Optimizing Kallithea performance

When serving a large amount of big repositories, Kallithea can start performing slower than expected. Because of the demanding nature of handling large amounts of data from version control systems, here are some tips on how to get the best performance.

• Kallithea is often I/O bound, and hence a fast disk (SSD/SAN) is usually more important than a fast CPU.
• Sluggish loading of the front page can easily be fixed by grouping repositories or by increasing cache size (see below). This includes using the lightweight dashboard option and vcs_full_cache setting in .ini file.

Follow these few steps to improve performance of Kallithea system.

1. Increase cache

   Tweak beaker cache settings in the ini file. The actual effect of that is questionable.

2. Switch from SQLite to PostgreSQL or MySQL

   SQLite is a good option when having a small load on the system. But due to locking issues with SQLite, it is not recommended to use it for larger deployments. Switching to MySQL or PostgreSQL will result in an immediate performance increase. A tool like SQLAlchemyGrate can be used for migrating to another database platform.

3. Scale Kallithea horizontally

   Scaling horizontally can give huge performance benefits when dealing with large amounts of traffic (many users, CI servers, etc.). Kallithea can be scaled horizontally on one (recommended) or multiple machines.

   It is generally possible to run WSGI applications multithreaded, so that several HTTP requests are served from the same Python process at once. That can in principle give better utilization of internal caches and less process overhead.

   One danger of running multithreaded is that program execution becomes much more complex; programs must be written to consider all combinations of events and problems might depend on timing and be impossible to reproduce.

   Kallithea can't promise to be thread-safe, just like the embedded Mercurial backend doesn't make any strong promises when used as Kallithea uses it. Instead, we recommend scaling by using multiple server processes.

   Web servers with multiple worker processes (such as mod_wsgi with the WSGIDaemonProcess processes parameter) will work out of the box.

   In order to scale horizontally on multiple machines, you need to do the following:

   • Each instance needs its own .ini file and unique instance_id set.
   • Each instance's data storage needs to be configured to be stored on a shared disk storage, preferably together with repositories. This data dir contains template caches, sessions, whoosh index and is used for task locking (so it is safe across multiple instances). Set the cache_dir, index_dir, beaker.cache.data_dir, beaker.cache.lock_dir variables in each .ini file to a shared location across Kallithea instances
   • If using several Celery instances, the message broker should be common to all of them (e.g., one shared RabbitMQ server)
   • Load balance using round robin or IP hash, recommended is writing LB rules that will separate regular user traffic from automated processes like CI servers or build bots.