Add a post on exceptions in Gradle

_posts/2023-03-09-The-Catch-With-Exceptions.md

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+---
+layout: post
+title:  The Catch in Using Exceptions for Control Flow
+header: header-eng
+---
+
+Continuing my series of posts about profiling Gradle, I'd like to share how I
+found a shortcoming in Gradle that was costing androidx configuration phase to
+be 110 seconds slower than needed.
+
+It all started with an observation that Gradle would be seemingly stuck when
+it got to `100% CONFIGURING [Xm Ys]`.
+
+![Screenshot of Gradle in 100% configuring state](/assets/2023-03-09-configuring.png)
+
+The time continued to update, but nothing seemed to be happening. To investigate
+I started looking at what is happening using a profiler. As it was recording
+I was paging through and stopped at `Exceptions` tab, spotting this:
+
+![Screenshot of Yourkit exceptions tab profiling androidx build](/assets/2023-03-09-exceptions.png){:width="600px"}
+
+Narrator: that surely seems like a lot of `NoMatchingVariantSelectionException` exceptions.
+
+Going through the timeline, this is the Gradle code that was running when in
+`100% CONFIGURING` state.
+
+![Screenshot of Yourkit stacktrace in the androidx build](/assets/2023-03-09-stack.png){:width="600px"}
+
+AndroidX does use configuration caching feature, so it is not surprising that
+we need to serialize it, but looking at `configuration-cache/*/work.bin` it is
+only 387MB, so spending 136 seconds writing that out feels excessive.
+
+Digging through this stack, it can be seen that we are spending about
+103 seconds in `AttributeMatchingVariantSelector`. Then I remembered seeing
+287K `NoMatchingVariantSelectionException` and the stacks for those matched
+up. Turns out Gradle was using exceptions for control flow. In AndroidX
+we hit this code path a lot so had to create 287K of them and that's not fast
+even on a desktop.
+
+I [created an issue report to Gradle](https://github.com/gradle/gradle/issues/24103)
+and within couple days they landed [a fix](https://github.com/gradle/gradle/pull/24242).
+It will ship in Gradle 8.1. Testing out the fixed version with AndroidX resulted
+in configuration cache serialization going from 136 to 22 seconds while also
+making the memory usage lower.
+
+I hope this is enough of a motivation for you to also profile your builds and
+help me improve the whole ecosystem. 
+
+As a side note, I also created [a feature request for Gradle to show the
+configuration serialization step in CLI](https://github.com/gradle/gradle/issues/24274)
+so hopefully such issues would become easier to spot in the future.