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  1. 30 Dec, 2013 3 commits
    • Vicent Marti's avatar
      pack-bitmap: implement optional name_hash cache · ae4f07fb
      Vicent Marti authored
      When we use pack bitmaps rather than walking the object
      graph, we end up with the list of objects to include in the
      packfile, but we do not know the path at which any tree or
      blob objects would be found.
      
      In a recently packed repository, this is fine. A fetch would
      use the paths only as a heuristic in the delta compression
      phase, and a fully packed repository should not need to do
      much delta compression.
      
      As time passes, though, we may acquire more objects on top
      of our large bitmapped pack. If clients fetch frequently,
      then they never even look at the bitmapped history, and all
      works as usual. However, a client who has not fetched since
      the last bitmap repack will have "have" tips in the
      bitmapped history, but "want" newer objects.
      
      The bitmaps themselves degrade gracefully in this
      circumstance. We manually walk the more recent bits of
      history, and then use bitmaps when we hit them.
      
      But we would also like to perform delta compression between
      the newer objects and the bitmapped objects (both to delta
      against what we know the user already has, but also between
      "new" and "old" objects that the user is fetching). The lack
      of pathnames makes our delta heuristics much less effective.
      
      This patch adds an optional cache of the 32-bit name_hash
      values to the end of the bitmap file. If present, a reader
      can use it to match bitmapped and non-bitmapped names during
      delta compression.
      
      Here are perf results for p5310:
      
      Test                      origin/master       HEAD^                      HEAD
      -------------------------------------------------------------------------------------------------
      5310.2: repack to disk    36.81(37.82+1.43)   47.70(48.74+1.41) +29.6%   47.75(48.70+1.51) +29.7%
      5310.3: simulated clone   30.78(29.70+2.14)   1.08(0.97+0.10) -96.5%     1.07(0.94+0.12) -96.5%
      5310.4: simulated fetch   3.16(6.10+0.08)     3.54(10.65+0.06) +12.0%    1.70(3.07+0.06) -46.2%
      5310.6: partial bitmap    36.76(43.19+1.81)   6.71(11.25+0.76) -81.7%    4.08(6.26+0.46) -88.9%
      
      You can see that the time spent on an incremental fetch goes
      down, as our delta heuristics are able to do their work.
      And we save time on the partial bitmap clone for the same
      reason.
      Signed-off-by: default avatarVicent Marti <tanoku@gmail.com>
      Signed-off-by: default avatarJeff King <peff@peff.net>
      Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
      ae4f07fb
    • Vicent Marti's avatar
      pack-objects: implement bitmap writing · 7cc8f971
      Vicent Marti authored
      This commit extends more the functionality of `pack-objects` by allowing
      it to write out a `.bitmap` index next to any written packs, together
      with the `.idx` index that currently gets written.
      
      If bitmap writing is enabled for a given repository (either by calling
      `pack-objects` with the `--write-bitmap-index` flag or by having
      `pack.writebitmaps` set to `true` in the config) and pack-objects is
      writing a packfile that would normally be indexed (i.e. not piping to
      stdout), we will attempt to write the corresponding bitmap index for the
      packfile.
      
      Bitmap index writing happens after the packfile and its index has been
      successfully written to disk (`finish_tmp_packfile`). The process is
      performed in several steps:
      
          1. `bitmap_writer_set_checksum`: this call stores the partial
             checksum for the packfile being written; the checksum will be
             written in the resulting bitmap index to verify its integrity
      
          2. `bitmap_writer_build_type_index`: this call uses the array of
             `struct object_entry` that has just been sorted when writing out
             the actual packfile index to disk to generate 4 type-index bitmaps
             (one for each object type).
      
             These bitmaps have their nth bit set if the given object is of
             the bitmap's type. E.g. the nth bit of the Commits bitmap will be
             1 if the nth object in the packfile index is a commit.
      
