Significant KVO Performance Regression in iOS 9

Originator:dillan.laughlin
Number:rdar://22718949 Date Originated:
Status:Open Resolved:
Product:iOS SDK Product Version:
Classification:Serious Bug Reproducible:Always
 
Summary:
There is a significant increase in time that it takes for -[NSObject addObserver:forKeyPath:options:context:] to complete. I've seen as much as an 8x increase in execution time.

Steps to Reproduce:
1. Create a large number of model objects with many attributes.
2. Observe the attributes of each model object.

Expected Results:
Similar performance characteristics on iOS 8 and iOS 9.

Actual Results:
Drastically slower performance on iOS 9.

Version:
iOS 9 (13A340)

Notes:

See also this issue on GitHub:
https://github.com/facebook/KVOController/issues/72

Configuration:
iPhone 6.

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