view tests/test-sparse-revlog.t @ 39493:3ca144f1c8dd

snapshot: search for unrelated but reusable full-snapshot

# New Strategy Step: Reusing Snapshot Outside Of Parents' Chain.

If no suitable bases were found in the parent's chains, see if we could reuse
a full snapshot not directly related to the current revision. Such search can
be expensive, so we only search for snapshots appended to the revlog *after*
the bases used by the parents of the current revision (the one we just
tested). We assume the parent's bases were created because the previous
snapshots were unsuitable, so there are low odds they would be useful now.

This search gives a chance to reuse a delta chain unrelated to the current
revision. Without this re-use, topological branches would keep reopening new
full chains. Creating more and more snapshots as the repository grow.

In repositories with many topological branches, the lack of delta reuse can
create too many snapshots reducing overall compression to nothing. This
results in a very large repository and other usability issues.

For now, we still focus on creating level-1 snapshots. However, this principle
will play a large part in how we avoid snapshot explosion once we have more
snapshot levels.

# Effects On The Test Repository

In the test repository we created, we can see the beneficial effect of such
reuse. We need very few level-0 snapshots and the overall revlog size has
decreased.

The `hg debugrevlog` call, show a "lvl-2" snapshot. It comes from the existing
delta logic using the `prev` revision (revlog's tip) as the base. In this
specific case, it turns out the tip was a level-1 snapshot. This is a
coincidence that can be ignored.

Finding and testing against all these unrelated snapshots can have a
performance impact at write time. We currently focus on building good deltas
chain we build. Performance concern will be dealt with later in another
series.

====================================
Test delta choice with sparse revlog
====================================

Sparse-revlog usually shows the most gain on Manifest. However, it is simpler
to general an appropriate file, so we test with a single file instead. The
goal is to observe intermediate snapshot being created.

We need a large enough file. Part of the content needs to be replaced
repeatedly while some of it changes rarely.

  $ bundlepath="$TESTDIR/artifacts/cache/big-file-churn.hg"

  $ expectedhash=`cat "$bundlepath".md5`
  $ if [ ! -f "$bundlepath" ]; then
  >     echo 'skipped: missing artifact, run "'"$TESTDIR"'/artifacts/scripts/generate-churning-bundle.py"'
  >     exit 80
  > fi
  $ currenthash=`f -M "$bundlepath" | cut -d = -f 2`
  $ if [ "$currenthash" != "$expectedhash" ]; then
  >     echo 'skipped: outdated artifact, md5 "'"$currenthash"'" expected "'"$expectedhash"'" run "'"$TESTDIR"'/artifacts/scripts/generate-churning-bundle.py"'
  >     exit 80
  > fi

  $ cat >> $HGRCPATH << EOF
  > [format]
  > sparse-revlog = yes
  > [storage]
  > revlog.optimize-delta-parent-choice = yes
  > EOF
  $ hg init sparse-repo
  $ cd sparse-repo
  $ hg unbundle $bundlepath
  adding changesets
  adding manifests
  adding file changes
  added 5001 changesets with 5001 changes to 1 files (+89 heads)
  new changesets 9706f5af64f4:d9032adc8114 (5001 drafts)
  (run 'hg heads' to see heads, 'hg merge' to merge)
  $ hg up
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  updated to "d9032adc8114: commit #5000"
  89 other heads for branch "default"

  $ hg log --stat -r 0:3
  changeset:   0:9706f5af64f4
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     initial commit
  
   SPARSE-REVLOG-TEST-FILE |  10500 ++++++++++++++++++++++++++++++++++++++++++++++
   1 files changed, 10500 insertions(+), 0 deletions(-)
  
  changeset:   1:724907deaa5e
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     commit #1
  
   SPARSE-REVLOG-TEST-FILE |  1068 +++++++++++++++++++++++-----------------------
   1 files changed, 534 insertions(+), 534 deletions(-)
  
  changeset:   2:62c41bce3e5d
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     commit #2
  
   SPARSE-REVLOG-TEST-FILE |  1068 +++++++++++++++++++++++-----------------------
   1 files changed, 534 insertions(+), 534 deletions(-)
  
  changeset:   3:348a9cbd6959
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     commit #3
  
   SPARSE-REVLOG-TEST-FILE |  1068 +++++++++++++++++++++++-----------------------
   1 files changed, 534 insertions(+), 534 deletions(-)
  

  $ f -s .hg/store/data/*.d
  .hg/store/data/_s_p_a_r_s_e-_r_e_v_l_o_g-_t_e_s_t-_f_i_l_e.d: size=67810463
  $ hg debugrevlog *
  format : 1
  flags  : generaldelta
  
  revisions     :     5001
      merges    :      625 (12.50%)
      normal    :     4376 (87.50%)
  revisions     :     5001
      empty     :        0 ( 0.00%)
                     text  :        0 (100.00%)
                     delta :        0 (100.00%)
      snapshot  :      126 ( 2.52%)
        lvl-0   :              4 ( 0.08%)
        lvl-1   :            120 ( 2.40%)
        lvl-2   :              2 ( 0.04%)
      deltas    :     4875 (97.48%)
  revision size : 67810463
      snapshot  : 14373347 (21.20%)
        lvl-0   :         804235 ( 1.19%)
        lvl-1   :       13535903 (19.96%)
        lvl-2   :          33209 ( 0.05%)
      deltas    : 53437116 (78.80%)
  
  chunks        :     5001
      0x78 (x)  :     5001 (100.00%)
  chunks size   : 67810463
      0x78 (x)  : 67810463 (100.00%)
  
  avg chain length  :       18
  max chain length  :       45
  max chain reach   : 25808240
  compression ratio :       25
  
  uncompressed data size (min/max/avg) : 346468 / 346472 / 346471
  full revision size (min/max/avg)     : 201014 / 201116 / 201058
  inter-snapshot size (min/max/avg)    : 11623 / 173150 / 111222
      level-1   (min/max/avg)          : 11623 / 173150 / 112799
      level-2   (min/max/avg)          : 14151 / 19058 / 16604
  delta size (min/max/avg)             : 10649 / 101790 / 10961
  
  deltas against prev  : 4207 (86.30%)
      where prev = p1  : 4164     (98.98%)
      where prev = p2  :    0     ( 0.00%)
      other            :   43     ( 1.02%)
  deltas against p1    :  653 (13.39%)
  deltas against p2    :   15 ( 0.31%)
  deltas against other :    0 ( 0.00%)