[PATCH 09/16] documentation: add documentation for the bitmap format
This is the technical documentation and design rationale for the new
Bitmap v2 on-disk format.
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Documentation/technical/bitmap-format.txt | 235 +++++++++++++++++++++++++++++
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+GIT bitmap v2 format & rationale
+================================
+
+ - A header appears at the beginning, using the same format
+ as JGit's original bitmap indexes.
+
+ 4-byte signature: {'B', 'I', 'T', 'M'}
+
+ 2-byte version number (network byte order)
+ The current implementation only supports version 2
+ of the bitmap index. The rationale for this is explained
+ in this document.
+
+ 2-byte flags (network byte order)
+
+ The folowing flags are supported:
+
+ - BITMAP_OPT_FULL_DAG (0x1) REQUIRED
+ This flag must always be present. It implies that the bitmap
+ index has been generated for a packfile with full closure
+ (i.e. where every single object in the packfile can find
+ its parent links inside the same packfile). This is a
+ requirement for the bitmap index format, also present in JGit,
+ that greatly reduces the complexity of the implementation.
+
+ - BITMAP_OPT_LE_BITMAPS (0x2)
+ If present, this implies that that the EWAH bitmaps in this
+ index has been serialized to disk in little-endian byte order.
+ Note that this only applies to the actual bitmaps, not to the
+ Git data structures in the index, which are always in Network
+ Byte order as it's costumary.
+
+ - BITMAP_OPT_BE_BITMAPS (0x4)
+ If present, this implies that the EWAH bitmaps have been serialized
+ using big-endian byte order (NWO). If the flag is missing, **the
+ default is to assume that the bitmaps are in big-endian**.
+
+ - BITMAP_OPT_HASH_CACHE (0x8)
+ If present, a hash cache for finding delta bases will be available
+ right after the header block in this index. See the following
+ section for details.
+
+ 4-byte entry count (network byte order)
+
+ The total count of entries (bitmapped commits) in this bitmap index.
+
+ 20-byte checksum
+
+ The SHA1 checksum of the pack this bitmap index belongs to.
+
+ - An OPTIONAL delta cache follows the header.
+
+ The cache is formed by `n` 4-byte hashes in a row, where `n` is
+ the amount of objects in the indexed packfile. Note that this amount
+ is the **total number of objects** and is not related to the
+ number of commits that have been selected and indexed in the
+ bitmap index.
+
+ The hashes are stored in Network Byte Order and they are the same
+ values generated by a normal revision walk during the `pack-objects`
+ phase.
+
+ The `n`nth hash in the cache is the name hash for the `n`th object
+ in the index for the indexed packfile.
+
+ [RATIONALE]:
+
+ The bitmap index allows us to skip the Counting Objects phase
+ during `pack-objects` and yield all the OIDs that would be reachable
+ ("WANTS") when generating the pack.
+
+ This optimization, however, means that we're adding objects to the
+ packfile straight from the packfile index, and hence we are lacking
+ path information for the objects that would normally be generated
+ during the "Counting Objects" phase.
+
+ This path information for each object is hashed and used as a very
+ effective way to find good delta bases when compressing the packfile;
+ without these hashes, the resulting packfiles are much less optimal.
+
+ By storing all the hashes in a cache together with the bitmapsin
+ the bitmap index, we can yield not only the SHA1 of all the reachable
+ objects, but also their hashes, and allow Git to be much smarter when
+ finding delta bases for packing.
+
+ If the delta cache is not available, the bitmap index will obviously
+ be smaller in disk, but the packfiles generated using this index will
+ be between 20% and 30% bigger, because of the lack of name/path
+ information when finding delta bases.
+
+ - 4 EWAH bitmaps that act as type indexes
+
+ Type indexes are serialized after the hash cache in the shape
+ of four EWAH bitmaps stored consecutively (see Appendix A for
+ the serialization format of an EWAH bitmap).
+
+ There is a bitmap for each Git object type, stored in the following
+ order:
+
+ - Commits
+ - Trees
+ - Blobs
+ - Tags
+
+ In each bitmap, the `n`th bit is set to true if the `n`th object
+ in the packfile index is of that type.
+
+ The obvious consequence is that the XOR of all 4 bitmaps will result
+ in a full set (all bits sets), and the AND of all 4 bitmaps will
+ result in an empty bitmap (no bits set).
+
+ - N EWAH bitmaps, one for each indexed commit
+
+ Where `N` is the total amount of entries in this bitmap index.
+ See Appendix A for the serialization format of an EWAH bitmap.
+
+ - An entry index with `N` entries for the indexed commits
+
+ Index entries are stored consecutively, and each entry has the
+ following format:
+
+ - 20-byte SHA1
+ The SHA1 of the commit that this bitmap indexes
+
+ - 4-byte offset (Network Byte Order)
+ The offset **from the beginning of the file** where the
+ bitmap for this commit is stored.
