update falls
into an already
+ * processed bucket this update will be lost. We accept that for the sake
of update performance.
+ * OTOH, if the update falls into a bucket that is yet to be processed, it
will be added
+ * with a weight that is consistent with the rescaleFactor used by
rescale, so the new rescaled
+ * buckets will all have values consistent with the new decay landmark.
+ */
+ private DecayingBuckets rescale(long now)
{
- // despite striping its safe to rescale each bucket individually
- final double rescaleFactor = forwardDecayWeight(now);
- for (int i = 0; i < decayingBuckets.length(); i++)
+ final double rescaleFactor = decayingBuckets.forwardDecayWeight(now);
Review Comment:
We are reading the `decayingBuckets` three times here and if the `rebase`
method is called, this could potentially cause a problem. So I would save the
decayingBuckets to a local ref and read it.
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