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
setup repo
$ hg init t
$ cd t
$ echo a > a
$ hg commit -Am'add a'
adding a
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 1 changesets with 1 changes to 1 files
$ hg parents
changeset: 0:1f0dee641bb7
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: add a
rollback to null revision
$ hg status
$ hg rollback
repository tip rolled back to revision -1 (undo commit)
working directory now based on revision -1
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 0 changesets with 0 changes to 0 files
$ hg parents
$ hg status
A a
Two changesets this time so we rollback to a real changeset
$ hg commit -m'add a again'
$ echo a >> a
$ hg commit -m'modify a'
Test issue 902 (current branch is preserved)
$ hg branch test
marked working directory as branch test
(branches are permanent and global, did you want a bookmark?)
$ hg rollback
repository tip rolled back to revision 0 (undo commit)
working directory now based on revision 0
$ hg branch
default
Test issue 1635 (commit message saved)
$ cat .hg/last-message.txt ; echo
modify a
Test rollback of hg before issue 902 was fixed
$ hg commit -m "test3"
$ hg branch test
marked working directory as branch test
(branches are permanent and global, did you want a bookmark?)
$ rm .hg/undo.branch
$ hg rollback
repository tip rolled back to revision 0 (undo commit)
named branch could not be reset: current branch is still 'test'
working directory now based on revision 0
$ hg branch
test
working dir unaffected by rollback: do not restore dirstate et. al.
$ hg log --template '{rev} {branch} {desc|firstline}\n'
0 default add a again
$ hg status
M a
$ hg bookmark foo
$ hg commit -m'modify a again'
$ echo b > b
$ hg bookmark bar -r default #making bar active, before the transaction
$ hg commit -Am'add b'
adding b
$ hg log --template '{rev} {branch} {desc|firstline}\n'
2 test add b
1 test modify a again
0 default add a again
$ hg update bar
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
(activating bookmark bar)
$ cat .hg/undo.branch ; echo
test
$ hg rollback -f
repository tip rolled back to revision 1 (undo commit)
$ hg id -n
0
$ hg branch
default
$ cat .hg/bookmarks.current ; echo
bar
$ hg bookmark --delete foo bar
rollback by pretxncommit saves commit message (issue1635)
$ echo a >> a
$ hg --config hooks.pretxncommit=false commit -m"precious commit message"
transaction abort!
rollback completed
abort: pretxncommit hook exited with status * (glob)
[255]
$ cat .hg/last-message.txt ; echo
precious commit message
same thing, but run $EDITOR
$ cat > editor.sh << '__EOF__'
> echo "another precious commit message" > "$1"
> __EOF__
$ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg --config hooks.pretxncommit=false commit 2>&1
note: commit message saved in .hg/last-message.txt
transaction abort!
rollback completed
abort: pretxncommit hook exited with status * (glob)
[255]
$ cat .hg/last-message.txt
another precious commit message
test rollback on served repository
#if serve
$ hg commit -m "precious commit message"
$ hg serve -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log
$ cat hg.pid >> $DAEMON_PIDS
$ cd ..
$ hg clone http://localhost:$HGPORT u
requesting all changes
adding changesets
adding manifests
adding file changes
added 3 changesets with 2 changes to 1 files (+1 heads)
new changesets 23b0221f3370:068774709090
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd u
$ hg id default
068774709090
now rollback and observe that 'hg serve' reloads the repository and
presents the correct tip changeset:
$ hg -R ../t rollback
repository tip rolled back to revision 1 (undo commit)
working directory now based on revision 0
$ hg id default
791dd2169706
$ killdaemons.py
#endif
update to older changeset and then refuse rollback, because
that would lose data (issue2998)
$ cd ../t
$ hg -q update
$ rm `hg status -un`
$ template='{rev}:{node|short} [{branch}] {desc|firstline}\n'
$ echo 'valuable new file' > b
$ echo 'valuable modification' >> a
$ hg commit -A -m'a valuable change'
adding b
$ hg update 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg rollback
abort: rollback of last commit while not checked out may lose data
(use -f to force)
[255]
$ hg tip -q
2:4d9cd3795eea
$ hg rollback -f
repository tip rolled back to revision 1 (undo commit)
$ hg status
$ hg log --removed b # yep, it's gone
same again, but emulate an old client that doesn't write undo.desc
$ hg -q update
$ echo 'valuable modification redux' >> a
$ hg commit -m'a valuable change redux'
$ rm .hg/undo.desc
$ hg update 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg rollback
rolling back unknown transaction
$ cat a
a
corrupt journal test
$ echo "foo" > .hg/store/journal
$ hg recover
rolling back interrupted transaction
couldn't read journal entry 'foo\n'!
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 2 changesets with 2 changes to 1 files
rollback disabled by config
$ cat >> $HGRCPATH <<EOF
> [ui]
> rollback = false
> EOF
$ echo narf >> pinky-sayings.txt
$ hg add pinky-sayings.txt
$ hg ci -m 'First one.'
$ hg rollback
abort: rollback is disabled because it is unsafe
(see `hg help -v rollback` for information)
[255]
$ cd ..