+
+ - 1-byte XOR-offset
+ The xor offset used to compress this bitmap. For an entry
+ in position `x`, a XOR offset of `y` means that the actual
+ bitmap representing for this commit is composed by XORing the
+ bitmap for this entry with the bitmap in entry `x-y` (i.e.
+ the bitmap `y` entries before this one).
+
+ Note that this compression can be recursive. In order to
+ XOR this entry with a previous one, the previous entry needs
+ to be decompressed first, and so on.
+
+ The hard-limit for this offset is 160 (an entry can only be
+ xor'ed against one of the 160 entries preceding it). This
+ number is always positivea, and hence entries are always xor'ed
+ with **previous** bitmaps, not bitmaps that will come afterwards
+ in the index.
+
+ - 1-byte flags for this bitmap
+ At the moment the only available flag is `0x1`, which hints
+ that this bitmap can be re-used when rebuilding bitmap indexes
+ for the repository.
+
+ - 2 bytes of RESERVED data (used right now for better packing).
+
+== Rationale for changes from the Bitmap Format v1
+
+- Serialized EWAH bitmaps can be stored in Little-Endian byte order,
+ if defined by the BITMAP_OPT_LE_BITMAPS flag in the header.
+
+ The original JGit implementation stored bitmaps in Big-Endian byte
+ order (NWO) because it was unable to `mmap` the serialized format,
+ and hence always required a full parse of the bitmap index to memory,
+ where the BE->LE conversion could be performed.
+
+ This full parse, however, requires prohibitive loading times in LE
+ machines (i.e. all modern server hardware): a repository like
+ `torvalds/linux` can have about 8mb of bitmap indexes, resulting
+ in roughly 400ms of parse time.
+
+ This is not an issue in JGit, which is capable of serving repositories
+ from a single-process daemon running on the JVM, but `git-daemon` in
+ git has been implemented with a process-based design (a new
+ `pack-objects` is spawned for each request), and the boot times
+ of parsing the bitmap index every time `pack-objects` is spawned can
+ seriously slow down requests (particularly for small fetches, where we'd
+ spend about 1.5s booting up and 300ms performing the Counting Objects
+ phase).
+
+ By storing the bitmaps in Little-Endian, we're able to `mmap` their
+ compressed data straight in memory without parsing it beforehand, and
+ since most queries don't require accessing all the serialized bitmaps,
+ we'll only page in the minimal amount of bitmaps necessary to perform
+ the reachability analysis as they are accessed.
+
+- An index of all the bitmapped commits is written at the end of the packfile,
+ instead of interpersed with the serialized bitmaps in the middle of the
+ file.
+
+ Again, the old design implied a full parse of the whole bitmap index
+ (which JGit can afford because its daemon is single-process), but it made
+ impossible `mmaping` the bitmap index file and accessing only the parts
+ required to actually solve the query.
+
+ With an index at the end of the file, we can load only this index in memory,
+ allowing for very efficient access to all the available bitmaps lazily (we
+ have their offsets in the mmaped file).
+
+- The ordering of the objects in each bitmap has changed from
+ packfile-order (the nth bit in the bitmap is the nth object in the
+ packfile) to index-order (the nth bit in the bitmap is the nth object
+ in the INDEX of the packfile).
+
+ There is not a noticeable performance difference when actually converting
+ from bitmap position to SHA1 and from SHA1 to bitmap position, but when
+ using packfile ordering like JGit does, queries need to go through the
+ reverse index (pack-revindex.c).
+
+ Generating this reverse index at runtime is **not** free (around 900ms
+ generation time for a repository like `torvalds/linux`), and once again,
+ this generation time needs to happen every time `pack-objects` is
+ spawned.
+
+ With index-ordering, the only requirement for SHA1 -> Bitmap conversions
+ is the packfile index, which we essentially load for free.
+
+
+== Appendix A: Serialization format for an EWAH bitmap
+
+Ewah bitmaps are serialized in the protocol as the JAVAEWAH
+library, making them backwards compatible with the JGit
+implementation:
+
+ - 4-byte number of bits of the resulting UNCOMPRESSED bitmap
+
+ - 4-byte number of words of the COMPRESSED bitmap, when stored
+
+ - N x 8-byte words, as specified by the previous field
+
+ This is the actual content of the compressed bitmap.
+
+ - 4-byte position of the current RLW for the compressed
+ bitmap
+
+Note that the byte order for this serialization is not defined by
+default. The byte order for all the content in a serialized EWAH
+bitmap can be known by the byte order flags in the header of the
+bitmap index file.
--
1.7.9.5
Vicent Marti <tanoku@gmail.com>