Optimizing Packfiles and Memory Utilization in Git Fast Import

writers, run the repack in the background and let it finish when it finishes. There is no reason to wait to explore your new Git project! If you choose to wait for the repack, don't try to run benchmarks or performance tests until repacking is completed. fast-import outputs suboptimal packfiles that are simply never seen in real use situations. Repacking Historical Data ~~~~~~~~~~~~~~~~~~~~~~~~~ If you are repacking very old imported data (e.g. older than the last year), consider expending some extra CPU time and supplying --window=50 (or higher) when you run 'git repack'. This will take longer, but will also produce a smaller packfile. You only need to expend the effort once, and everyone using your project will benefit from the smaller repository. Include Some Progress Messages ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Every once in a while have your frontend emit a `progress` message to fast-import. The contents of the messages are entirely free-form, so one suggestion would be to output the current month and year each time the current commit date moves into the next month. Your users will feel better knowing how much of the data stream has been processed. PACKFILE OPTIMIZATION --------------------- When packing a blob fast-import always attempts to deltify against the last blob written. Unless specifically arranged for by the frontend, this will probably not be a prior version of the same file, so the generated delta will not be the smallest possible. The resulting packfile will be compressed, but will not be optimal. Frontends which have efficient access to all revisions of a single file (for example reading an RCS/CVS ,v file) can choose to supply all revisions of that file as a sequence of consecutive `blob` commands. This allows fast-import to deltify the different file revisions against each other, saving space in the final packfile. Marks can be used to later identify individual file revisions during a sequence of `commit` commands. The packfile(s) created by fast-import do not encourage good disk access patterns. This is caused by fast-import writing the data in the order it is received on standard input, while Git typically organizes data within packfiles to make the most recent (current tip) data appear before historical data. Git also clusters commits together, speeding up revision traversal through better cache locality. For this reason it is strongly recommended that users repack the repository with `git repack -a -d` after fast-import completes, allowing Git to reorganize the packfiles for faster data access. If blob deltas are suboptimal (see above) then also adding the `-f` option to force recomputation of all deltas can significantly reduce the final packfile size (30-50% smaller can be quite typical). Instead of running `git repack` you can also run `git gc --aggressive`, which will also optimize other things after an import (e.g. pack loose refs). As noted in the "AGGRESSIVE" section in linkgit:git-gc[1] the `--aggressive` option will find new deltas with the `-f` option to linkgit:git-repack[1]. For the reasons elaborated on above using `--aggressive` after a fast-import is one of the few cases where it's known to be worthwhile. MEMORY UTILIZATION ------------------ There are a number of factors which affect how much memory fast-import requires to perform an import. Like critical sections of core Git, fast-import uses its own memory allocators to amortize any overheads associated with malloc. In practice fast-import tends to amortize any malloc overheads to 0, due to its use of large block allocations. per object ~~~~~~~~~~ fast-import maintains an in-memory

This section discusses strategies for optimizing packfiles created by Git fast-import, including repacking historical data, emitting progress messages, and using options like --window and --aggressive to reduce packfile size and improve data access patterns, as well as factors that affect memory utilization during the import process.