[release/10.0] [mono][sgen] Fix incorrect condition when checking if we should do a major collection #120532
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…major collection When we are in this method, major sweep hasn't yet finished so we check with some imprecise conservative values. min_heap_size is a conservative lower limit of the actual heap size. Allowance represents how much more we allow the heap to grow after the previous major collection. We were comparing the min_heap_size with the allowance by mistake, when we should have compared it with the next major trigger size (which is the heap_size after the last collection plus the allowance).
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Tagging subscribers to this area: @BrzVlad |
BrzVlad
approved these changes
Oct 10, 2025
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This is going in 10.0.1. The milestone hasn't been created yet (I lack the ability to do it). |
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Backport of #120432 to release/10.0
/cc @BrzVlad
Customer Impact
This impacts all platforms using mono. A customer reported a 40x regression on an XML Serialization benchmark when moving from .NET8 to .NET9. This regression is caused by the runtime triggering excessive GCs due to a bug in the heuristic. This is more likely to happen if the GC concurrent sweep phase takes longer and the application allocates large objects frequently. This likely impacts many applications, but with less noticeable performance impact.
Regression
This regressed in .NET9
Testing
Tested the fix on sample benchmark that it works correctly.
Risk
Low risk. The fix is localized and the previous condition was blatantly wrong by mistake.