view tests/test-convert-tagsbranch-topology.t @ 33207:895ecec31c70

revlog: add an experimental option to mitigated delta issues (issue5480)

The general delta heuristic to select a delta do not scale with the number of
branch. The delta base is frequently too far away to be able to reuse a chain
according to the "distance" criteria. This leads to insertion of larger delta (or
even full text) that themselves push the bases for the next delta further away
leading to more large deltas and full texts. This full text and frequent
recomputation throw Mercurial performance in disarray.

For example of a slightly large repository

  280 000 files (2 150 000 versions)
  430 000 changesets (10 000 topological heads)

Number below compares repository with and without the distance criteria:

manifest size:
    with:    21.4 GB
    without:  0.3 GB

store size:
    with:    28.7 GB
    without   7.4 GB

bundle last 15 00 revisions:
    with:    800 seconds
             971 MB
    without:  50 seconds
              73 MB

unbundle time (of the last 15K revisions):
    with:    1150 seconds (~19 minutes)
    without:   35 seconds

Similar issues has been observed in other repositories.


Adding a new option or "feature" on stable is uncommon. However, given that this
issues is making Mercurial practically unusable, I'm exceptionally targeting
this patch for stable.

What is actually needed is a full rework of the delta building and reading
logic. However, that will be a longer process and churn not suitable for stable.

In the meantime, we introduces a quick and dirty mitigation of this in the
'experimental' config space. The new option introduces a way to set the maximum
amount of memory usable to store a diff in memory. This extend the ability for
Mercurial to create chains without removing all safe guard regarding memory
access. The option should be phased out when core has a more proper solution
available.

Setting the limit to '0' remove all limits, setting it to '-1' use the default
limit (textsize x 4).

#require git

  $ echo "[core]" >> $HOME/.gitconfig
  $ echo "autocrlf = false" >> $HOME/.gitconfig
  $ echo "[core]" >> $HOME/.gitconfig
  $ echo "autocrlf = false" >> $HOME/.gitconfig
  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > convert =
  > [convert]
  > hg.usebranchnames = True
  > hg.tagsbranch = tags-update
  > EOF
  $ GIT_AUTHOR_NAME='test'; export GIT_AUTHOR_NAME
  $ GIT_AUTHOR_EMAIL='[e-mail]'; export GIT_AUTHOR_EMAIL
  $ GIT_AUTHOR_DATE="2007-01-01 00:00:00 +0000"; export GIT_AUTHOR_DATE
  $ GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME"; export GIT_COMMITTER_NAME
  $ GIT_COMMITTER_EMAIL="$GIT_AUTHOR_EMAIL"; export GIT_COMMITTER_EMAIL
  $ GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"; export GIT_COMMITTER_DATE
  $ count=10
  $ action()
  > {
  >     GIT_AUTHOR_DATE="2007-01-01 00:00:$count +0000"
  >     GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"
  >     git "$@" >/dev/null 2>/dev/null || echo "git command error"
  >     count=`expr $count + 1`
  > }
  $ glog()
  > {
  >     hg log -G --template '{rev} "{desc|firstline}" files: {files}\n' "$@"
  > }
  $ convertrepo()
  > {
  >     hg convert --datesort git-repo hg-repo
  > }

Build a GIT repo with at least 1 tag

  $ mkdir git-repo
  $ cd git-repo
  $ git init >/dev/null 2>&1
  $ echo a > a
  $ git add a
  $ action commit -m "rev1"
  $ action tag -m "tag1" tag1
  $ cd ..

Convert without tags

  $ hg convert git-repo hg-repo --config convert.skiptags=True
  initializing destination hg-repo repository
  scanning source...
  sorting...
  converting...
  0 rev1
  updating bookmarks
  $ hg -R hg-repo tags
  tip                                0:d98c8ad3a4cf
  $ rm -rf hg-repo

Do a first conversion

  $ convertrepo
  initializing destination hg-repo repository
  scanning source...
  sorting...
  converting...
  0 rev1
  updating tags
  updating bookmarks

Simulate upstream  updates after first conversion

  $ cd git-repo
  $ echo b > a
  $ git add a
  $ action commit -m "rev2"
  $ action tag -m "tag2" tag2
  $ cd ..

Perform an incremental conversion

  $ convertrepo
  scanning source...
  sorting...
  converting...
  0 rev2
  updating tags
  updating bookmarks

Print the log

  $ cd hg-repo
  $ glog
  o  3 "update tags" files: .hgtags
  |
  | o  2 "rev2" files: a
  | |
  o |  1 "update tags" files: .hgtags
   /
  o  0 "rev1" files: a
  

  $ cd ..