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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.
author | Boris Feld <boris.feld@octobus.net> |
---|---|
date | Fri, 07 Sep 2018 11:17:30 -0400 |
parents | a33f394b2bfd |
children | e72130f58f5d |
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==================================== 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%)