changelog: disable delta chains
This patch disables delta chains on changelogs. After this patch, new
entries on changelogs - including existing changelogs - will be stored
as the fulltext of that data (likely compressed). No delta computation
will be performed.
An overview of delta chains and data justifying this change follows.
Revlogs try to store entries as a delta against a previous entry (either
a parent revision in the case of generaldelta or the previous physical
revision when not using generaldelta). Most of the time this is the
correct thing to do: it frequently results in less CPU usage and smaller
storage.
Delta chains are most effective when the base revision being deltad
against is similar to the current data. This tends to occur naturally
for manifests and file data, since only small parts of each tend to
change with each revision. Changelogs, however, are a different story.
Changelog entries represent changesets/commits. And unless commits in a
repository are homogonous (same author, changing same files, similar
commit messages, etc), a delta from one entry to the next tends to be
relatively large compared to the size of the entry. This means that
delta chains tend to be short. How short? Here is the full vs delta
revision breakdown on some real world repos:
Repo % Full % Delta Max Length
hg 45.8 54.2 6
mozilla-central 42.4 57.6 8
mozilla-unified 42.5 57.5 17
pypy 46.1 53.9 6
python-zstandard 46.1 53.9 3
(I threw in python-zstandard as an example of a repo that is homogonous.
It contains a small Python project with changes all from the same
author.)
Contrast this with the manifest revlog for these repos, where 99+% of
revisions are deltas and delta chains run into the thousands.
So delta chains aren't as useful on changelogs. But even a short delta
chain may provide benefits. Let's measure that.
Delta chains may require less CPU to read revisions if the CPU time
spent reading smaller deltas is less than the CPU time used to
decompress larger individual entries. We can measure this via
`hg perfrevlog -c -d 1` to iterate a revlog to resolve each revision's
fulltext. Here are the results of that command on a repo using delta
chains in its changelog and on a repo without delta chains:
hg (forward)
! wall 0.407008 comb 0.410000 user 0.410000 sys 0.000000 (best of 25)
! wall 0.390061 comb 0.390000 user 0.390000 sys 0.000000 (best of 26)
hg (reverse)
! wall 0.515221 comb 0.520000 user 0.520000 sys 0.000000 (best of 19)
! wall 0.400018 comb 0.400000 user 0.390000 sys 0.010000 (best of 25)
mozilla-central (forward)
! wall 4.508296 comb 4.490000 user 4.490000 sys 0.000000 (best of 3)
! wall 4.370222 comb 4.370000 user 4.350000 sys 0.020000 (best of 3)
mozilla-central (reverse)
! wall 5.758995 comb 5.760000 user 5.720000 sys 0.040000 (best of 3)
! wall 4.346503 comb 4.340000 user 4.320000 sys 0.020000 (best of 3)
mozilla-unified (forward)
! wall 4.957088 comb 4.950000 user 4.940000 sys 0.010000 (best of 3)
! wall 4.660528 comb 4.650000 user 4.630000 sys 0.020000 (best of 3)
mozilla-unified (reverse)
! wall 6.119827 comb 6.110000 user 6.090000 sys 0.020000 (best of 3)
! wall 4.675136 comb 4.670000 user 4.670000 sys 0.000000 (best of 3)
pypy (forward)
! wall 1.231122 comb 1.240000 user 1.230000 sys 0.010000 (best of 8)
! wall 1.164896 comb 1.160000 user 1.160000 sys 0.000000 (best of 9)
pypy (reverse)
! wall 1.467049 comb 1.460000 user 1.460000 sys 0.000000 (best of 7)
! wall 1.160200 comb 1.170000 user 1.160000 sys 0.010000 (best of 9)
The data clearly shows that it takes less wall and CPU time to resolve
revisions when there are no delta chains in the changelogs, regardless
of the direction of traversal. Furthermore, not using a delta chain
means that fulltext resolution in reverse is as fast as iterating
forward. So not using delta chains on the changelog is a clear CPU win
for reading operations.
An example of a user-visible operation showing this speed-up is revset
evaluation. Here are results for
`hg perfrevset 'author(gps) or author(mpm)'`:
hg
! wall 1.655506 comb 1.660000 user 1.650000 sys 0.010000 (best of 6)
! wall 1.612723 comb 1.610000 user 1.600000 sys 0.010000 (best of 7)
mozilla-central
! wall 17.629826 comb 17.640000 user 17.600000 sys 0.040000 (best of 3)
! wall 17.311033 comb 17.300000 user 17.260000 sys 0.040000 (best of 3)
What about 00changelog.i size?
