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authorJunio C Hamano <gitster@pobox.com>2014-02-27 22:01:48 (GMT)
committerJunio C Hamano <gitster@pobox.com>2014-02-27 22:01:48 (GMT)
commit0f9e62e0847c075678a7a5a748567d1e881d16f8 (patch)
tree8ef8989069ae40eef891b0964fc2cb8036a74e48 /Documentation
parent6784fab0ac1f973f22f1d4252f0e513d61be6c6b (diff)
parent6b5b3a27b7faf9d72efec28fa017408daf45cd00 (diff)
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Merge branch 'jk/pack-bitmap'
Borrow the bitmap index into packfiles from JGit to speed up enumeration of objects involved in a commit range without having to fully traverse the history. * jk/pack-bitmap: (26 commits) ewah: unconditionally ntohll ewah data ewah: support platforms that require aligned reads read-cache: use get_be32 instead of hand-rolled ntoh_l block-sha1: factor out get_be and put_be wrappers do not discard revindex when re-preparing packfiles pack-bitmap: implement optional name_hash cache t/perf: add tests for pack bitmaps t: add basic bitmap functionality tests count-objects: recognize .bitmap in garbage-checking repack: consider bitmaps when performing repacks repack: handle optional files created by pack-objects repack: turn exts array into array-of-struct repack: stop using magic number for ARRAY_SIZE(exts) pack-objects: implement bitmap writing rev-list: add bitmap mode to speed up object lists pack-objects: use bitmaps when packing objects pack-objects: split add_object_entry pack-bitmap: add support for bitmap indexes documentation: add documentation for the bitmap format ewah: compressed bitmap implementation ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/config.txt25
-rw-r--r--Documentation/git-repack.txt9
-rw-r--r--Documentation/git-rev-list.txt1
-rw-r--r--Documentation/rev-list-options.txt8
-rw-r--r--Documentation/technical/bitmap-format.txt164
5 files changed, 206 insertions, 1 deletions
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 7eec746..040197b 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1870,6 +1870,31 @@ pack.packSizeLimit::
Common unit suffixes of 'k', 'm', or 'g' are
supported.
+pack.useBitmaps::
+ When true, git will use pack bitmaps (if available) when packing
+ to stdout (e.g., during the server side of a fetch). Defaults to
+ true. You should not generally need to turn this off unless
+ you are debugging pack bitmaps.
+
+pack.writebitmaps::
+ When true, git will write a bitmap index when packing all
+ objects to disk (e.g., when `git repack -a` is run). This
+ index can speed up the "counting objects" phase of subsequent
+ packs created for clones and fetches, at the cost of some disk
+ space and extra time spent on the initial repack. Defaults to
+ false.
+
+pack.writeBitmapHashCache::
+ When true, git will include a "hash cache" section in the bitmap
+ index (if one is written). This cache can be used to feed git's
+ delta heuristics, potentially leading to better deltas between
+ bitmapped and non-bitmapped objects (e.g., when serving a fetch
+ between an older, bitmapped pack and objects that have been
+ pushed since the last gc). The downside is that it consumes 4
+ bytes per object of disk space, and that JGit's bitmap
+ implementation does not understand it, causing it to complain if
+ Git and JGit are used on the same repository. Defaults to false.
+
pager.<cmd>::
If the value is boolean, turns on or off pagination of the
output of a particular Git subcommand when writing to a tty.
diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt
index 509cf73..002cfd5 100644
--- a/Documentation/git-repack.txt
+++ b/Documentation/git-repack.txt
@@ -9,7 +9,7 @@ git-repack - Pack unpacked objects in a repository
SYNOPSIS
--------
[verse]
-'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [--window=<n>] [--depth=<n>]
+'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [--window=<n>] [--depth=<n>]
DESCRIPTION
-----------
@@ -110,6 +110,13 @@ other objects in that pack they already have locally.
The default is unlimited, unless the config variable
`pack.packSizeLimit` is set.
+-b::
+--write-bitmap-index::
+ Write a reachability bitmap index as part of the repack. This
+ only makes sense when used with `-a` or `-A`, as the bitmaps
+ must be able to refer to all reachable objects. This option
+ overrides the setting of `pack.writebitmaps`.
