changegroup: remove reordering control (BC)
This logic - including the experimental bundle.reorder option -
was originally added in
a8e3931e3fb5 in 2011 and then later ported
to changegroup.py.
The intent of this option and associated logic is to control
the ordering of revisions in deltagroups in changegroups. At the
time it was implemented, only changegroup version 1 existed
and generaldelta revlogs were just coming into the world. Changegroup
version 1 requires that deltas be made against the last revision
sent over the wire. Used with generaldelta, this created an
impedance mismatch of sorts and resulted in changegroup producers
spending a lot of time recomputing deltas.
Revision reordering was introduced so outgoing revisions would be
sent in "generaldelta order" and producers would be able to
reuse internal deltas from storage.
Later on, we introduced changegroup version 2. It supported denoting
which revision a delta was against. So we no longer needed to
sort outgoing revisions to ensure optimal delta generation from the
producer. So, subsequent changegroup versions disabled reordering.
We also later made the changelog not store deltas by default. And
we also made the changelog send out deltas in storage order. Why we
do this for changelog, I'm not sure. Maybe we want to preserve revision
order across clones? It doesn't really matter for this commit.
Fast forward to 2018. We want to abstract storage backends. And having
changegroup code require knowledge about how deltas are stored
internally interferes with that goal.
This commit removes reordering control from changegroup generation.
After this commit, the reordering behavior is:
* The changelog is always sent out in storage order (no behavior
change).
* Non-changelog generaldelta revlogs are reordered to always be in DAG
topological order (previously, generaldelta revlogs would be emitted
in storage order for version 2 and 3 changegroups).
* Non-changelog non-generaldelta revlogs are sent in storage order (no
behavior change).
* There exists no config option to override behavior.
The big difference here is that generaldelta revlogs now *always* have
their revisions sorted in DAG order before going out over the wire. This
behavior was previously only done for changegroup version 1. Version 2
and version 3 changegroups disabled reordering because the interchange
format supported encoding arbitrary delta parents, so reordering wasn't
strictly necessary.
I can think of a few significant implications for this change.
Because changegroup receivers will now see non-changelog revisions
in DAG order instead of storage order, the internal storage order of
manifests and files may differ substantially between producer and
consumer. I don't think this matters that much, since the storage
order of manifests and files is largely hidden from users. Only
the storage order of changelog matters (because `hg log` shows the
changelog in storage order). I don't think there should be any
controversy here.
The reordering of revisions has implications for changegroup producers.
Previously, generaldelta revlogs would be emitted in storage order.
And in the common case, the internally-stored delta could effectively
be copied from disk into the deltagroup delta. This meant that emitting
delta groups for generaldelta revlogs would be mostly linear read I/O.
This is desirable for performance. With us now reordering generaldelta
revlog revisions in DAG order, the read operations may use more random
I/O instead of sequential I/O. This could result in performance
loss. But with the prevalence of SSDs and fast random I/O, I'm not
too worried. (Note: the optimal emission order for revlogs is actually
delta encoding order. But the changegroup code wasn't doing that before
or after this change. We could potentially implement that in a later
commit.)
Changegroups in DAG order will have implications for receivers.
Previously, receiving storage order might mean seeing a number of
interleaved branches. This would mean long delta chains, sparse
I/O, and possibly more fulltext revisions instead of deltas, blowing
up storage storage. (This is the same set of problems that sparse
revlogs aims to address.) With the producer now sending revisions in DAG
order, the receiver also stores revisions in DAG order. That means
revisions for the same DAG branch are all grouped together. And this
should yield better storage outcomes. In other words, sending the
reordered changegroup allows the receiver to have better storage
order and for the producer to not propagate its (possibly sub-optimal)
internal storage order.
On the mozilla-unified repository, this change influences bundle
generation:
$ hg bundle -t none-v2 -a
before: time: real 355.680 secs (user 256.790+0.000 sys 16.820+0.000)
after: time: real 382.950 secs (user 281.700+0.000 sys 17.690+0.000)
before: 7,150,228,967 bytes (uncompressed)
after: 7,041,556,273 bytes (uncompressed)
before: 1,669,063,234 bytes (zstd l=3)
after: 1,628,598,830 bytes (zstd l=3)
$ hg unbundle
before: time: real 511.910 secs (user 466.750+0.000 sys 32.680+0.000)
after: time: real 487.790 secs (user 443.940+0.000 sys 30.840+0.000)
00manifest.d size:
source: 274,924,292 bytes
before: 304,741,626 bytes
after: 245,252,087 bytes
.hg/store total file size:
source: 2,649,133,490
before: 2,680,888,130
after: 2,627,875,673
We see the bundle size drop. That's probably because if a revlog
internally isn't storing a delta, it will choose to delta against
the last emitted revision. And on repos with interleaved branches
(like mozilla-unified), the previous revision could be an unrelated
branch and therefore be a large delta. But with this patch, the
previous revision is likely p1 or p2 and a delta should be small.