Repo Delta Chains No Delta Chains
hg 7,033,250 6,976,771
mozilla-central 82,978,748 81,574,623
mozilla-unified 88,112,349 86,702,162
pypy 20,740,699 20,659,741
The data shows that removing delta chains from the changelog makes the
changelog smaller.
Delta chains are also used during changegroup generation. This
operation essentially converts a series of revisions to one large
delta chain. And changegroup generation is smart: if the delta in
the revlog matches what the changegroup is emitting, it will reuse
the delta instead of recalculating it. We can measure the impact
removing changelog delta chains has on changegroup generation via
`hg perfchangegroupchangelog`:
hg
! wall 1.589245 comb 1.590000 user 1.590000 sys 0.000000 (best of 7)
! wall 1.788060 comb 1.790000 user 1.790000 sys 0.000000 (best of 6)
mozilla-central
! wall 17.382585 comb 17.380000 user 17.340000 sys 0.040000 (best of 3)
! wall 20.161357 comb 20.160000 user 20.120000 sys 0.040000 (best of 3)
mozilla-unified
! wall 18.722839 comb 18.720000 user 18.680000 sys 0.040000 (best of 3)
! wall 21.168075 comb 21.170000 user 21.130000 sys 0.040000 (best of 3)
pypy
! wall 4.828317 comb 4.830000 user 4.820000 sys 0.010000 (best of 3)
! wall 5.415455 comb 5.420000 user 5.410000 sys 0.010000 (best of 3)
The data shows eliminating delta chains makes the changelog part of
changegroup generation slower. This is expected since we now have to
compute deltas for revisions where we could recycle the delta before.
It is worth putting this regression into context of overall changegroup
times. Here is the rough total CPU time spent in changegroup generation
for various repos while using delta chains on the changelog:
Repo CPU Time (s) CPU Time w/ compression
hg 4.50 7.05
mozilla-central 111.1 222.0
pypy 28.68 75.5
Before compression, removing delta chains from the changegroup adds
~4.4% overhead to hg changegroup generation, 1.3% to mozilla-central,
and 2.0% to pypy. When you factor in zlib compression, these percentages
are roughly divided by 2.
While the increased CPU usage for changegroup generation is unfortunate,
I think it is acceptable because the percentage is small, server
operators (those likely impacted most by this) have other mechanisms
to mitigate CPU consumption (namely reducing zlib compression level and
pre-generated clone bundles), and because there is room to optimize this
in the future. For example, we could use the nullid as the base revision,
effectively encoding the full revision for each entry in the changegroup.
When doing this, `hg perfchangegroupchangelog` nearly halves:
mozilla-unified
! wall 21.168075 comb 21.170000 user 21.130000 sys 0.040000 (best of 3)
! wall 11.196461 comb 11.200000 user 11.190000 sys 0.010000 (best of 3)
This looks very promising as a future optimization opportunity.
It's worth that the changes in test-acl.t to the changegroup part size.
This is because revision 6 in the changegroup had a delta chain of
length 2 before and after this patch the base revision is nullrev.
When the base revision is nullrev, cg2packer.deltaparent() hardcodes
the *previous* revision from the changegroup as the delta parent.
This caused the delta in the changegroup to switch base revisions,
the delta to change, and the size to change accordingly. While the
size increased in this case, I think sizes will remain the same
on average, as the delta base for changelog revisions doesn't matter
too much (as this patch shows). So, I don't consider this a regression.
test that a commit clears the merge state.
