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.
Show all commands except debug commands
$ hg debugcomplete
add
addremove
annotate
archive
backout
bisect
bookmarks
branch
branches
bundle
cat
clone
commit
config
copy
diff
export
files
forget
graft
grep
heads
help
identify
import
incoming
init
locate
log
manifest
merge
outgoing
parents
paths
phase
pull
push
recover
remove
rename
resolve
revert
rollback
root
serve
status
summary
tag
tags
tip
unbundle
update
verify
version
Show all commands that start with "a"
$ hg debugcomplete a
add
addremove
annotate
archive
Do not show debug commands if there are other candidates
$ hg debugcomplete d
diff
Show debug commands if there are no other candidates
$ hg debugcomplete debug
debugancestor
debugapplystreamclonebundle
debugbuilddag
debugbundle
debugcheckstate
debugcommands
debugcomplete
debugconfig
debugcreatestreamclonebundle
debugdag
debugdata
debugdate
debugdeltachain
debugdirstate
debugdiscovery
debugextensions
debugfileset
debugfsinfo
debuggetbundle
debugignore
debugindex
debugindexdot
debuginstall
debugknown
debuglabelcomplete
debuglocks
debugmergestate
debugnamecomplete
debugobsolete
debugpathcomplete
debugpushkey
debugpvec
debugrebuilddirstate
debugrebuildfncache
debugrename
debugrevlog
debugrevspec
debugsetparents
debugsub
debugsuccessorssets
debugtemplate
debugwalk
debugwireargs
Do not show the alias of a debug command if there are other candidates
(this should hide rawcommit)
$ hg debugcomplete r
recover
remove
rename
resolve
revert
rollback
root
Show the alias of a debug command if there are no other candidates
$ hg debugcomplete rawc
Show the global options
$ hg debugcomplete --options | sort
--config
--cwd
--debug
--debugger
--encoding
--encodingmode
--help
--hidden
--noninteractive
--profile
--quiet
--repository
--time
--traceback
--verbose
--version
-R
-h
-q
-v
-y
Show the options for the "serve" command
$ hg debugcomplete --options serve | sort
--accesslog
--address
--certificate
--cmdserver
--config
--cwd
--daemon
--daemon-postexec
--debug
--debugger
--encoding
--encodingmode
--errorlog
--help
--hidden
--ipv6
--name
--noninteractive
--pid-file
--port
--prefix
--profile
--quiet
--repository
--stdio
--style
--templates
--time
--traceback
--verbose
--version
--web-conf
-6
-A
-E
-R
-a
-d
-h
-n
-p
-q
-t
-v
-y
Show an error if we use --options with an ambiguous abbreviation
$ hg debugcomplete --options s
hg: command 's' is ambiguous:
serve showconfig status summary
[255]
Show all commands + options
$ hg debugcommands
add: include, exclude, subrepos, dry-run
annotate: rev, follow, no-follow, text, user, file, date, number, changeset, line-number, ignore-all-space, ignore-space-change, ignore-blank-lines, include, exclude, template
clone: noupdate, updaterev, rev, branch, pull, uncompressed, ssh, remotecmd, insecure
commit: addremove, close-branch, amend, secret, edit, interactive, include, exclude, message, logfile, date, user, subrepos
diff: rev, change, text, git, nodates, noprefix, show-function, reverse, ignore-all-space, ignore-space-change, ignore-blank-lines, unified, stat, root, include, exclude, subrepos
export: output, switch-parent, rev, text, git, nodates
forget: include, exclude
init: ssh, remotecmd, insecure
log: follow, follow-first, date, copies, keyword, rev, removed, only-merges, user, only-branch, branch, prune, patch, git, limit, no-merges, stat, graph, style, template, include, exclude
merge: force, rev, preview, tool
pull: update, force, rev, bookmark, branch, ssh, remotecmd, insecure
push: force, rev, bookmark, branch, new-branch, ssh, remotecmd, insecure
remove: after, force, subrepos, include, exclude
serve: accesslog, daemon, daemon-postexec, errorlog, port, address, prefix, name, web-conf, webdir-conf, pid-file, stdio, cmdserver, templates, style, ipv6, certificate
status: all, modified, added, removed, deleted, clean, unknown, ignored, no-status, copies, print0, rev, change, include, exclude, subrepos, template
