Pulling 100k revisions 100× faster

Mercurial is a distributed version control system whose codebase combines Python, C and Rust. Over its twenty years of development, significant effort has been put into its scaling and overall performance.

In the recent 7.2 version, the performance of exchanging data between repositories (e.g. push and pull) has been significantly improved, with some of our most complicated benchmark cases moving from almost four hours down to 2 minutes, a speedup of over 100x.

This talk uses this work as a case study of the multiple places where performance improvements lie. It goes over the challenges that arise from exchanging data in a DVCS, and the levers we can pull to overcome them: higher level logic changes, lower level algorithmic improvements, programming language strengths, modern CPU architecture, network protocol design, etc.

Despite the great results, exchanging data in version control remains a complex matter, and we lastly expose our ideas to further tackle its inherent complexity.