KEP-1040: Weighted borrowing #4733
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MikeSpreitzer
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Jun 15, 2024
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MikeSpreitzer
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- One-line PR description: Add weighting to the borrowing between APF priority levels
- Issue link:
- Other comments: This PR includes the commit of PR KEP-1040: Prescribe less extreme borrowing by exempt priority levels #4623, which I hope will merge first. Once that is merged, I will rebase this one. To review just the change that this PR is about, look at the second of the two commits that are currently here.
Signed-off-by: Mike Spreitzer <[email protected]>
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Signed-off-by: Mike Spreitzer <[email protected]>
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| the next version, the common fields of | ||
| `LimitedPriorityLevelConfiguration` and | ||
| `ExemptPriorityLevelConfiguration` will move to their common ancestor | ||
| `PriorityLevelConfigurationSpec`. |
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Cutting this out because this plan did not come to fruition.
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| CurrentCL(i) = MinCL(i) + ( MinCurrentCL(i) - MinCL(i) ) * Weight(i) * | ||
| (RemainingServerCL-MinCLSum) / WeightedDiffSum | ||
| WeightedDiffSum = sum[non-exempt priority level i] | ||
| ( MinCurrentCL(i) - MinCL(i) ) * Weight(i) |
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I might not have fully understood those formulae, but questions:
- why can't we just apply the previous logic -- scaling down the CurrentCL proportional to
RemainingServerCL / LowerBoundSum? why do the weights have to impact this if no borrowing is happening here. - A more direct question: I tested APF a while ago and learned a rule of "the owner keeps the shares if inuse", now would the weights break this rule? i.e. PLs with lower weights will be borrowed even when their demands are higher than their NominalCL?
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First, remember that the PR that you are looking at is actually combining two changes: one is #4623 , which aims to heal the long-standing divergence between code and KEP by moving both to a middle ground, and then there is the change to add weighting to the priority levels.
It is true that when MinCLSum < RemainingServerCL < MinCurrentCLSum there is no borrowing between non-exempt priority levels, but the exempt level(s) have borrowed so much from the non-exempt ones that each must give up some concurrency. So it is actually a borrowing scenario.
Remember that you have not tested the behavior described in #4623 unless you tested its implementation in kubernetes/kubernetes#124736 .
Regarding "the owner keeps the shares if in use", I would be a little more precise: the rule is that each non-exempt level gets its nominal concurrency limit if at least that much has been in use AND the borrowing by exempt levels is not so great as to cut into that. The formulae that you have commented on concern the case where the exempt level(s) have borrowed so much that the non-exempt ones can not get their nominal concurrency limits.