$ hg init repo
$ cd repo
$ echo foo > file1
$ echo foo > file2
$ hg commit -Am 'add files'
adding file1
adding file2
$ echo bar >> file1
$ echo bar >> file2
$ hg commit -Am 'append bar to files'
create a second head with conflicting edits
$ hg up -C 0
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo baz >> file1
$ echo baz >> file2
$ hg commit -Am 'append baz to files'
created new head
create a third head with no conflicting edits
$ hg up -qC 0
$ echo foo > file3
$ hg commit -Am 'add non-conflicting file'
adding file3
created new head
failing merge
$ hg up -qC 2
$ hg merge --tool=internal:fail 1
0 files updated, 0 files merged, 0 files removed, 2 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
[1]
resolve -l should contain unresolved entries
$ hg resolve -l
U file1
U file2
$ hg resolve -l --no-status
file1
file2
resolving an unknown path should emit a warning, but not for -l
$ hg resolve -m does-not-exist
arguments do not match paths that need resolving
$ hg resolve -l does-not-exist
tell users how they could have used resolve
$ mkdir nested
$ cd nested
$ hg resolve -m file1
arguments do not match paths that need resolving
(try: hg resolve -m path:file1)
$ hg resolve -m file1 filez
arguments do not match paths that need resolving
(try: hg resolve -m path:file1 path:filez)
$ hg resolve -m path:file1 path:filez
$ hg resolve -l
R file1
U file2
$ hg resolve -m filez file2
arguments do not match paths that need resolving
(try: hg resolve -m path:filez path:file2)
$ hg resolve -m path:filez path:file2
(no more unresolved files)
$ hg resolve -l
R file1
R file2
cleanup
$ hg resolve -u
$ cd ..
$ rmdir nested
don't allow marking or unmarking driver-resolved files
$ cat > $TESTTMP/markdriver.py << EOF
> '''mark and unmark files as driver-resolved'''
> from mercurial import cmdutil, merge, scmutil
> cmdtable = {}
> command = cmdutil.command(cmdtable)
> @command('markdriver',
> [('u', 'unmark', None, '')],
> 'FILE...')
> def markdriver(ui, repo, *pats, **opts):
> wlock = repo.wlock()
> try:
> ms = merge.mergestate.read(repo)
> m = scmutil.match(repo[None], pats, opts)
> for f in ms:
> if not m(f):
> continue
> if not opts['unmark']:
> ms.mark(f, 'd')
> else:
> ms.mark(f, 'u')
> ms.commit()
> finally:
> wlock.release()
> EOF
$ hg --config extensions.markdriver=$TESTTMP/markdriver.py markdriver file1
$ hg resolve --list
D file1
U file2
$ hg resolve --mark file1
not marking file1 as it is driver-resolved
this should not print out file1
$ hg resolve --mark --all
(no more unresolved files -- run "hg resolve --all" to conclude)
$ hg resolve --mark 'glob:file*'
(no more unresolved files -- run "hg resolve --all" to conclude)
$ hg resolve --list
D file1
R file2
$ hg resolve --unmark file1
not unmarking file1 as it is driver-resolved
(no more unresolved files -- run "hg resolve --all" to conclude)
$ hg resolve --unmark --all
$ hg resolve --list
D file1
U file2
$ hg --config extensions.markdriver=$TESTTMP/markdriver.py markdriver --unmark file1
$ hg resolve --list
U file1
U file2
resolve the failure
$ echo resolved > file1
$ hg resolve -m file1
resolve -l should show resolved file as resolved
$ hg resolve -l
R file1
U file2
$ hg resolve -l -Tjson
[
{
"path": "file1",
"status": "R"
},
{
"path": "file2",
"status": "U"
}
]
resolve -m without paths should mark all resolved
$ hg resolve -m
(no more unresolved files)
$ hg commit -m 'resolved'
resolve -l should be empty after commit
$ hg resolve -l
$ hg resolve -l -Tjson
[
]
resolve --all should abort when no merge in progress
$ hg resolve --all
abort: resolve command not applicable when not merging
[255]
resolve -m should abort when no merge in progress
$ hg resolve -m
abort: resolve command not applicable when not merging
[255]
can not update or merge when there are unresolved conflicts
$ hg up -qC 0
$ echo quux >> file1
$ hg up 1
merging file1
warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
1 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
[1]
$ hg up 0
abort: outstanding merge conflicts
[255]
$ hg merge 2
abort: outstanding merge conflicts
[255]
$ hg merge --force 2
abort: outstanding merge conflicts
[255]
set up conflict-free merge
$ hg up -qC 3
$ hg merge 1
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
resolve --all should do nothing in merge without conflicts
$ hg resolve --all
(no more unresolved files)
resolve -m should do nothing in merge without conflicts
$ hg resolve -m
