view tests/test-init.t @ 30155:b7a966ce89ed

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.

This test tries to exercise the ssh functionality with a dummy script

(enable general delta early)

  $ cat << EOF >> $HGRCPATH
  > [format]
  > usegeneraldelta=yes
  > EOF

  $ checknewrepo()
  > {
  >    name=$1
  >    if [ -d "$name"/.hg/store ]; then
  >    echo store created
  >    fi
  >    if [ -f "$name"/.hg/00changelog.i ]; then
  >    echo 00changelog.i created
  >    fi
  >    cat "$name"/.hg/requires
  > }

creating 'local'

  $ hg init local
  $ checknewrepo local
  store created
  00changelog.i created
  dotencode
  fncache
  generaldelta
  revlogv1
  store
  $ echo this > local/foo
  $ hg ci --cwd local -A -m "init"
  adding foo

test custom revlog chunk cache sizes

  $ hg --config format.chunkcachesize=0 log -R local -pv
  abort: revlog chunk cache size 0 is not greater than 0!
  [255]
  $ hg --config format.chunkcachesize=1023 log -R local -pv
  abort: revlog chunk cache size 1023 is not a power of 2!
  [255]
  $ hg --config format.chunkcachesize=1024 log -R local -pv
  changeset:   0:08b9e9f63b32
  tag:         tip
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  files:       foo
  description:
  init
  
  
  diff -r 000000000000 -r 08b9e9f63b32 foo
  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
  +++ b/foo	Thu Jan 01 00:00:00 1970 +0000
  @@ -0,0 +1,1 @@
  +this
  

creating repo with format.usestore=false

  $ hg --config format.usestore=false init old
  $ checknewrepo old
  generaldelta
  revlogv1

creating repo with format.usefncache=false

  $ hg --config format.usefncache=false init old2
  $ checknewrepo old2
  store created
  00changelog.i created
  generaldelta
  revlogv1
  store

creating repo with format.dotencode=false

  $ hg --config format.dotencode=false init old3
  $ checknewrepo old3
  store created
  00changelog.i created
  fncache
  generaldelta
  revlogv1
  store

creating repo with format.dotencode=false

  $ hg --config format.generaldelta=false --config format.usegeneraldelta=false init old4
  $ checknewrepo old4
  store created
  00changelog.i created
  dotencode
  fncache
  revlogv1
  store

test failure

  $ hg init local
  abort: repository local already exists!
  [255]

init+push to remote2

  $ hg init -e "python \"$TESTDIR/dummyssh\"" ssh://user@dummy/remote2
  $ hg incoming -R remote2 local
  comparing with local
  changeset:   0:08b9e9f63b32
  tag:         tip
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     init
  

  $ hg push -R local -e "python \"$TESTDIR/dummyssh\"" ssh://user@dummy/remote2
  pushing to ssh://user@dummy/remote2
  searching for changes
  remote: adding changesets
  remote: adding manifests
  remote: adding file changes
  remote: added 1 changesets with 1 changes to 1 files

clone to remote1

  $ hg clone -e "python \"$TESTDIR/dummyssh\"" local ssh://user@dummy/remote1
  searching for changes
  remote: adding changesets
  remote: adding manifests
  remote: adding file changes
  remote: added 1 changesets with 1 changes to 1 files

The largefiles extension doesn't crash
  $ hg clone -e "python \"$TESTDIR/dummyssh\"" local ssh://user@dummy/remotelf --config extensions.largefiles=
  searching for changes
  remote: adding changesets
  remote: adding manifests
  remote: adding file changes
  remote: added 1 changesets with 1 changes to 1 files

init to existing repo

  $ hg init -e "python \"$TESTDIR/dummyssh\"" ssh://user@dummy/remote1
  abort: repository remote1 already exists!
  abort: could not create remote repo!
  [255]

clone to existing repo

  $ hg clone -e "python \"$TESTDIR/dummyssh\"" local ssh://user@dummy/remote1
  abort: repository remote1 already exists!
  abort: could not create remote repo!
  [255]

output of dummyssh

  $ cat dummylog
  Got arguments 1:user@dummy 2:hg init remote2
  Got arguments 1:user@dummy 2:hg -R remote2 serve --stdio
  Got arguments 1:user@dummy 2:hg -R remote2 serve --stdio
  Got arguments 1:user@dummy 2:hg init remote1
  Got arguments 1:user@dummy 2:hg -R remote1 serve --stdio
  Got arguments 1:user@dummy 2:hg init remotelf
  Got arguments 1:user@dummy 2:hg -R remotelf serve --stdio
  Got arguments 1:user@dummy 2:hg init remote1
  Got arguments 1:user@dummy 2:hg init remote1

comparing repositories

  $ hg tip -q -R local
  0:08b9e9f63b32
  $ hg tip -q -R remote1
  0:08b9e9f63b32
  $ hg tip -q -R remote2
  0:08b9e9f63b32

check names for repositories (clashes with URL schemes, special chars)

  $ for i in bundle file hg http https old-http ssh static-http "with space"; do
  >   printf "hg init \"$i\"... "
  >   hg init "$i"
  >   test -d "$i" -a -d "$i/.hg" && echo "ok" || echo "failed"
  > done
  hg init "bundle"... ok
  hg init "file"... ok
  hg init "hg"... ok
  hg init "http"... ok
  hg init "https"... ok
  hg init "old-http"... ok
  hg init "ssh"... ok
  hg init "static-http"... ok
  hg init "with space"... ok
#if eol-in-paths
/* " " is not a valid name for a directory on Windows */
  $ hg init " "
  $ test -d " "
  $ test -d " /.hg"
#endif

creating 'local/sub/repo'

  $ hg init local/sub/repo
  $ checknewrepo local/sub/repo
  store created
  00changelog.i created
  dotencode
  fncache
  generaldelta
  revlogv1
  store

prepare test of init of url configured from paths

  $ echo '[paths]' >> $HGRCPATH
  $ echo "somewhere = `pwd`/url from paths" >> $HGRCPATH
  $ echo "elsewhere = `pwd`/another paths url" >> $HGRCPATH

init should (for consistency with clone) expand the url

  $ hg init somewhere
  $ checknewrepo "url from paths"
  store created
  00changelog.i created
  dotencode
  fncache
  generaldelta
  revlogv1
  store

verify that clone also expand urls

  $ hg clone somewhere elsewhere
  updating to branch default
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ checknewrepo "another paths url"
  store created
  00changelog.i created
  dotencode
  fncache
  generaldelta
  revlogv1
  store

clone bookmarks

  $ hg -R local bookmark test
  $ hg -R local bookmarks
   * test                      0:08b9e9f63b32
  $ hg clone -e "python \"$TESTDIR/dummyssh\"" local ssh://user@dummy/remote-bookmarks
  searching for changes
  remote: adding changesets
  remote: adding manifests
  remote: adding file changes
  remote: added 1 changesets with 1 changes to 1 files
  exporting bookmark test
  $ hg -R remote-bookmarks bookmarks
     test                      0:08b9e9f63b32