OpenJDK has completed the transition from Mercurial to GitHub as planned in September 2020. GitHub offers various benefits such as increased performance and support for code reviews. InfoQ reported in June about the change in more detail.
One of the reasons behind the migration was to move to a more widely used version control solution - Mercurial is much less widely used than git, despite both being distributed version control systems with essentially equivalent capabilities. The switch to git is intended to make it easier for developers to contribute to the development of OpenJDK and to remove roadblocks resulting from the use of Mercurial.
Although the transition was completed recently, the effects are already positive. Since the migration to GitHub, the number of users of OpenJDK development infrastructure tripled.
The migration and the increasing number of users also resulted in the discovery of various bugs, which are registered in the Skara project. The transition to Git and GitHub was a big effort where more than 65 authors created more than a thousand pull requests.
Better support for code reviews was one of the reasons to move from Mercurial to GitHub. The first responses to the new feature are positive. As for instance, Uwe Schindler mentioned:
I also got my membership in the GitHub openjdk project yesterday. I can now also review PRs and already made use of that. Much better than webrevs!
The Mercurial (hg) repositories are in read-only mode now the migration to GitHub is completed. The changes in GitHub will not be reflected in Mercurial.
Project Skara provides a FAQ about Git and GitHub for contributors.