(no more unresolved files)
get back to conflicting state
$ hg up -qC 2
$ hg merge --tool=internal:fail 1
0 files updated, 0 files merged, 0 files removed, 2 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
[1]
resolve without arguments should suggest --all
$ hg resolve
abort: no files or directories specified
(use --all to re-merge all unresolved files)
[255]
resolve --all should re-merge all unresolved files
$ hg resolve --all
merging file1
merging file2
warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
warning: conflicts while merging file2! (edit, then use 'hg resolve --mark')
[1]
$ cat file1.orig
foo
baz
$ cat file2.orig
foo
baz
.orig files should exists where specified
$ hg resolve --all --verbose --config 'ui.origbackuppath=.hg/origbackups'
merging file1
creating directory: $TESTTMP/repo/.hg/origbackups (glob)
merging file2
warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
warning: conflicts while merging file2! (edit, then use 'hg resolve --mark')
[1]
$ ls .hg/origbackups
file1.orig
file2.orig
$ grep '<<<' file1 > /dev/null
$ grep '<<<' file2 > /dev/null
resolve <file> should re-merge file
$ echo resolved > file1
$ hg resolve -q file1
warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
[1]
$ grep '<<<' file1 > /dev/null
test .orig behavior with resolve
$ hg resolve -q file1 --tool "sh -c 'f --dump \"$TESTTMP/repo/file1.orig\"'"
$TESTTMP/repo/file1.orig: (glob)
>>>
foo
baz
<<<
resolve <file> should do nothing if 'file' was marked resolved
$ echo resolved > file1
$ hg resolve -m file1
$ hg resolve -q file1
$ cat file1
resolved
insert unsupported advisory merge record
$ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -x
$ hg debugmergestate
* version 2 records
local: 57653b9f834a4493f7240b0681efcb9ae7cab745
other: dc77451844e37f03f5c559e3b8529b2b48d381d1
labels:
local: working copy
other: merge rev
unrecognized entry: x advisory record
file extras: file1 (ancestorlinknode = 99726c03216e233810a2564cbc0adfe395007eac)
file: file1 (record type "F", state "r", hash 60b27f004e454aca81b0480209cce5081ec52390)
local path: file1 (flags "")
ancestor path: file1 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
other path: file1 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
file extras: file2 (ancestorlinknode = 99726c03216e233810a2564cbc0adfe395007eac)
file: file2 (record type "F", state "u", hash cb99b709a1978bd205ab9dfd4c5aaa1fc91c7523)
local path: file2 (flags "")
ancestor path: file2 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
other path: file2 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
$ hg resolve -l
R file1
U file2
insert unsupported mandatory merge record
$ hg --config extensions.fakemergerecord=$TESTDIR/fakemergerecord.py fakemergerecord -X
$ hg debugmergestate
* version 2 records
local: 57653b9f834a4493f7240b0681efcb9ae7cab745
other: dc77451844e37f03f5c559e3b8529b2b48d381d1
labels:
local: working copy
other: merge rev
file extras: file1 (ancestorlinknode = 99726c03216e233810a2564cbc0adfe395007eac)
file: file1 (record type "F", state "r", hash 60b27f004e454aca81b0480209cce5081ec52390)
local path: file1 (flags "")
ancestor path: file1 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
other path: file1 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
file extras: file2 (ancestorlinknode = 99726c03216e233810a2564cbc0adfe395007eac)
file: file2 (record type "F", state "u", hash cb99b709a1978bd205ab9dfd4c5aaa1fc91c7523)
local path: file2 (flags "")
ancestor path: file2 (node 2ed2a3912a0b24502043eae84ee4b279c18b90dd)
other path: file2 (node 6f4310b00b9a147241b071a60c28a650827fb03d)
unrecognized entry: X mandatory record
$ hg resolve -l
abort: unsupported merge state records: X
(see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
[255]
$ hg resolve -ma
abort: unsupported merge state records: X
(see https://mercurial-scm.org/wiki/MergeStateRecords for more information)
[255]
$ hg summary
warning: merge state has unsupported record types: X
parent: 2:57653b9f834a
append baz to files
parent: 1:dc77451844e3
append bar to files
branch: default
commit: 2 modified, 2 unknown (merge)
update: 2 new changesets (update)
phases: 5 draft
update --clean shouldn't abort on unsupported records
$ hg up -qC 1
$ hg debugmergestate
no merge state found
test crashed merge with empty mergestate
$ mkdir .hg/merge
$ touch .hg/merge/state
resolve -l should be empty
$ hg resolve -l
$ cd ..