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Created attachment 7113 [details]
A synthetic benchmark for the parallel work issue.
The improvements made by #18880 over the synthetic benchmark have improved the performance, but there seem to still be some important contention in mono_loader_lock, particularly during the startup of an app that run on multiple threads.
This line is an example of location where the time is spent : https://github.com/mono/mono/blob/078dc0321d53f9e161957656550fd10cc41db618/mono/metadata/metadata.c#L2808
The attached sample demonstrates the issue, showing that on an iPad2, 1 Thread=268ms, 2 threads=380ms.
In a bigger app, the time spent in the lock is significantly larger, particularly when the UI thread is performing work at the same time as background threads.
This has already been fixed in mono dd72864f564231c324248c3fd75b55054debc1e4, but that commit is not yet in the stable xamarin.ios release.
$ git branch --contains dd72864f564231c324248c3fd75b55054debc1e4
This fix is publicly available inside XI 7.2.6 (alpha right now) if you want to test it.