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.
(various thing I heard about please
better control on compression,
skipping useless buffering,
zstd for storage,
zstd for diff,
clone bundle for pull