             This is a very cheap operation because the bitmap writing code has
             access to the metadata stored in the `struct object_entry` array,
             and hence the real type for each object in the packfile.
      
          3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap
             index for one of the packfiles we're trying to repack, this call
             will efficiently rebuild the existing bitmaps so they can be
             reused on the new index. All the existing bitmaps will be stored
             in a `reuse` hash table, and the commit selection phase will
             prioritize these when selecting, as they can be written directly
             to the new index without having to perform a revision walk to
             fill the bitmap. This can greatly speed up the repack of a
             repository that already has bitmaps.
      
          4. `bitmap_writer_select_commits`: if bitmap writing is enabled for
             a given `pack-objects` run, the sequence of commits generated
             during the Counting Objects phase will be stored in an array.
      
             We then use that array to build up the list of selected commits.
             Writing a bitmap in the index for each object in the repository
             would be cost-prohibitive, so we use a simple heuristic to pick
             the commits that will be indexed with bitmaps.
      
             The current heuristics are a simplified version of JGit's
             original implementation. We select a higher density of commits
             depending on their age: the 100 most recent commits are always
             selected, after that we pick 1 commit of each 100, and the gap
             increases as the commits grow older. On top of that, we make sure
             that every single branch that has not been merged (all the tips
             that would be required from a clone) gets their own bitmap, and
             when selecting commits between a gap, we tend to prioritize the
             commit with the most parents.
      
             Do note that there is no right/wrong way to perform commit
             selection; different selection algorithms will result in
             different commits being selected, but there's no such thing as
             "missing a commit". The bitmap walker algorithm implemented in
             `prepare_bitmap_walk` is able to adapt to missing bitmaps by
             performing manual walks that complete the bitmap: the ideal
             selection algorithm, however, would select the commits that are
             more likely to be used as roots for a walk in the future (e.g.
             the tips of each branch, and so on) to ensure a bitmap for them
             is always available.
      
          5. `bitmap_writer_build`: this is the computationally expensive part
             of bitmap generation. Based on the list of commits that were
             selected in the previous step, we perform several incremental
             walks to generate the bitmap for each commit.
      
             The walks begin from the oldest commit, and are built up
             incrementally for each branch. E.g. consider this dag where A, B,
             C, D, E, F are the selected commits, and a, b, c, e are a chunk
             of simplified history that will not receive bitmaps.
      
                  A---a---B--b--C--c--D
                           \
                            E--e--F
      
             We start by building the bitmap for A, using A as the root for a
             revision walk and marking all the objects that are reachable
             until the walk is over. Once this bitmap is stored, we reuse the
             bitmap walker to perform the walk for B, assuming that once we
             reach A again, the walk will be terminated because A has already
             been SEEN on the previous walk.
      
             This process is repeated for C, and D, but when we try to
             generate the bitmaps for E, we can reuse neither the current walk
             nor the bitmap we have generated so far.
      
             What we do now is resetting both the walk and clearing the
             bitmap, and performing the walk from scratch using E as the
             origin. This new walk, however, does not need to be completed.
             Once we hit B, we can lookup the bitmap we have already stored
             for that commit and OR it with the existing bitmap we've composed
             so far, allowing us to limit the walk early.
      
             After all the bitmaps have been generated, another iteration
             through the list of commits is performed to find the best XOR
             offsets for compression before writing them to disk. Because of
             the incremental nature of these bitmaps, XORing one of them with
             its predecesor results in a minimal "bitmap delta" most of the
             time. We can write this delta to the on-disk bitmap index, and
             then re-compose the original bitmaps by XORing them again when
             loaded.
      
             This is a phase very similar to pack-object's `find_delta` (using
             bitmaps instead of objects, of course), except the heuristics
             have been greatly simplified: we only check the 10 bitmaps before
             any given one to find best compressing one. This gives good
             results in practice, because there is locality in the ordering of
             the objects (and therefore bitmaps) in the packfile.
      