+
Configuration
-------------
diff --git a/Documentation/git-rev-list.txt b/Documentation/git-rev-list.txt
index 045b37b..7a1585d 100644
--- a/Documentation/git-rev-list.txt
+++ b/Documentation/git-rev-list.txt
@@ -55,6 +55,7 @@ SYNOPSIS
[ \--reverse ]
[ \--walk-reflogs ]
[ \--no-walk ] [ \--do-walk ]
+ [ \--use-bitmap-index ]
<commit>... [ \-- <paths>... ]
DESCRIPTION
diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 03533af..9a3da36 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -257,6 +257,14 @@ See also linkgit:git-reflog[1].
Output excluded boundary commits. Boundary commits are
prefixed with `-`.
+ifdef::git-rev-list[]
+--use-bitmap-index::
+
+ Try to speed up the traversal using the pack bitmap index (if
+ one is available). Note that when traversing with `--objects`,
+ trees and blobs will not have their associated path printed.
+endif::git-rev-list[]
+
--
History Simplification
diff --git a/Documentation/technical/bitmap-format.txt b/Documentation/technical/bitmap-format.txt
new file mode 100644
index 0000000..f8c18a0
--- /dev/null
+++ b/Documentation/technical/bitmap-format.txt
@@ -0,0 +1,164 @@
+GIT bitmap v1 format
+====================
+
+ - A header appears at the beginning:
+
+ 4-byte signature: {'B', 'I', 'T', 'M'}
+
+ 2-byte version number (network byte order)
+ The current implementation only supports version 1
+ of the bitmap index (the same one as JGit).
+
+ 2-byte flags (network byte order)
+
+ The following 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_HASH_CACHE (0x4)
+ If present, the end of the bitmap file contains
+ `N` 32-bit name-hash values, one per object in the
+ pack. The format and meaning of the name-hash is
+ described below.
+
+ 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.
+
+ - 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 is of that type.
+
+ The obvious consequence is that the OR of all 4 bitmaps will result
+ in a full set (all bits set), and the AND of all 4 bitmaps will
+ result in an empty bitmap (no bits set).
+
+ - N entries with compressed bitmaps, one for each indexed commit
+
+ Where `N` is the total amount of entries in this bitmap index.
+ Each entry contains the following:
+
+ - 4-byte object position (network byte order)
+ The position **in the index for the packfile** where the
+ bitmap for this commit is found.
+
+ - 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 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 positive, 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.
+
+ - The compressed bitmap itself, see Appendix A.
+
+== Appendix A: Serialization format for an EWAH bitmap
+
+Ewah bitmaps are serialized in the same 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
+
+All words are stored in network byte order for their corresponding
+sizes.
+
+The compressed bitmap is stored in a form of run-length encoding, as
+follows. It consists of a concatenation of an arbitrary number of
+chunks. Each chunk consists of one or more 64-bit words
+
+ H L_1 L_2 L_3 .... L_M
+
+H is called RLW (run length word). It consists of (from lower to higher
+order bits):
+
+ - 1 bit: the repeated bit B
+
+ - 32 bits: repetition count K (unsigned)
+
+ - 31 bits: literal word count M (unsigned)
+
+The bitstream represented by the above chunk is then:
+
+ - K repetitions of B
+
+ - The bits stored in `L_1` through `L_M`. Within a word, bits at
+ lower order come earlier in the stream than those at higher
+ order.
+
+The next word after `L_M` (if any) must again be a RLW, for the next
+chunk. For efficient appending to the bitstream, the EWAH stores a
+pointer to the last RLW in the stream.
+
+
+== Appendix B: Optional Bitmap Sections
+
+These sections may or may not be present in the `.bitmap` file; their
+presence is indicated by the header flags section described above.
+
+Name-hash cache
+---------------
+
+If the BITMAP_OPT_HASH_CACHE flag is set, the end of the bitmap contains
+a cache of 32-bit values, one per object in the pack. The value at
+position `i` is the hash of the pathname at which the `i`th object
+(counting in index order) in the pack can be found. This can be fed
+into the delta heuristics to compare objects with similar pathnames.
+
+The hash algorithm used is:
+
+ hash = 0;
+ while ((c = *name++))
+ if (!isspace(c))
+ hash = (hash >> 2) + (c << 24);
+
+Note that this hashing scheme is tied to the BITMAP_OPT_HASH_CACHE flag.
+If implementations want to choose a different hashing scheme, they are
+free to do so, but MUST allocate a new header flag (because comparing
+hashes made under two different schemes would be pointless).