While investigating a benign Coverity warning on the new pseudo-merge
implementation, I was struggling to understand the (paraphrased) below:
ofs = index_end - 24 - (index->pseudo_merges.nr * sizeof(uint64_t));
for (i = 0; i < index->pseudo_merges.nr; i++) {
index->pseudo_merges.v[i].at = get_be64(ofs);
ofs += sizeof(uint64_t);
}
, in pack-bitmap.c::load_bitmap_header(). Looking at the documentation,
the diagram describing the on-disk format (prior to this patch)
suggested that the optional extended lookup table immediately preceded
the trailing metadata portion.
If that were the case, that would make the above code from
load_bitmap_header() incorrect, as we'd be blindly reading into the
extended offset table.
But later on in the documentation there is a description of the
pseudo-merge position table as immediately preceding the trailing
metadata portion of the extension. And indeed, we do write the position
table in pack-bitmap-write.c:
/* write positions for all pseudo merges */
for (i = 0; i < writer->pseudo_merges_nr; i++)
hashwrite_be64(f, pseudo_merge_ofs[i]);
hashwrite_be32(f, writer->pseudo_merges_nr);
hashwrite_be32(f, kh_size(writer->pseudo_merge_commits));
hashwrite_be64(f, table_start - start);
hashwrite_be64(f, hashfile_total(f) - start + sizeof(uint64_t));
So this is purely a case of the diagram being out of sync with the
textual description and actual implementation of the format
specification.
Add the missing component back to the format diagram to avoid further
confusion in this area.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Prepare to implement pseudo-merge bitmaps over the next several commits
by first describing the serialization format which will store the new
pseudo-merge bitmaps themselves.
This format is implemented as an optional extension within the bitmap v1
format, making it compatible with previous versions of Git, as well as
the original .bitmap implementation within JGit.
The format is described in detail in the patch contents below, but the
high-level description is as follows:
- An array of pseudo-merge bitmaps, each containing a pair of EWAH
bitmaps: one describing the set of pseudo-merge "parents", and
another describing the set of object(s) reachable from those
parents.
- A lookup table to determine which pseudo-merge(s) a given commit
appears in. An optional extended lookup table follows when there is
at least one commit which appears in multiple pseudo-merge groups.
- Trailing metadata, including the number of pseudo-merge(s), number
of unique parents, the offset within the .bitmap file for the
pseudo-merge commit lookup table, and the size of the optional
extension itself.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
When reading bitmap file, Git loads each and every bitmap one by one
even if all the bitmaps are not required. A "bitmap lookup table"
extension to the bitmap format can reduce the overhead of loading
bitmaps which stores a list of bitmapped commit id pos (in the midx
or pack, along with their offset and xor offset. This way Git can
load only the necessary bitmaps without loading the previous bitmaps.
Older versions of Git ignore the lookup table extension and don't
throw any kind of warning or error while parsing the bitmap file.
Add some information for the new "bitmap lookup table" extension in the
bitmap-format documentation.
Mentored-by: Taylor Blau <me@ttaylorr.com>
Co-Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-Authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Bitmap file has a trailing checksum at the end of the file. However
there is no information in the bitmap-format documentation about it.
Add a trailer section to include the trailing checksum info in the
`Documentation/technical/bitmap-format.txt` file.
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The asciidoc generated html for `Documentation/technical/bitmap-
format.txt` is broken. This is mainly because `-` is used for nested
lists (which is not allowed in asciidoc) instead of `*`.
Fix these and also reformat it for better readability of the html page.
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Update the technical documentation to describe the multi-pack bitmap
format. This patch merely introduces the new format, and describes its
high-level ideas. Git does not yet know how to read nor write these
multi-pack variants, and so the subsequent patches will:
- Introduce code to interpret multi-pack bitmaps, according to this
document.
- Then, introduce code to write multi-pack bitmaps from the 'git
multi-pack-index write' sub-command.
Finally, the implementation will gain tests in subsequent patches (as
opposed to inline with the patch teaching Git how to write multi-pack
bitmaps) to avoid a cyclic dependency.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This is the technical documentation for the JGit-compatible Bitmap v1
on-disk format.
Signed-off-by: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Taylor Blau