• Jeff King's avatar
    reachable: only mark local objects as recent · 1385bb7b
    Jeff King authored
    When pruning and repacking a repository that has an
    alternate object store configured, we may traverse a large
    number of objects in the alternate. This serves no purpose,
    and may be expensive to do. A longer explanation is below.
    
    Commits d3038d22 and abcb8655 taught prune and pack-objects
    (respectively) to treat "recent" objects as tips for
    reachability, so that we keep whole chunks of history. They
    built on the object traversal in 660c889e (sha1_file: add
    for_each iterators for loose and packed objects,
    2014-10-15), which covers both local and alternate objects.
    
    In both cases, covering alternate objects is unnecessary, as
    both commands can only drop objects from the local
    repository. In the case of prune, we traverse only the local
    object directory. And in the case of repacking, while we may
    or may not include local objects in our pack, we will never
    reach into the alternate with "repack -d". The "-l" option
    is only a question of whether we are migrating objects from
    the alternate into our repository, or leaving them
    untouched.
    
    It is possible that we may drop an object that is depended
    upon by another object in the alternate. For example,
    imagine two repositories, A and B, with A pointing to B as
    an alternate. Now imagine a commit that is in B which
    references a tree that is only in A. Traversing from recent
    objects in B might prevent A from dropping that tree. But
    this case isn't worth covering. Repo B should take
    responsibility for its own objects. It would never have had
    the commit in the first place if it did not also have the
    tree, and assuming it is using the same "keep recent chunks
    of history" scheme, then it would itself keep the tree, as
    well.
    
    So checking the alternate objects is not worth doing, and
    come with a significant performance impact. In both cases,
    we skip any recent objects that have already been marked
    SEEN (i.e., that we know are already reachable for prune, or
    included in the pack for a repack). So there is a slight
    waste of time in opening the alternate packs at all, only to
    notice that we have already considered each object. But much
    worse, the alternate repository may have a large number of
    objects that are not reachable from the local repository at
    all, and we end up adding them to the traversal.
    
    We can fix this by considering only local unseen objects.
    Signed-off-by: default avatarJeff King <peff@peff.net>
    Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
    1385bb7b
reachable.c 4.7 KB