This page is primarily intended for Mercurial's developers.
Performance Improvement Plan
Status: In progress
This is a speculative project and does not represent any firm decisions on future behavior.
The goal of this page is to gather data about known performance bottleneck and ideas about how to solve them
(I'm creating this page with clone/push/pull in mind) XXX fill me more
2. Detailed description
All kind of stuff can go here, solution description / alternative solution etc
2.1. Server-side Changegroup Performance
Servers tend to spend a lot of CPU and bandwidth computing and transferring changegroup data.
The most effective way to alleviate this resource usage is by serving static, pre-generated changegroup data instead of dynamically generating it at request time. A server-side cache of changegroup data would fall into this bucket. The "clone bundles" feature which serves initial clones from URLs is one implementation of this. But it only addresses the initial clone case. Subsequent pulls still result in significant load on the server. There is support for a "remote changegroup" bundle2 part that allows servers to advertise the URL of a pre-generated changegroup. But there are no extensions or features relying on this.
There is plenty of potential to optimize the server for changegroup generation. As of Mercurial 4.0, changegroups (with exception of the changelog) are effectively collections of single delta chains per revlogs. For generaldelta repos, many deltas on disk are reused. However, the server still needs to decompress the revlog entries on disk to obtain the raw deltas then recompress them as part of the changegroup compression context. Furthermore, if there are multiple delta chains in the revlog, the server will need to compute a new delta for those entries. This contributes to overhead, especially the decompression and recompression. Switching away from zlib for both revlog storage and wire protocol compression will help tremendously, as zstd can be 2x more efficient in both decompression and compression.
While server efficiency could be increased by increasing the efficiency of compression, it would be better to avoid compression altogether. There exists an "streaming clone" feature that essentially does a file copy of revlogs from server to client. However, this only applies to initial clone. It should be possible to extend this feature to subsequent pulls. So instead of transferring a changegroup with on-the-fly computed delta chains, the server would transfer the raw data in its revlogs, including compression. This feature would not be suitable for all environments, as the transfer size would likely increase and clients would need to support and effectively inherit the settings of the server. However, it would substantially reduce server-side CPU requirements.
(various thing I heard about please
better control on compression,
skipping useless buffering,
zstd for storage,
zstd for diff,
clone bundle for pull