We also see the manifest size drop by ~50 MB. It's worth noting that
the manifest actually *increased* in size by ~25 MB in the old
strategy and decreased ~25 MB from its source in the new strategy.
Again, my explanation for this is that the DAG ordering in the
changegroup is resulting in better grouping of revisions in the
receiver, which results in more compact delta chains and higher
storage efficiency.
Unbundle time also dropped. I suspect this is due to the revlog having
to work less to compute deltas since the incoming deltas are more
optimal. i.e. the receiver spends less time resolving fulltext
revisions as incoming deltas bounce around between DAG branches and
delta chains.
We also see bundle generation time increase. This is not desirable.
However, the regression is only significant on the original repository:
if we generate a bundle from the repository created from the new,
always reordered bundles, we're close to baseline (if not at it with
expected noise):
$ hg bundle -t none-v2 -a
before (original): time: real 355.680 secs (user 256.790+0.000 sys 16.820+0.000)
after (original): time: real 382.950 secs (user 281.700+0.000 sys 17.690+0.000)
after (new repo): time: real 362.280 secs (user 260.300+0.000 sys 17.700+0.000)
This regression is a bit worrying because it will impact serving
canonical repositories (that don't have optimal internal storage
unless they are reordered - possibly as part of running
`hg debugupgraderepo`). However, this regression will only be
noticed by very large changegroups. And I'm guessing/hoping that
any repository that large is using clonebundles to mitigate server
load.
Again, sending DAG order isn't the optimal send order for servers:
sending in storage-delta order is. But in order to enable
storage-optimal send order, we'll need a storage API that handles
sorting. Future commits will introduce such an API.
Differential Revision: https://phab.mercurial-scm.org/D4721
$ hg init a
$ cd a
$ echo a > a
$ hg ci -A -d'1 0' -m a
adding a
$ cd ..
$ hg init b
$ cd b
$ echo b > b
$ hg ci -A -d'1 0' -m b
adding b
$ cd ..
$ hg clone a c
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd c
$ cat >> .hg/hgrc <<EOF
> [paths]
> relative = ../a
> EOF
$ hg pull -f ../b
pulling from ../b
searching for changes
warning: repository is unrelated
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
new changesets b6c483daf290
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg merge
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ cd ..
Testing -R/--repository:
$ hg -R a tip
changeset: 0:8580ff50825a
tag: tip
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: a
$ hg --repository b tip
changeset: 0:b6c483daf290
tag: tip
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: b
-R with a URL:
$ hg -R file:a identify
8580ff50825a tip
$ hg -R file://localhost/`pwd`/a/ identify
8580ff50825a tip
-R with path aliases:
$ cd c
$ hg -R default identify
8580ff50825a tip
$ hg -R relative identify
8580ff50825a tip
$ echo '[paths]' >> $HGRCPATH
$ echo 'relativetohome = a' >> $HGRCPATH
$ HOME=`pwd`/../ hg -R relativetohome identify
8580ff50825a tip
$ cd ..
#if no-outer-repo
Implicit -R:
$ hg ann a/a
0: a
$ hg ann a/a a/a
0: a
$ hg ann a/a b/b
abort: no repository found in '$TESTTMP' (.hg not found)!
[255]
$ hg -R b ann a/a
abort: a/a not under root '$TESTTMP/b'
(consider using '--cwd b')
[255]
$ hg log
abort: no repository found in '$TESTTMP' (.hg not found)!
[255]
#endif
Abbreviation of long option:
$ hg --repo c tip
changeset: 1:b6c483daf290
tag: tip
parent: -1:000000000000
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: b
earlygetopt with duplicate options (36d23de02da1):
$ hg --cwd a --cwd b --cwd c tip
changeset: 1:b6c483daf290
tag: tip
parent: -1:000000000000
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: b
$ hg --repo c --repository b -R a tip
changeset: 0:8580ff50825a
tag: tip
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: a
earlygetopt short option without following space:
$ hg -q -Rb tip
0:b6c483daf290
earlygetopt with illegal abbreviations:
$ hg --confi "foo.bar=baz"
abort: option --config may not be abbreviated!
