view tests/test-rebase-base-flag.t @ 34824:e2ad93bcc084

revlog: introduce an experimental flag to slice chunks reads when too sparse

Delta chains can become quite sparse if there is a lot of unrelated data between
relevant pieces. Right now, revlog always reads all the necessary data for the
delta chain in one single read. This can lead to a lot of unrelated data to be
read (see issue5482 for more details).
One can use the `experimental.maxdeltachainspan` option with a large value
(or -1) to easily produce a very sparse delta chain.

This change introduces the ability to slice the chunks retrieval into  multiple
reads, skipping large sections of unrelated data. Preliminary testing shows
interesting results. For example the peak memory consumption to read a manifest
on a large repository is reduced from 600MB to 250MB (200MB without
maxdeltachainspan). However, the slicing itself and the multiple reads can have
an negative impact on performance. This is why the new feature is hidden behind
an experimental flag.

Future changesets will add various parameters to control the slicing heuristics.
We hope to experiment a wide variety of repositories during 4.4 and hopefully
turn the feature on by default in 4.5.
As a first try, the algorithm itself is prone to deep changes. However, we wish
to define APIs and have a baseline to work on.

Test the "--base" flag of the rebase command. (Tests unrelated to the "--base"
flag should probably live in somewhere else)

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > rebase=
  > drawdag=$TESTDIR/drawdag.py
  > 
  > [phases]
  > publish=False
  > 
  > [alias]
  > tglog = log -G --template "{rev}: {desc}"
  > EOF

  $ rebasewithdag() {
  >   N=`$PYTHON -c "print($N+1)"`
  >   hg init repo$N && cd repo$N
  >   hg debugdrawdag
  >   hg rebase "$@" > _rebasetmp
  >   r=$?
  >   grep -v 'saved backup bundle' _rebasetmp
  >   [ $r -eq 0 ] && hg tglog
  >   cd ..
  >   return $r
  > }

Single branching point, without merge:

  $ rebasewithdag -b D -d Z <<'EOS'
  >     D E
  >     |/
  > Z B C   # C: branching point, E should be picked
  >  \|/    # B should not be picked
  >   A
  >   |
  >   R
  > EOS
  rebasing 3:d6003a550c2c "C" (C)
  rebasing 5:4526cf523425 "D" (D)
  rebasing 6:b296604d9846 "E" (E tip)
  o  6: E
  |
  | o  5: D
  |/
  o  4: C
  |
  o  3: Z
  |
  | o  2: B
  |/
  o  1: A
  |
  o  0: R
  
Multiple branching points caused by selecting a single merge changeset:

  $ rebasewithdag -b E -d Z <<'EOS'
  >     E
  >    /|
  >   B C D  # B, C: multiple branching points
  >   | |/   # D should not be picked
  > Z | /
  >  \|/
  >   A
  >   |
  >   R
  > EOS
  rebasing 2:c1e6b162678d "B" (B)
  rebasing 3:d6003a550c2c "C" (C)
  rebasing 6:54c8f00cb91c "E" (E tip)
  o    6: E
  |\
  | o  5: C
  | |
  o |  4: B
  |/
  o  3: Z
  |
  | o  2: D
  |/
  o  1: A
  |
  o  0: R
  
Rebase should not extend the "--base" revset using "descendants":

  $ rebasewithdag -b B -d Z <<'EOS'
  >     E
  >    /|
  > Z B C  # descendants(B) = B+E. With E, C will be included incorrectly
  >  \|/
  >   A
  >   |
  >   R
  > EOS
  rebasing 2:c1e6b162678d "B" (B)
  rebasing 5:54c8f00cb91c "E" (E tip)
  o    5: E
  |\
  | o  4: B
  | |
  | o  3: Z
  | |
  o |  2: C
  |/
  o  1: A
  |
  o  0: R
  
Rebase should not simplify the "--base" revset using "roots":

  $ rebasewithdag -b B+E -d Z <<'EOS'
  >     E
  >    /|
  > Z B C  # roots(B+E) = B. Without E, C will be missed incorrectly
  >  \|/
  >   A
  >   |
  >   R
  > EOS
  rebasing 2:c1e6b162678d "B" (B)
  rebasing 3:d6003a550c2c "C" (C)
  rebasing 5:54c8f00cb91c "E" (E tip)
  o    5: E
  |\
  | o  4: C
  | |
  o |  3: B
  |/
  o  2: Z
  |
  o  1: A
  |
  o  0: R
  
The destination is one of the two branching points of a merge:

  $ rebasewithdag -b F -d Z <<'EOS'
  >     F
  >    / \
  >   E   D
  >  /   /
  > Z   C
  >  \ /
  >   B
  >   |
  >   A
  > EOS
  nothing to rebase
  [1]

Multiple branching points caused by multiple bases (issue5420):

  $ rebasewithdag -b E1+E2+C2+B1 -d Z <<'EOS'
  >   Z    E2
  >   |   /
  >   F E1 C2
  >   |/  /
  >   E C1 B2
  >   |/  /
  >   C B1
  >   |/
  >   B
  >   |
  >   A
  >   |
  >   R
  > EOS
  rebasing 3:a113dbaa660a "B1" (B1)
  rebasing 5:06ce7b1cc8c2 "B2" (B2)
  rebasing 6:0ac98cce32d3 "C1" (C1)
  rebasing 8:781512f5e33d "C2" (C2)
  rebasing 9:428d8c18f641 "E1" (E1)
  rebasing 11:e1bf82f6b6df "E2" (E2)
  o  12: E2
  |
  o  11: E1
  |
  | o  10: C2
  | |
  | o  9: C1
  |/
  | o  8: B2
  | |
  | o  7: B1
  |/
  o  6: Z
  |
  o  5: F
  |
  o  4: E
  |
  o  3: C
  |
  o  2: B
  |
  o  1: A
  |
  o  0: R
  
