EF9 introduced a change in how we construct ValueComparers for some of our types (specifically collection of scalars/references), in preparation for AOT work. The way the change was implemented may cause a severe performance regression during SaveChanges operation involving multiple entities using collections of primitives (one of our highly requested features).

Customer impact

Customers performing data manipulation operations on entities with collections of primitives may experience significant performance regressions. This may also happen when no data has been changed, but sufficiently large entity graph has been loaded into change tracker. There is no workaround for this issue, apart from changing the model to not use primitive collections (which is unacceptable for majority of customers)

How found

Multiple customer reports on EF 9

Regression

Yes, from EF8. Note: this is a perf regression only, not a functional regression.

Testing

Ad hoc performance test using BenchmarkDotNet. Functional testing already covered by existing tests.

Risk

Low. The patch fix has been limited in scope to reduce the risk. Changes should only affect models with primitive collections. Added quirks just to be sure.

Perf numbers

Over 20x regression between 8 and 9 for the tested scenario.

8.0.11

Method	Mean	Error	StdDev
SaveChangesTest	172.1 ms	1.78 ms	1.58 ms

9.0

Method	Mean	Error	StdDev
SaveChangesTest	5.487 s	0.0621 s	0.0551 s

9.0.2 (with fix)

Method	Mean	Error	StdDev
SaveChangesTest	179.8 ms	1.71 ms	1.43 ms

maumar changed the title~~Fix to #35239 - EF9: SaveChanges() is significantly slower in .NET9 vs. .NET8 when using .ToJson() Mapping vs. PostgreSQL Legacy POCO mapping~~[release/9.0-staging] Fix to #35239 - EF9: SaveChanges() is significantly slower in .NET9 vs. .NET8 when using .ToJson() Mapping vs. PostgreSQL Legacy POCO mapping

Dec 19, 2024

AndriySvyryd reviewed

Dec 19, 2024

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Dec 19, 2024

WIP on ConvertingValueComparer#35354

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Fix to#35239- EF9: SaveChanges() is significantly slower in .NET9 v…

f2c36d8

…s. .NET8 when using .ToJson() Mapping vs. PostgreSQL Legacy POCO mapping

maumar force-pushed thefix35239_90 branch fromd058551 tof2c36d8Compare

December 20, 2024 01:54

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Dec 20, 2024

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December 20, 2024 23:04

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December 27, 2024 23:08

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maumar commentedDec 27, 2024

@AndriySvyryd update: added changes to the cosmos StringDictionaryComparer. It does support nested collections and the legacy functions can be invoked (rather than just being constructed like in relational). I added targeted fix similar to what I initially did for main.