Commits on Aug 4, 2021

1. fetch-pack: speed up loading of refs via commit graph …

   When doing reference negotiation, git-fetch-pack(1) is loading all refs
   from disk in order to determine which commits it has in common with the
   remote repository. This can be quite expensive in repositories with many
   references though: in a real-world repository with around 2.2 million
   refs, fetching a single commit by its ID takes around 44 seconds.
   
   Dominating the loading time is decompression and parsing of the objects
   which are referenced by commits. Given the fact that we only care about
   commits (or tags which can be peeled to one) in this context, there is
   thus an easy performance win by switching the parsing logic to make use
   of the commit graph in case we have one available. Like this, we avoid
   hitting the object database to parse these commits but instead only load
   them from the commit-graph. This results in a significant performance
   boost when executing git-fetch in said repository with 2.2 million refs:
   
       Benchmark #1: HEAD~: git fetch $remote $commit
         Time (mean ± σ):     44.168 s ±  0.341 s    [User: 42.985 s, System: 1.106 s]
         Range (min … max):   43.565 s … 44.577 s    10 runs
   
       Benchmark #2: HEAD: git fetch $remote $commit
         Time (mean ± σ):     19.498 s ±  0.724 s    [User: 18.751 s, System: 0.690 s]
         Range (min … max):   18.629 s … 20.454 s    10 runs
   
       Summary
         'HEAD: git fetch $remote $commit' ran
           2.27 ± 0.09 times faster than 'HEAD~: git fetch $remote $commit'
   
   Signed-off-by: Patrick Steinhardt <ps@pks.im>
   Signed-off-by: Junio C Hamano <gitster@pobox.com>

   

   pks-t authored and gitster committed Aug 4, 2021
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   3e5e6c6 View commit details

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