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https://issues.apache.org/jira/browse/FLINK-35285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17849352#comment-17849352
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Trystan commented on FLINK-35285:
---------------------------------
[~gyfora] is there maybe another setting that can help tune this? At least on
1.7.0, I often find that a max scale down factor of 0.5 (which seems to be
essentially mandatory given the current computations) leads to an overshoot -
so then it scales back up. For example 40 -> 20 -> 40 -> 24.
I'd prefer to be conservative on the scale down and set it to 0.2 or 0.3. In
the case of maxParallelism=120, 0.3 would work for _this_ scale down, but 0.2
would not - we would effectively have a minParallelism of 40 and never go below
it. Yet in the case of current=120, max=120, maxScaleDown=.2, it works just
fine - it'll scale to 96. The "good" minScaleFactors seem highly dependent on
both the maxParallelism and currentParallelism.
It seems that the problem lies in this loop:
https://github.com/apache/flink-kubernetes-operator/blob/fe3d24e4500d6fcaed55250ccc816546886fd1cf/flink-autoscaler/src/main/java/org/apache/flink/autoscaler/JobVertexScaler.java#L299-L303
If we add
{code:java}
&& p < currentParallelism {code}
to the loop we get the expected behavior on scale down. Of course, then other
keygroup-optimized scale ups break. Perhaps there needs to be different loops
for scale up / scale down?
Is there something obvious that I'm missing, something I can tune better?
> Autoscaler key group optimization can interfere with scale-down.max-factor
> --------------------------------------------------------------------------
>
> Key: FLINK-35285
> URL: https://issues.apache.org/jira/browse/FLINK-35285
> Project: Flink
> Issue Type: Bug
> Components: Kubernetes Operator
> Reporter: Trystan
> Priority: Minor
>
> When setting a less aggressive scale down limit, the key group optimization
> can prevent a vertex from scaling down at all. It will hunt from target
> upwards to maxParallelism/2, and will always find currentParallelism again.
>
> A simple test trying to scale down from a parallelism of 60 with a
> scale-down.max-factor of 0.2:
> {code:java}
> assertEquals(48, JobVertexScaler.scale(60, inputShipStrategies, 360, .8, 8,
> 360)); {code}
>
> It seems reasonable to make a good attempt to spread data across subtasks,
> but not at the expense of total deadlock. The problem is that during scale
> down it doesn't actually ensure that newParallelism will be <
> currentParallelism. The only workaround is to set a scale down factor large
> enough such that it finds the next lowest divisor of the maxParallelism.
>
> Clunky, but something to ensure it can make at least some progress. There is
> another test that now fails, but just to illustrate the point:
> {code:java}
> for (int p = newParallelism; p <= maxParallelism / 2 && p <= upperBound; p++)
> {
> if ((scaleFactor < 1 && p < currentParallelism) || (scaleFactor > 1 && p
> > currentParallelism)) {
> if (maxParallelism % p == 0) {
> return p;
> }
> }
> } {code}
>
> Perhaps this is by design and not a bug, but total failure to scale down in
> order to keep optimized key groups does not seem ideal.
>
> Key group optimization block:
> [https://github.com/apache/flink-kubernetes-operator/blob/fe3d24e4500d6fcaed55250ccc816546886fd1cf/flink-autoscaler/src/main/java/org/apache/flink/autoscaler/JobVertexScaler.java#L296C1-L303C10]
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