CASSANDRA-20333 Decaying histogram reservoir uses thread locals to store updates #4003
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netudima
reviewed
Apr 7, 2025
| if (decaying.decayLandmark != decayingEstimatedBuckets.decayLandmark) | ||
| decayingEstimatedBuckets.flush(this, decaying); | ||
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| decayingRef.get().update(index, now); |
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do we need to do this get always twice?
netudima
reviewed
Apr 7, 2025
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| public void update(int index, long now) | ||
| { | ||
| if (lastSampledClock != now) |
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forwardDecayWeight actually uses seconds granularity, so we can reuse the value for much longer..
netudima
reviewed
Apr 7, 2025
| // try to use int[] instead of long[] to reduce memory usage, and move to the sum array when overflow | ||
| private final AtomicReference<DecayingArray> decayingRef; | ||
| private final long[] estimated; | ||
| private volatile boolean writing; |
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potential false sharing with the array reference and decayingRef..
netudima
reviewed
Apr 7, 2025
| { | ||
| // This is only called by the thread that owns the thread local, so we don't need to worry about contention. | ||
| // Once the rescaling has occurred, we need to flush the values to the decayingBucket and report that the values are no longer in use. | ||
| writing = true; |
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so, in the current implementation we have 2 volatile writes vs 2 addAndGet operations in the original one, without a contention they cost may be quite similar...
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