[255]
$ hg --cw a tip
abort: option --cwd may not be abbreviated!
[255]
$ hg --rep a tip
abort: option -R has to be separated from other options (e.g. not -qR) and --repository may only be abbreviated as --repo!
[255]
$ hg --repositor a tip
abort: option -R has to be separated from other options (e.g. not -qR) and --repository may only be abbreviated as --repo!
[255]
$ hg -qR a tip
abort: option -R has to be separated from other options (e.g. not -qR) and --repository may only be abbreviated as --repo!
[255]
$ hg -qRa tip
abort: option -R has to be separated from other options (e.g. not -qR) and --repository may only be abbreviated as --repo!
[255]
Testing --cwd:
$ hg --cwd a parents
changeset: 0:8580ff50825a
tag: tip
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: a
Testing -y/--noninteractive - just be sure it is parsed:
$ hg --cwd a tip -q --noninteractive
0:8580ff50825a
$ hg --cwd a tip -q -y
0:8580ff50825a
Testing -q/--quiet:
$ hg -R a -q tip
0:8580ff50825a
$ hg -R b -q tip
0:b6c483daf290
$ hg -R c --quiet parents
0:8580ff50825a
1:b6c483daf290
Testing -v/--verbose:
$ hg --cwd c head -v
changeset: 1:b6c483daf290
tag: tip
parent: -1:000000000000
user: test
date: Thu Jan 01 00:00:01 1970 +0000
files: b
description:
b
changeset: 0:8580ff50825a
user: test
date: Thu Jan 01 00:00:01 1970 +0000
files: a
description:
a
$ hg --cwd b tip --verbose
changeset: 0:b6c483daf290
tag: tip
user: test
date: Thu Jan 01 00:00:01 1970 +0000
files: b
description:
b
Testing --config:
$ hg --cwd c --config paths.quuxfoo=bar paths | grep quuxfoo > /dev/null && echo quuxfoo
quuxfoo
$ hg --cwd c --config '' tip -q
abort: malformed --config option: '' (use --config section.name=value)
[255]
$ hg --cwd c --config a.b tip -q
abort: malformed --config option: 'a.b' (use --config section.name=value)
[255]
$ hg --cwd c --config a tip -q
abort: malformed --config option: 'a' (use --config section.name=value)
[255]
$ hg --cwd c --config a.= tip -q
abort: malformed --config option: 'a.=' (use --config section.name=value)
[255]
$ hg --cwd c --config .b= tip -q
abort: malformed --config option: '.b=' (use --config section.name=value)
[255]
Testing --debug:
$ hg --cwd c log --debug
changeset: 1:b6c483daf2907ce5825c0bb50f5716226281cc1a
tag: tip
phase: public
parent: -1:0000000000000000000000000000000000000000
parent: -1:0000000000000000000000000000000000000000
manifest: 1:23226e7a252cacdc2d99e4fbdc3653441056de49
user: test
date: Thu Jan 01 00:00:01 1970 +0000
files+: b
extra: branch=default
description:
b
changeset: 0:8580ff50825a50c8f716709acdf8de0deddcd6ab
phase: public
parent: -1:0000000000000000000000000000000000000000
parent: -1:0000000000000000000000000000000000000000
manifest: 0:a0c8bcbbb45c63b90b70ad007bf38961f64f2af0
user: test
date: Thu Jan 01 00:00:01 1970 +0000
files+: a
extra: branch=default
description:
a
Testing --traceback:
#if no-chg
$ hg --cwd c --config x --traceback id 2>&1 | grep -i 'traceback'
Traceback (most recent call last):
#else
Traceback for '--config' errors not supported with chg.