summary: remote
update: clean, check, date, rev, tool
addremove: similarity, subrepos, include, exclude, dry-run
archive: no-decode, prefix, rev, type, subrepos, include, exclude
backout: merge, commit, no-commit, parent, rev, edit, tool, include, exclude, message, logfile, date, user
bisect: reset, good, bad, skip, extend, command, noupdate
bookmarks: force, rev, delete, rename, inactive, template
branch: force, clean
branches: active, closed, template
bundle: force, rev, branch, base, all, type, ssh, remotecmd, insecure
cat: output, rev, decode, include, exclude
config: untrusted, edit, local, global, template
copy: after, force, include, exclude, dry-run
debugancestor:
debugapplystreamclonebundle:
debugbuilddag: mergeable-file, overwritten-file, new-file
debugbundle: all, spec
debugcheckstate:
debugcommands:
debugcomplete: options
debugcreatestreamclonebundle:
debugdag: tags, branches, dots, spaces
debugdata: changelog, manifest, dir
debugdate: extended
debugdeltachain: changelog, manifest, dir, template
debugdirstate: nodates, datesort
debugdiscovery: old, nonheads, ssh, remotecmd, insecure
debugextensions: template
debugfileset: rev
debugfsinfo:
debuggetbundle: head, common, type
debugignore:
debugindex: changelog, manifest, dir, format
debugindexdot: changelog, manifest, dir
debuginstall: template
debugknown:
debuglabelcomplete:
debuglocks: force-lock, force-wlock
debugmergestate:
debugnamecomplete:
debugobsolete: flags, record-parents, rev, index, delete, date, user, template
debugpathcomplete: full, normal, added, removed
debugpushkey:
debugpvec:
debugrebuilddirstate: rev, minimal
debugrebuildfncache:
debugrename: rev
debugrevlog: changelog, manifest, dir, dump
debugrevspec: optimize, show-stage, no-optimized, verify-optimized
debugsetparents:
debugsub: rev
debugsuccessorssets:
debugtemplate: rev, define
debugwalk: include, exclude
debugwireargs: three, four, five, ssh, remotecmd, insecure
files: rev, print0, include, exclude, template, subrepos
graft: rev, continue, edit, log, force, currentdate, currentuser, date, user, tool, dry-run
grep: print0, all, text, follow, ignore-case, files-with-matches, line-number, rev, user, date, template, include, exclude
heads: rev, topo, active, closed, style, template
help: extension, command, keyword, system
identify: rev, num, id, branch, tags, bookmarks, ssh, remotecmd, insecure
import: strip, base, edit, force, no-commit, bypass, partial, exact, prefix, import-branch, message, logfile, date, user, similarity
incoming: force, newest-first, bundle, rev, bookmarks, branch, patch, git, limit, no-merges, stat, graph, style, template, ssh, remotecmd, insecure, subrepos
locate: rev, print0, fullpath, include, exclude
manifest: rev, all, template
outgoing: force, rev, newest-first, bookmarks, branch, patch, git, limit, no-merges, stat, graph, style, template, ssh, remotecmd, insecure, subrepos
parents: rev, style, template
paths: template
phase: public, draft, secret, force, rev
recover:
rename: after, force, include, exclude, dry-run
resolve: all, list, mark, unmark, no-status, tool, include, exclude, template
revert: all, date, rev, no-backup, interactive, include, exclude, dry-run
rollback: dry-run, force
root:
tag: force, local, rev, remove, edit, message, date, user
tags: template
tip: patch, git, style, template
unbundle: update
verify:
version: template
$ hg init a
$ cd a
$ echo fee > fee
$ hg ci -q -Amfee
$ hg tag fee
$ mkdir fie
$ echo dead > fie/dead
$ echo live > fie/live
$ hg bookmark fo
$ hg branch -q fie
$ hg ci -q -Amfie
$ echo fo > fo
$ hg branch -qf default
$ hg ci -q -Amfo
$ echo Fum > Fum
$ hg ci -q -AmFum
$ hg bookmark Fum
Test debugpathcomplete
$ hg debugpathcomplete f
fee
fie
fo
$ hg debugpathcomplete -f f
fee
fie/dead
fie/live
fo
$ hg rm Fum
$ hg debugpathcomplete -r F
Fum
Test debugnamecomplete
$ hg debugnamecomplete
Fum
default
fee
fie
fo
tip
$ hg debugnamecomplete f
fee
fie
fo