sparse-revlog: implement algorithm to write sparse delta chains (
issue5480)
The classic behavior of revlog._isgooddeltainfo is to consider the span size
of the whole delta chain, and limit it to 4 * textlen.
Once sparse-revlog writing is allowed (and enforced with a requirement),
revlog._isgooddeltainfo considers the span of the largest chunk as the
distance used in the verification, instead of using the span of the whole
delta chain.
In order to compute the span of the largest chunk, we need to slice into
chunks a chain with the new revision at the top of the revlog, and take the
maximal span of these chunks. The sparse read density is a parameter to the
slicing, as it will stop when the global read density reaches this threshold.
For instance, a density of 50% means that 2 of 4 read bytes are actually used
for the reconstruction of the revision (the others are part of other chains).
This allows a new revision to be potentially stored with a diff against
another revision anywhere in the history, instead of forcing it in the last 4
* textlen. The result is a much better compression on repositories that have
many concurrent branches. Here are a comparison between using deltas from
current upstream (aggressive-merge-deltas on by default) and deltas from a
sparse-revlog
Comparison of `.hg/store/` size:
mercurial (6.74% merges):
before: 46,831,873 bytes
after: 46,795,992 bytes (no relevant change)
pypy (8.30% merges):
before: 333,524,651 bytes
after: 308,417,511 bytes -8%
netbeans (34.21% merges):
before: 1,141,847,554 bytes
after: 1,131,093,161 bytes -1%
mozilla-central (4.84% merges):
before: 2,344,248,850 bytes
after: 2,328,459,258 bytes -1%
large-private-repo-A (merge 19.73%)
before: 41,510,550,163 bytes
after: 8,121,763,428 bytes -80%
large-private-repo-B (23.77%)
before: 58,702,221,709 bytes
after: 8,351,588,828 bytes -76%
Comparison of `00manifest.d` size:
mercurial (6.74% merges):
before: 6,143,044 bytes
after: 6,107,163 bytes
pypy (8.30% merges):
before: 52,941,780 bytes
after: 27,834,082 bytes -48%
netbeans (34.21% merges):
before: 130,088,982 bytes
after: 119,337,636 bytes -10%
mozilla-central (4.84% merges):
before: 215,096,339 bytes
after: 199,496,863 bytes -8%
large-private-repo-A (merge 19.73%)
before: 33,725,285,081 bytes
after: 390,302,545 bytes -99%
large-private-repo-B (23.77%)
before: 49,457,701,645 bytes
after: 1,366,752,187 bytes -97%
The better delta chains provide a performance boost in relevant repositories:
pypy, bundling 1000 revisions:
before: 1.670s
after: 1.149s -31%
Unbundling got a bit slower. probably because the sparse algorithm is still
pure
python.
pypy, unbundling 1000 revisions:
before: 4.062s
after: 4.507s +10%
Performance of bundle/unbundle in repository with few concurrent branches (eg:
mercurial) are unaffected.
No significant differences have been noticed then timing `hg push` and `hg
pull` locally. More state timings are being gathered.
Same as for aggressive-merge-delta, better delta comes with longer delta
chains. Longer chains have a performance impact. For example. The length of
the chain needed to get the manifest of pypy's tip moves from 82 item to 1929
items. This moves the restore time from 3.88ms to 11.3ms.
Delta chain length is an independent issue that affects repository without
this changes. It will be dealt with independently.
No significant differences have been observed on repositories where
`sparse-revlog` have not much effect (mercurial, unity, netbeans). On pypy,
small differences have been observed on some operation affected by delta chain
building and retrieval.
pypy, perfmanifest
before: 0.006162s
after: 0.017899s +190%
pypy, commit:
before: 0.382
after: 0.376 -1%
pypy, status:
before: 0.157
after: 0.168 +7%
More comprehensive and stable timing comparisons are in progress.
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# same user, different group
trusted
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untrusted
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# different user, same group
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# different user, same group, but we trust the group
trusted
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# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
trusted
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untrusted
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# different user, different group, but we trust the user
trusted
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# different user, different group, but we trust the user and the group
trusted
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# we trust all users
# different user, different group
trusted
global = /some/path
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untrusted
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# we trust all groups
# different user, different group
trusted
global = /some/path
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untrusted
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# we trust all users and groups
# different user, different group
trusted
global = /some/path
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untrusted
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# we don't get confused by users and groups with the same name
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
trusted
global = /some/path
untrusted
. . global = /some/path
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# list of user names
# different user, different group, but we trust the user
trusted
global = /some/path
local = /another/path
untrusted
. . global = /some/path
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# list of group names
# different user, different group, but we trust the group
trusted
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untrusted
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# Can't figure out the name of the user running this process
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
trusted
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untrusted
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# prints debug warnings
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
trusted
ignoring untrusted configuration option paths.local = /another/path
global = /some/path
untrusted
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. ignoring untrusted configuration option paths.local = /another/path
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# report_untrusted enabled without debug hides warnings
# different user, different group
trusted
global = /some/path
untrusted
. . global = /some/path
. . local = /another/path
# report_untrusted enabled with debug shows warnings
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
trusted
ignoring untrusted configuration option paths.local = /another/path
global = /some/path
untrusted
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# ui.readconfig sections
quux
# read trusted, untrusted, new ui, trusted
not trusting file foobar from untrusted user abc, group def
trusted:
ignoring untrusted configuration option foobar.baz = quux
None
untrusted:
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# error handling
# file doesn't exist
# same user, same group
# different user, different group
# parse error
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
('foo', '.hg/hgrc:1')
# same user, same group
('foo', '.hg/hgrc:1')
# access typed information
# different user, different group
not trusting file .hg/hgrc from untrusted user abc, group def
# suboptions, trusted and untrusted
(None, []) ('main', [('one', 'one'), ('two', 'two')])
# path, trusted and untrusted
None .hg/monty/python
# bool, trusted and untrusted
False True
# int, trusted and untrusted
0 42
# bytes, trusted and untrusted
0 84934656
# list, trusted and untrusted
[] ['spam', 'ham', 'eggs']