I/O errors on stdio are handled properly (issue5658)
$ cat > badui.py << EOF
> import errno
> from mercurial.i18n import _
> from mercurial import (
> error,
> registrar,
> ui as uimod,
> )
>
> configtable = {}
> configitem = registrar.configitem(configtable)
>
> configitem(b'ui', b'ioerrors',
> default=list,
> )
>
> def pretxncommit(ui, repo, **kwargs):
> ui.warn(b'warn during pretxncommit\n')
>
> def pretxnclose(ui, repo, **kwargs):
> ui.warn(b'warn during pretxnclose\n')
>
> def txnclose(ui, repo, **kwargs):
> ui.warn(b'warn during txnclose\n')
>
> def txnabort(ui, repo, **kwargs):
> ui.warn(b'warn during abort\n')
>
> class fdproxy(object):
> def __init__(self, ui, o):
> self._ui = ui
> self._o = o
>
> def __getattr__(self, attr):
> return getattr(self._o, attr)
>
> def write(self, msg):
> errors = set(self._ui.configlist(b'ui', b'ioerrors'))
> pretxncommit = msg == b'warn during pretxncommit\n'
> pretxnclose = msg == b'warn during pretxnclose\n'
> txnclose = msg == b'warn during txnclose\n'
> txnabort = msg == b'warn during abort\n'
> msgabort = msg == _(b'transaction abort!\n')
> msgrollback = msg == _(b'rollback completed\n')
>
> if pretxncommit and b'pretxncommit' in errors:
> raise IOError(errno.EPIPE, 'simulated epipe')
> if pretxnclose and b'pretxnclose' in errors:
> raise IOError(errno.EIO, 'simulated eio')
> if txnclose and b'txnclose' in errors:
> raise IOError(errno.EBADF, 'simulated badf')
> if txnabort and b'txnabort' in errors:
> raise IOError(errno.EPIPE, 'simulated epipe')
> if msgabort and b'msgabort' in errors:
> raise IOError(errno.EBADF, 'simulated ebadf')
> if msgrollback and b'msgrollback' in errors:
> raise IOError(errno.EIO, 'simulated eio')
>
> return self._o.write(msg)
>
> def uisetup(ui):
> class badui(ui.__class__):
> def write_err(self, *args, **kwargs):
> olderr = self.ferr
> try:
> self.ferr = fdproxy(self, olderr)
> return super(badui, self).write_err(*args, **kwargs)
> finally:
> self.ferr = olderr
>
> ui.__class__ = badui
>
> def reposetup(ui, repo):
> ui.setconfig(b'hooks', b'pretxnclose.badui', pretxnclose, b'badui')
> ui.setconfig(b'hooks', b'txnclose.badui', txnclose, b'badui')
> ui.setconfig(b'hooks', b'pretxncommit.badui', pretxncommit, b'badui')
> ui.setconfig(b'hooks', b'txnabort.badui', txnabort, b'badui')
> EOF
$ cat >> $HGRCPATH << EOF
> [extensions]
> badui = $TESTTMP/badui.py
> EOF
An I/O error during pretxncommit is handled
$ hg init ioerror-pretxncommit
$ cd ioerror-pretxncommit
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=pretxncommit commit -m 'error during pretxncommit'
warn during pretxnclose
warn during txnclose
$ hg commit -m 'commit 1'
nothing changed
[1]
$ cd ..
An I/O error during pretxnclose is handled
$ hg init ioerror-pretxnclose
$ cd ioerror-pretxnclose
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=pretxnclose commit -m 'error during pretxnclose'
warn during pretxncommit
warn during txnclose
$ hg commit -m 'commit 1'
nothing changed
[1]
$ cd ..
An I/O error during txnclose is handled
$ hg init ioerror-txnclose
$ cd ioerror-txnclose
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=txnclose commit -m 'error during txnclose'
warn during pretxncommit
warn during pretxnclose
$ hg commit -m 'commit 1'
nothing changed
[1]
$ cd ..
An I/O error writing "transaction abort" is handled
$ hg init ioerror-msgabort
$ cd ioerror-msgabort
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=msgabort --config hooks.pretxncommit=false commit -m 'error during abort message'
warn during abort
rollback completed
abort: pretxncommit hook exited with status 1
[255]
$ hg commit -m 'commit 1'
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ cd ..
An I/O error during txnabort should still result in rollback
$ hg init ioerror-txnabort
$ cd ioerror-txnabort
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=txnabort --config hooks.pretxncommit=false commit -m 'error during abort'
transaction abort!
rollback completed
abort: pretxncommit hook exited with status 1
[255]
$ hg commit -m 'commit 1'
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ cd ..
An I/O error writing "rollback completed" is handled
$ hg init ioerror-msgrollback
$ cd ioerror-msgrollback
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=msgrollback --config hooks.pretxncommit=false commit -m 'error during rollback message'
transaction abort!
warn during abort
abort: pretxncommit hook exited with status 1
[255]
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 1 changesets with 1 changes to 1 files
$ cd ..
Multiple I/O errors after transaction open are handled.
This is effectively what happens if a peer disconnects in the middle
of a transaction.
$ hg init ioerror-multiple
$ cd ioerror-multiple
$ echo 0 > foo
$ hg -q commit -A -m initial
warn during pretxncommit
warn during pretxnclose
warn during txnclose
$ echo 1 > foo
$ hg --config ui.ioerrors=pretxncommit,pretxnclose,txnclose,txnabort,msgabort,msgrollback commit -m 'multiple errors'
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 2 changesets with 2 changes to 1 files
$ cd ..