revlog: change generaldelta delta parent heuristic
The old generaldelta heuristic was "if p1 (or p2) was closer than the last full text,
use it, otherwise use prev". This was problematic when a repo contained multiple
branches that were very different. If commits to branch A were pushed, and the
last full text was branch B, it would generate a fulltext. Then if branch B was
pushed, it would generate another fulltext. The problem is that the last
fulltext (and delta'ing against `prev` in general) has no correlation with the
contents of the incoming revision, and therefore will always have degenerate
cases.
According to the blame, that algorithm was chosen to minimize the chain length.
Since there is already code that protects against that (the delta-vs-fulltext
code), and since it has been improved since the original generaldelta algorithm
went in (2011), I believe the chain length criteria will still be preserved.
The new algorithm always diffs against p1 (or p2 if it's closer), unless the
resulting delta will fail the delta-vs-fulltext check, in which case we delta
against prev.
Some before and after stats on manifest.d size.
internal large repo
old heuristic - 2.0 GB
new heuristic - 1.2 GB
mozilla-central
old heuristic - 242 MB
new heuristic - 261 MB
The regression in mozilla central is due to the new heuristic choosing p2r as
the delta when it's closer to the tip. Switching the algorithm to always prefer
p1r brings the size back down (242 MB). This is result of the way in which
mozilla does merges and pushes, and the result could easily swing the other
direction in other repos (depending on if they merge X into Y or Y into X), but
will never be as degenerate as before.
I future patch will address the regression by introducing an optional, even more
aggressive delta heuristic which will knock the mozilla manifest size down
dramatically.
Create repo with old manifest
$ hg init existing
$ cd existing
$ echo footext > foo
$ hg add foo
$ hg commit -m initial
We're using v1, so no manifestv2 entry is in requires yet.
$ grep manifestv2 .hg/requires
[1]
Let's clone this with manifestv2 enabled to switch to the new format for
future commits.
$ cd ..
$ hg clone --pull existing new --config experimental.manifestv2=1
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd new
Check that entry was added to .hg/requires.
$ grep manifestv2 .hg/requires
manifestv2
Make a new commit.
$ echo newfootext > foo
$ hg commit -m new
Check that the manifest actually switched to v2.
$ hg debugdata -m 0
foo\x0021e958b1dca695a60ee2e9cf151753204ee0f9e9 (esc)
$ hg debugdata -m 1
\x00 (esc)
\x00foo\x00 (esc)
I\xab\x7f\xb8(\x83\xcas\x15\x9d\xc2\xd3\xd3:5\x08\xbad5_ (esc)
Check that manifestv2 is used if the requirement is present, even if it's
disabled in the config.
$ echo newerfootext > foo
$ hg --config experimental.manifestv2=False commit -m newer
$ hg debugdata -m 2
\x00 (esc)
\x00foo\x00 (esc)
\xa6\xb1\xfb\xef]\x91\xa1\x19`\xf3.#\x90S\xf8\x06 \xe2\x19\x00 (esc)
Check that we can still read v1 manifests.
$ hg files -r 0
foo
$ cd ..
Check that entry is added to .hg/requires on repo creation
$ hg --config experimental.manifestv2=True init repo
$ cd repo
$ grep manifestv2 .hg/requires
manifestv2
Set up simple repo
$ echo a > file1
$ echo b > file2
$ echo c > file3
$ hg ci -Aqm 'initial'
$ echo d > file2
$ hg ci -m 'modify file2'
Check that 'hg verify', which uses manifest.readdelta(), works
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
3 files, 2 changesets, 4 total revisions
Check that manifest revlog is smaller than for v1
$ hg debugindex -m
rev offset length base linkrev nodeid p1 p2
0 0 81 0 0 57361477c778 000000000000 000000000000
1 81 33 0 1 aeaab5a2ef74 57361477c778 000000000000