maedhroz commented on code in PR #3102:
URL: https://github.com/apache/cassandra/pull/3102#discussion_r1579961777
##########
src/java/org/apache/cassandra/metrics/DecayingEstimatedHistogramReservoir.java:
##########
@@ -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);
+ DecayingBuckets newBuckets = new DecayingBuckets(now);
+ for (int i = 0; i < decayingBuckets.buckets.length(); i++)
{
- long newValue = Math.round(decayingBuckets.get(i) / rescaleFactor);
- decayingBuckets.set(i, newValue);
+ long newValue = Math.round(decayingBuckets.buckets.get(i) /
rescaleFactor);
+ newBuckets.buckets.set(i, newValue);
}
+ return newBuckets;
}
private boolean needRescale(long now)
{
- return (now - decayLandmark) > LANDMARK_RESET_INTERVAL_IN_NS;
+ return (now - decayingBuckets.decayLandmark) >
LANDMARK_RESET_INTERVAL_IN_NS;
}
@VisibleForTesting
public void clear()
{
- final int bucketCount = decayingBuckets.length();
+ final int bucketCount = buckets.length();
for (int i = 0; i < bucketCount; i++)
{
- decayingBuckets.set(i, 0L);
buckets.set(i, 0L);
}
+ decayingBuckets = new DecayingBuckets(decayingBuckets.decayLandmark);
Review Comment:
I guess you could wrap this up in some crazy to avoid the landmark going
backward in time...
```
while (true)
{
if (rescaling.compareAndSet(false, true))
{
try
{
long landmark = decayingBuckets.decayLandmark;
decayingBuckets = new DecayingBuckets(landmark);
}
finally
{
rescaling.set(false);
}
break;
}
}
```
...but it's only used for tests...we don't really care :D
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