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Git9 was sloppy about telling git what commits we have.
We would list the commits at the tip of the branch, but not
walk down it, which means we would request too much data if
our local branches were ahead of the remote.
This patch changes that, sending the tips *and* the first
256 commits after them, so that git can produce a better
pack for us, with fewer redundant commits.
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Inspired by some changes made in game of trees, I've
implemented a number of speedups in git9.
First, hashing the chunks during deltification with
murmurhash instead of sha1 speeds up the delta search
significantly.
The stretch function was micro-optimized a bit as well,
since that was taking a large portion of the time when
chunking.
Finally, the full path is not stored. We only care about
grouping files with the same name and path. We don't care
about the ordering. Therefore, only the hash of the path
xored with the hash of the diretory is kept, which saves
a bunch of mallocs and string munging.
This reduces the time spent repacking some test repos
significantly.
9front:
% time git/repack
deltifying 97473 objects: 100%
writing 97473 objects: 100%
indexing 97473 objects: 100%
58.85u 1.39s 61.82r git/repack
% time /sys/src/cmd/git/6.repack
deltifying 97473 objects: 100%
writing 97473 objects: 100%
indexing 97473 objects: 100%
43.86u 1.29s 47.51r /sys/src/cmd/git/6.repack
openbsd:
% time git/repack
deltifying 2092325 objects: 100%
writing 2092325 objects: 100%
indexing 2092325 objects: 100%
1589.48u 45.03s 1729.18r git/repack
% time /sys/src/cmd/git/6.repack
deltifying 2092325 objects: 100%
writing 2092325 objects: 100%
indexing 2092325 objects: 100%
1238.68u 41.49s 1373.15r /sys/src/cmd/git/6.repack
go:
% time git/repack
deltifying 529507 objects: 100%
writing 529507 objects: 100%
indexing 529507 objects: 100%
345.32u 7.71s 369.25r git/repack
% time /sys/src/cmd/git/6.repack
deltifying 529507 objects: 100%
writing 529507 objects: 100%
indexing 529507 objects: 100%
248.07u 4.47s 257.59r /sys/src/cmd/git/6.repack
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Although git9 always uses the same commit date and author date, other
implementation do make a distinction. Since commit date is more
representative of the commit graph order, use this as a traversal hint
instead of author date.
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We now keep track of 3 sets during traversal:
- keep: commits we've reached from head commits
- drop: commits we've reached from tail commits
- skip: ancestors of commits in both 'keep' and 'drop'
Commits in 'keep' and/or 'drop' may be added later to the 'skip' set
if we discover later that they are part of a common subgraph of the
head and tail commits.
From these sets we can calculate the commits we are interested in:
lca commits are those in 'keep' and 'drop', but not in 'skip'.
findtwixt commits are those in 'keep', but not in 'drop' or 'skip'.
The "LCA" commit returned is a common ancestor such that there are no
other common ancestors that can reach that commit. Although there can
be multiple commits that meet this criteria, where one is technically
lower on the commit-graph than the other, these cases only happen in
complex merge arrangements and any choice is likely a decent merge
base.
Repainting is now done in paint() directly. When we find a boundary
commit, we switch our paint color to 'skip'. 'skip' painting does
not stop when it hits another color; we continue until we are left
with only 'skip' commits on the queue.
This fixes several mishandled cases in the current algorithm:
1. If we hit the common subgraph from tail commits first (if the tail
commit was newer than the head commit), we ended up traversing the
entire commit graph. This is because we couldn't distinguish
between 'drop' commits that were part of the common subgraph, and
those that were still looking for it.
2. If we traversed through an initial part of the common subgraph from
head commits before reaching it from tail commits, these commits
were returned from findtwixt even though they were also reachable
from tail commits.
3. In the same case as 2, we might end up choosing an incorrect
commit as the LCA, which is an ancestor of the real LCA.
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git used to track cache size in object
count, rather than bytes. This had the
unfortunate effect of making memory use
depend on the size of objects -- repos
with lots of large objects could cause
out of memory deaths.
now, we track sizes in bytes, which should
keep our memory usage flatter.
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Due to the way LCA is defined, a using a strict LCA
on a graph like this:
<--a--b--c--d--e--f--g
\ /
+-----h-------
can lead to spurious requests to merge. This happens
because 'lca(b, g)' would return 'a', since it can be
reached in one step from 'b', and 2 steps from 'g', while
reaching 'b' from 'a' would be a longer path.
As a result, we need to implement an lca variant that
returns the starting node if one is reachable from the
other, even if it's already found the technically correct
least common ancestor.
This replaces our LCA algorithm with one based on the
painting we do while finding a twixt, making it give
the resutls we want.
git/query: fix spurious merge requests
Due to the way LCA is defined, a using a strict LCA
on a graph like this:
<--a--b--c--d--e--f--g
\ /
+-----h-------
can lead to spurious requests to merge. This happens
because 'lca(b, g)' would return 'a', since it can be
reached in one step from 'b', and 2 steps from 'g', while
reaching 'b' from 'a' would be a longer path.
As a result, we need to implement an lca variant that
returns the starting node if one is reachable from the
other, even if it's already found the technically correct
least common ancestor.
This replaces our LCA algorithm with one based on the
painting we do while finding a twixt.
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Add a snapshot of git9 to 9front.
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