$ hg --cwd c --config x --traceback id 2>&1 | grep -i 'traceback'
[1]
#endif
Testing --time:
$ hg --cwd a --time id
8580ff50825a tip
time: real * (glob)
Testing --version:
$ hg --version -q
Mercurial Distributed SCM * (glob)
hide outer repo
$ hg init
Testing -h/--help:
#if no-extraextensions
$ hg -h
Mercurial Distributed SCM
list of commands:
add add the specified files on the next commit
addremove add all new files, delete all missing files
annotate show changeset information by line for each file
archive create an unversioned archive of a repository revision
backout reverse effect of earlier changeset
bisect subdivision search of changesets
bookmarks create a new bookmark or list existing bookmarks
branch set or show the current branch name
branches list repository named branches
bundle create a bundle file
cat output the current or given revision of files
clone make a copy of an existing repository
commit commit the specified files or all outstanding changes
config show combined config settings from all hgrc files
copy mark files as copied for the next commit
diff diff repository (or selected files)
export dump the header and diffs for one or more changesets
files list tracked files
forget forget the specified files on the next commit
graft copy changes from other branches onto the current branch
grep search revision history for a pattern in specified files
heads show branch heads
help show help for a given topic or a help overview
identify identify the working directory or specified revision
import import an ordered set of patches
incoming show new changesets found in source
init create a new repository in the given directory
log show revision history of entire repository or files
manifest output the current or given revision of the project manifest
merge merge another revision into working directory
outgoing show changesets not found in the destination
paths show aliases for remote repositories
phase set or show the current phase name
pull pull changes from the specified source
push push changes to the specified destination
recover roll back an interrupted transaction
remove remove the specified files on the next commit
rename rename files; equivalent of copy + remove
resolve redo merges or set/view the merge status of files
revert restore files to their checkout state
root print the root (top) of the current working directory
serve start stand-alone webserver
status show changed files in the working directory
summary summarize working directory state
tag add one or more tags for the current or given revision
tags list repository tags
unbundle apply one or more bundle files
update update working directory (or switch revisions)
verify verify the integrity of the repository
version output version and copyright information
additional help topics:
bundlespec Bundle File Formats
color Colorizing Outputs
config Configuration Files
dates Date Formats
deprecated Deprecated Features
diffs Diff Formats
environment Environment Variables
extensions Using Additional Features
filesets Specifying File Sets
flags Command-line flags
glossary Glossary
hgignore Syntax for Mercurial Ignore Files
hgweb Configuring hgweb
internals Technical implementation topics
merge-tools Merge Tools
pager Pager Support
patterns File Name Patterns
phases Working with Phases
revisions Specifying Revisions
scripting Using Mercurial from scripts and automation
subrepos Subrepositories
templating Template Usage
urls URL Paths
(use 'hg help -v' to show built-in aliases and global options)
$ hg --help
Mercurial Distributed SCM
list of commands:
add add the specified files on the next commit
addremove add all new files, delete all missing files
annotate show changeset information by line for each file
archive create an unversioned archive of a repository revision
backout reverse effect of earlier changeset
bisect subdivision search of changesets
bookmarks create a new bookmark or list existing bookmarks
branch set or show the current branch name
branches list repository named branches
bundle create a bundle file
cat output the current or given revision of files
clone make a copy of an existing repository
commit commit the specified files or all outstanding changes
config show combined config settings from all hgrc files
copy mark files as copied for the next commit
diff diff repository (or selected files)
export dump the header and diffs for one or more changesets
files list tracked files
forget forget the specified files on the next commit
graft copy changes from other branches onto the current branch
grep search revision history for a pattern in specified files
heads show branch heads
help show help for a given topic or a help overview
identify identify the working directory or specified revision
import import an ordered set of patches
incoming show new changesets found in source
init create a new repository in the given directory
log show revision history of entire repository or files
manifest output the current or given revision of the project manifest
merge merge another revision into working directory
outgoing show changesets not found in the destination
paths show aliases for remote repositories
phase set or show the current phase name
pull pull changes from the specified source
push push changes to the specified destination
recover roll back an interrupted transaction
remove remove the specified files on the next commit
rename rename files; equivalent of copy + remove
resolve redo merges or set/view the merge status of files
revert restore files to their checkout state
root print the root (top) of the current working directory
serve start stand-alone webserver
status show changed files in the working directory
summary summarize working directory state
tag add one or more tags for the current or given revision
tags list repository tags
unbundle apply one or more bundle files
update update working directory (or switch revisions)
verify verify the integrity of the repository
version output version and copyright information
additional help topics:
bundlespec Bundle File Formats
color Colorizing Outputs
config Configuration Files
dates Date Formats
deprecated Deprecated Features
diffs Diff Formats
environment Environment Variables
extensions Using Additional Features
filesets Specifying File Sets
flags Command-line flags
glossary Glossary
hgignore Syntax for Mercurial Ignore Files
hgweb Configuring hgweb
internals Technical implementation topics
merge-tools Merge Tools
pager Pager Support
patterns File Name Patterns
phases Working with Phases
revisions Specifying Revisions
scripting Using Mercurial from scripts and automation
subrepos Subrepositories
templating Template Usage
urls URL Paths
(use 'hg help -v' to show built-in aliases and global options)
#endif
Not tested: --debugger