Fly-Style commented on code in PR #19378:
URL: https://github.com/apache/druid/pull/19378#discussion_r3149076801
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indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/WeightedCostFunction.java:
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@@ -24,17 +24,23 @@
/**
* Weighted cost function using compute time as the core metric.
- * Costs represent actual time in seconds, making them intuitive and
debuggable.
- * Uses linear scaling without mode inversions for predictable behavior.
+ * Lag cost is based on recovery time in seconds; idle cost is a penalty
derived from
+ * the predicted idle ratio.
+ *
+ * <p>Idle cost uses a U-shaped penalty with minimum at {@link
#IDEAL_IDLE_RATIO}.
+ * This penalizes both under-provisioning (low idle, no safety margin, lag
risk) and
+ * over-provisioning (high idle, wasted capacity), with asymmetric severity
controlled by
+ * {@link #UNDER_PROVISIONING_PENALTY} and {@link #OVER_PROVISIONING_PENALTY}.
*/
public class WeightedCostFunction
{
private static final Logger log = new Logger(WeightedCostFunction.class);
+
/**
* Multiplier for a lag amplification factor; it was carefully chosen
* during extensive testing as the most balanced multiplier for high-lag
recovery.
*/
- static final double LAG_AMPLIFICATION_MULTIPLIER = 0.05;
+ static final double LAG_AMPLIFICATION_MULTIPLIER = 0.4;
Review Comment:
I will note it in the patch notes. Generally, the intention was to find a
point where scale-up/scale-down decisions are 'normal' in terms of normal
distribution near `0.5/0.5` weights. `0.4` is a good amplification multiplier.
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