Multiple branching points with multiple merges:

  $ rebasewithdag -b G+P -d Z <<'EOS'
  > G   H   P
  > |\ /|   |\
  > F E D   M N
  >  \|/|  /| |\
  > Z C B I J K L
  >  \|/  |/  |/
  >   A   A   A
  > EOS
  rebasing 2:dc0947a82db8 "C" (C)
  rebasing 8:4e4f9194f9f1 "D" (D)
  rebasing 9:03ca77807e91 "E" (E)
  rebasing 10:afc707c82df0 "F" (F)
  rebasing 13:690dfff91e9e "G" (G)
  rebasing 14:2893b886bb10 "H" (H)
  rebasing 3:08ebfeb61bac "I" (I)
  rebasing 4:a0a5005cec67 "J" (J)
  rebasing 5:83780307a7e8 "K" (K)
  rebasing 6:e131637a1cb6 "L" (L)
  rebasing 11:d1f6d0c3c7e4 "M" (M)
  rebasing 12:7aaec6f81888 "N" (N)
  rebasing 15:325bc8f1760d "P" (P tip)
  o    15: P
  |\
  | o    14: N
  | |\
  o \ \    13: M
  |\ \ \
  | | | o  12: L
  | | | |
  | | o |  11: K
  | | |/
  | o /  10: J
  | |/
  o /  9: I
  |/
  | o    8: H
  | |\
  | | | o  7: G
  | | |/|
  | | | o  6: F
  | | | |
  | | o |  5: E
  | | |/
  | o |  4: D
  | |\|
  +---o  3: C
  | |
  o |  2: Z
  | |
  | o  1: B
  |/
  o  0: A
  
Slightly more complex merge case (mentioned in https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-November/091074.html):

  $ rebasewithdag -b A3+B3 -d Z <<'EOF'
  > Z     C1    A3     B3
  > |    /     / \    / \
  > M3 C0     A1  A2 B1  B2
  > | /       |   |  |   |
  > M2        M1  C1 C1  M3
  > |
  > M1
  > |
  > M0
  > EOF
  rebasing 4:8817fae53c94 "C0" (C0)
  rebasing 6:06ca5dfe3b5b "B2" (B2)
  rebasing 7:73508237b032 "C1" (C1)
  rebasing 9:fdb955e2faed "A2" (A2)
  rebasing 11:4e449bd1a643 "A3" (A3)
  rebasing 10:0a33b0519128 "B1" (B1)
  rebasing 12:209327807c3a "B3" (B3 tip)
  o    12: B3
  |\
  | o  11: B1
  | |
  | | o    10: A3
  | | |\
  | +---o  9: A2
  | | |
  | o |  8: C1
  | | |
  o | |  7: B2
  | | |
  | o |  6: C0
  |/ /
  o |  5: Z
  | |
  o |  4: M3
  | |
  o |  3: M2
  | |
  | o  2: A1
  |/
  o  1: M1
  |
  o  0: M0
  
Disconnected graph:

  $ rebasewithdag -b B -d Z <<'EOS'
  >   B
  >   |
  > Z A
  > EOS
  nothing to rebase from 112478962961 to 48b9aae0607f
  [1]

Multiple roots. Roots are ancestors of dest:

  $ rebasewithdag -b B+D -d Z <<'EOF'
  > D Z B
  >  \|\|
  >   C A
  > EOF
  rebasing 2:112478962961 "B" (B)
  rebasing 3:b70f76719894 "D" (D)
  o  4: D
  |
  | o  3: B
  |/
  o    2: Z
  |\
  | o  1: C
  |
  o  0: A
  
Multiple roots. One root is not an ancestor of dest:

  $ rebasewithdag -b B+D -d Z <<'EOF'
  > Z B D
  >  \|\|
  >   A C
  > EOF
  nothing to rebase from f675d5a1c6a4+b70f76719894 to 262e37e34f63
  [1]

Multiple roots. One root is not an ancestor of dest. Select using a merge:

  $ rebasewithdag -b E -d Z <<'EOF'
  >   E
  >   |\
  > Z B D
  >  \|\|
  >   A C
  > EOF
  rebasing 2:f675d5a1c6a4 "B" (B)
  rebasing 5:f68696fe6af8 "E" (E tip)
  o    5: E
  |\
  | o    4: B
  | |\
  | | o  3: Z
  | | |
  o | |  2: D
  |/ /
  o /  1: C
   /
  o  0: A
  
Multiple roots. Two children share two parents while dest has only one parent:

  $ rebasewithdag -b B+D -d Z <<'EOF'
  > Z B D
  >  \|\|\
  >   A C A
  > EOF
  rebasing 2:f675d5a1c6a4 "B" (B)
  rebasing 3:c2a779e13b56 "D" (D)
  o    4: D
  |\
  +---o  3: B
  | |/
  | o  2: Z
  | |
  o |  1: C
   /
  o  0: A