JDK-8227434 : G1 predictions may over/underflow with high variance input
- Type: Bug
- Component: hotspot
- Sub-Component: gc
- Affected Version: 14
- Priority: P3
- Status: Resolved
- Resolution: Fixed
- Submitted: 2019-07-09
- Updated: 2019-12-10
- Resolved: 2019-11-29
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| JDK 14 |
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14 b26Fixed  |
Related Reports
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Description
Currently G1 uses a predictor based on a decaying average that adds a safety margin using the sequences' variance/standard deviation. On workloads/situations with recent high variance, the predictors used in the G1Analytics class may currently return unexpected negative/positive values as they are not clamped to a useful range. As usual these errors can influence the prediction accuracy significantly (e.g. I have seen predictions for the amount of bytes survived to be in the ~2^63 range due to overflow, which are then propagated further to result in completely impossible overall time predictions). This is a day one bug as far as I understand; only in very few cases consumers of the predictions "manually" clamp values already, e.g. the G1Policy::predict_yg_surv_rate() method. It would be better if the G1Analytics predict_* would do value clamping already.
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