           6. `bitmap_writer_finish`: the last step in the process is
      	serializing to disk all the bitmap data that has been generated
      	in the two previous steps.
      
      	The bitmap is written to a tmp file and then moved atomically to
      	its final destination, using the same process as
      	`pack-write.c:write_idx_file`.
      Signed-off-by: default avatarVicent Marti <tanoku@gmail.com>
      Signed-off-by: default avatarJeff King <peff@peff.net>
      Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
      7cc8f971
    • Vicent Marti's avatar
      pack-bitmap: add support for bitmap indexes · fff42755
      Vicent Marti authored
      A bitmap index is a `.bitmap` file that can be found inside
      `$GIT_DIR/objects/pack/`, next to its corresponding packfile, and
      contains precalculated reachability information for selected commits.
      The full specification of the format for these bitmap indexes can be found
      in `Documentation/technical/bitmap-format.txt`.
      
      For a given commit SHA1, if it happens to be available in the bitmap
      index, its bitmap will represent every single object that is reachable
      from the commit itself. The nth bit in the bitmap is the nth object in
      the packfile; if it's set to 1, the object is reachable.
      
      By using the bitmaps available in the index, this commit implements
      several new functions:
      
      	- `prepare_bitmap_git`
      	- `prepare_bitmap_walk`
      	- `traverse_bitmap_commit_list`
      	- `reuse_partial_packfile_from_bitmap`
      
      The `prepare_bitmap_walk` function tries to build a bitmap of all the
      objects that can be reached from the commit roots of a given `rev_info`
      struct by using the following algorithm:
      
      - If all the interesting commits for a revision walk are available in
      the index, the resulting reachability bitmap is the bitwise OR of all
      the individual bitmaps.
      
      - When the full set of WANTs is not available in the index, we perform a
      partial revision walk using the commits that don't have bitmaps as
      roots, and limiting the revision walk as soon as we reach a commit that
      has a corresponding bitmap. The earlier OR'ed bitmap with all the
      indexed commits can now be completed as this walk progresses, so the end
      result is the full reachability list.
      
      - For revision walks with a HAVEs set (a set of commits that are deemed
      uninteresting), first we perform the same method as for the WANTs, but
      using our HAVEs as roots, in order to obtain a full reachability bitmap
      of all the uninteresting commits. This bitmap then can be used to:
      
      	a) limit the subsequent walk when building the WANTs bitmap
      	b) finding the final set of interesting commits by performing an
      	   AND-NOT of the WANTs and the HAVEs.
      
      If `prepare_bitmap_walk` runs successfully, the resulting bitmap is
      stored and the equivalent of a `traverse_commit_list` call can be
      performed by using `traverse_bitmap_commit_list`; the bitmap version
      of this call yields the objects straight from the packfile index
      (without having to look them up or parse them) and hence is several
      orders of magnitude faster.
      
      As an extra optimization, when `prepare_bitmap_walk` succeeds, the
      `reuse_partial_packfile_from_bitmap` call can be attempted: it will find
      the amount of objects at the beginning of the on-disk packfile that can
      be reused as-is, and return an offset into the packfile. The source
      packfile can then be loaded and the bytes up to `offset` can be written
      directly to the result without having to consider the entires inside the
      packfile individually.
      
      If the `prepare_bitmap_walk` call fails (e.g. because no bitmap files
      are available), the `rev_info` struct is left untouched, and can be used
      to perform a manual rev-walk using `traverse_commit_list`.
      
      Hence, this new set of functions are a generic API that allows to
      perform the equivalent of
      
      	git rev-list --objects [roots...] [^uninteresting...]
      
      for any set of commits, even if they don't have specific bitmaps
      generated for them.
      
      In further patches, we'll use this bitmap traversal optimization to
      speed up the `pack-objects` and `rev-list` commands.
      Signed-off-by: default avatarVicent Marti <tanoku@gmail.com>
      Signed-off-by: default avatarJeff King <peff@peff.net>
      Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
      fff42755